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 LIBRARY 
 
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 JOHNS HOPKINS UNIVERSITY 
 
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 WELC 
 
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 LIBRARY 
 
 WILLIAM H. WELCH MEDICAL LIBRARY 
 
 Presented by 
 
 
 
Digitized by the Internet Archive 
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37 
 
 AN 
 
 ACCOUNT 
 
 OF THE 
 
 STRUCTURE of the EYE, 
 
 WITH OCCASIONAL. 
 
 REMARKS on fome DISORDERS 
 of that ORGAN, 
 
 DELIVERED IN 
 
 LECTURES 
 
 AT THE 
 
 THEATRE of SURGEONS-HALL, 
 
 By THOMAS GAT AKER, 
 
 *1 
 
 Surgeon-Extraordinary to her Royal Highnefs the Princefs of WALES, 
 and Surgeon to St. GE ORGE’s-HO S PIT AL, 
 
 LONDON: 
 
 Printed for R. and J. Dqdesly, in Pall-Mall ; and 
 G. Hawkins, at Temple-Bar, 1761, 
 
(F -fd-Aer 
 
 l 
 
 
 
 
 GIFT 
 
 <aM / - 
 
 Y 
 
 VJU 7/U.Z a 
 
 J 
 
TO THE 
 
 Master, Governors, 
 
 AND 
 
 Court of Assistants 
 
 OF THE 
 
 INCORPORATED SOCIETY 
 
 * O F 
 
 SURGEONS of LONDON. 
 
 * 
 
 GENTLEMEN, 
 
 T H E obliging manner in which 
 you appointed me to the office 
 of reading thefe occafional Lectures was 
 efteemed by me as a mark of your re- 
 gard : nor had I lefs reafon to be fatis- 
 fyed with your favourable behaviour to 
 me at the Theatre: juffiy fenhble of 
 thefe civilities, I take this public oppor- 
 tunity of returning you my thanks. 
 
 In 
 
DEDICATION. 
 
 In compiling the anatomical part of 
 thefe Ledtures, I endeavoured to col- 
 led; the principal articles of this interefl- 
 ing fubjedt, and to form fuch an ac- 
 count of the eye and eye-lids as might 
 convey a clear and comprehenhve idea 
 of their ftrudure and ufes without being 
 tedioully circumdantial. 
 
 If I have fucceeded in my defign, as I 
 am encouraged to hope, the trouble I 
 took on the occafion may probably, by 
 this publication, be ufeful to thofe who 
 are in purfuit of the fame knowledge. 
 
 I am, 
 
 Gentlemen, 
 
 With due regard 
 Your mod obedient Servant, 
 
 THOMAS GATAKER. 
 
FIRST LECTURE. 
 
 THE 
 
 INTRODUCTION. 
 
 T H E fubjedt I have chofen for the 
 following Lectures, is the ftruc- 
 ture of the eye and eye-lids ; a fub- 
 jedt which, from its importance in regard to 
 the difeafes of thefe parts, and from the diffi- 
 culty of acquiring a perfect knowledge of it, 
 will, I imagine, very properly admit of being 
 often explained. 
 
 As the employments and the pleafures of life 
 depend, in a great degree, upon the faculty of 
 feeing , whatever may interrupt or deftroy the 
 ufe of that faculty defer TT °s our utmoft attention. 
 Agreeable to this opinion, we find in all 
 times, that the diforders which affedted the 
 fight were confidered as objedts of importance. 
 Anatomifts have taken great pains in difcover- 
 ing the ftrudture of the eye; the arts of phyfic 
 
 B and 
 
2 INTRODUCTION 
 
 and forgery have been employed to remove or 
 alleviate the difeafes of this organ ; and to thefe 
 afiiftances has been added the refult of philofo- 
 phical enquiries, which, by explaining the na- 
 ture of vifion, have pointed out the means 
 of relief in fome cafes, wherein neither medi- 
 cines nor manual operations avail. 
 
 With all thefe advantages, it may feem ex- 
 traordinary to alfert, that diforders of the eyes 
 are frequently treated with far lefs propriety 
 than many other complaints of a much lefs in- 
 terefling nature. 
 
 May I be allowed here to mention fome of 
 the reafons which induce me to form this 
 opinion ? 
 
 If we look into the accounts given of 
 the difeafes of the eyes, we fhall find, that 
 moft authors, who have written at large or 
 profefledly upon this fubjedl, feem fond of 
 making numerous diftindfions of the com- 
 plaints, and of bellowing particular appella- 
 tions upon them; multiplying them by this 
 means into fuch an ufelefs and perplexing va- 
 riety, that the diforders of the eyes and eye- 
 lids are made by fome writers almoft equal to 
 all the other diftempers which ever affedted the 
 4 * human 
 
TO THE FIRST LECTURE. 3 
 human body. But whatever indulgences of 
 this fort a creative fancy, or a motive lefs juf- 
 tifiable may induce a writer to take, it ought 
 to be confidered, that all diftindtions and fub- 
 divifions upon thefe occaflons, which are not 
 effentially ufeful, are in reality prejudicial: 
 they load the memory, and confound the judg- 
 ment of the unexperienced, and by filling the 
 mind with falfe and trifling ideas, they prevent 
 in fome degree the more plain and neceffary 
 circumftances from being properly attended to. 
 In the treatment of tumours formed upon the 
 eye-lids, of what confequence can it be to 
 know, whether any of them refemble a barley- 
 corn, a gravel-flone, or a hail-flone? In the 
 defcription of fome diforders of the eye itfelf, 
 what anunneceffaryand perplexing trouble mufl 
 it be to determine the figure and fize of cer- 
 tain appearances? as whether they are formed like 
 the nail of a man’s hand, or like the wing of a 
 bird ? or whether they refemble a grape-flone, 
 a fmall apple, the head of a nail, or the head 
 of a fly? Yet fuch kind of diftindtions are to 
 be met with in writers of the greatefl emi- 
 nence. 
 
 Another circumflance to which the want of 
 B 2 fuccefs 
 
4 INTRODUCTION 
 
 fuccefs in thefe cafes feems to be often owing, 
 is, that they are frequently of fo complicated a 
 nature as to be incapable of relief, without the 
 united afliftance of phyfic and furgery. Upon 
 fuch occafions, if a practitioner is unfkilled in 
 either of thofe profeflions, and wants oppor- 
 tunity or inclination to procure any additional 
 help to his own knowledge and judgment, the 
 patient muft neceffarily lofe part of that benefit 
 which the circumftances of his cafe would have 
 allowed. The fame difadvantage indeed may 
 prevail more or lefs in the treatment of com- 
 plaints incident to fome other parts of the body ; 
 but here the obfervation ferves at leaf! to account 
 for the little real advantage attending the gene- 
 ral pradices and extraordinary boafls of ocu- 
 lifts. The merit of moft practitioners of this 
 clafs confifts chiefly in knowing fomething of 
 the anatomy of the eye, and in the perform- 
 ance of one or two particular operations: In 
 refpeCt to the more comprehenfive branches of 
 phyfical knowledge, as they rarely have any 
 reafonable pretence to thefe, fo their conduCt is 
 feldom ingenuous enough to make them either 
 defire or deferve the advantage of fuch know- 
 ledge from thofe who poflefs it. 
 
 To 
 
TO THE FIRST LECTURE. 5 
 To the reafons already affigned may be 
 added the following one, which relates more 
 immediately than the others to the bufi- 
 nefs of the prefent ledtures. Some practi- 
 tioners may not have that thorough knowledge 
 in the ftrudture of the eye, as to enable them 
 in all cafes to treat the feveral diforders of this 
 organ with that judgment and propriety with 
 which they would execute many other parts of 
 their profeffion. Theftudy of anatomy, it muft 
 be owned, has of late years been very much cul- 
 tivated, and the methods of teaching it have 
 been much improved : but notwithftanding the 
 progrefs which many ftudents make in this 
 very ufeful branch of phylical education, many 
 of thofe who attend anatomical courfes cannot, 
 I conceive, be reafonably expedted to receive 
 from thence all the benefit that could be wifh- 
 ed. The time and opportunities which they 
 are allowed for this improvement, are very 
 often unequal to the defign. A multiplicity of 
 things are to be learnt ; many of them are very 
 minute; and it is extremely difficult to com- 
 prehend them perfectly, and to fix them all in 
 the memory from one or two tranfient views, 
 though accompanied with the molt exadt and 
 B 3 ingenious 
 
6 INTRODUCTION 
 
 ingenious defcriptions. A moderate fhare of 
 knowledge may be acquired by this means in the 
 grofs anatomy, but when the finer organs of the 
 body become the objedts of attention, greater 
 difficulties occur : repeated examinations and 
 explanations are then necefiary, in order to 
 underftand perfectly the ftrudture and ufes of 
 thefe delicate parts, and to imprefs a lading 
 idea of them upon the mind. Shall I afiert too 
 much in faying, that the want of thefe oppor- 
 tunities of improvement, by repeated infpec- 
 tion and explanations of the parts, is not fo 
 well fupplied by books as the fubjedt feems 
 capable of? For though there is very little 
 known in anatomy that has not been given 
 to the public, yet the manner in which this 
 knowledge has been communicated, makes 
 the acquifition of it in fome inftances very 
 difficult to perfons who are little acquainted 
 with the fubjedt. Thus, in refpedl to the 
 itrudture of the eye, we find fome writers have 
 been fo concife and imperfedt in certain ar^ 
 tides, as by no means to anfwer the full in- 
 tention of fuch defcriptions: others have been 
 more elaborate and exadt, and very much de^ 
 ferve our acknowledgments and attention; but 
 
 in 
 
TO THE FIRST LECTURE. 7 
 in fome particulars, I prefume, are more diffufe 
 and circumfcantial than is generally neceffary. 
 The account is alfo frequently complicated with 
 critical difcuffions; perplexed, like moft other 
 articles in anatomy, with an ufelefs multipli- 
 cation of names and diffinCtions ; and when par- 
 ticular parts of the eye are defcribed, as is often 
 done, among other things that have not any 
 immediate connection with it, there is not that 
 methodical arrangement of the feveral parts 
 which leads belt to a regular view of the whole. 
 
 Though unequal, I fear, to the talk of re- 
 moving every difficulty from the fubjeCt, I ffiall 
 endeavour, at leaft, that the following Lectures 
 may be as little liable as poffible to the objec- 
 tions which I have taken the liberty of making 
 in the preceding paragraph. 
 
 The fubftance of thefe LeCtures will con lift, 
 as was before intimated, of a defcription of the 
 eye and eye-lids ; to which will be occaftonally 
 added fome few remarks relating to the difeafes 
 of thefe parts, and an explanation of the nature 
 and ufes of eye-waters, ointments and lini- 
 ments for the eyes. 
 
 B 4 
 
 Defcription 
 
[ 8 ] 
 
 Defcription of the Parts. 
 THE EYE-BROWS. 
 
 P REVIOUS to the defcription of thofe 
 parts which more immediately and prin- 
 cipally constitute the organ of fight, it may be 
 proper to fay a few words on a neighbouring 
 part, the eye-brow. 
 
 The eye-brows are raifed alittle above the even 
 furface of the forehead, by the thicknefs of the 
 adipous membrane which lies underneath them, 
 and by the Shape of the frontal bone in that part. 
 The form and fubStance of the eye -brows are 
 in general too well known to require any par- 
 ticular defcription : In different perfons they 
 have indeed fome peculiarities in regard to their 
 form, and likewife to their fize and colour ; 
 but thefe differences, though they often relate 
 effentially to the beauty and expreffion of the 
 countenance, are not the proper objedts of the 
 prefent account. 
 
 The eye-brows admit of two motions, de- 
 preffion and elevation. The firfl of thefe is per- 
 formed 
 
THE EYE-BROWS. 9 
 formed by a mufcle peculiar to the eye-brow, 
 and is termed depreffor or corrugator fupercilii. 
 This mufcle is adherent to, and covered by the 
 inferior portion of the frontal mufcle, of which 
 it is reckoned by fome to be an elongation. It 
 takes its origin in the great angle of the eye, 
 from the fuperior part of the nafal bone, and 
 running obliquely upwards, is inferted tendi- 
 nous into the fkin of the brow. 
 
 The ufe of this mufcle, when it adts fingly, 
 is to move its refpedtive eye-brow downwards; 
 but when both thefe mufcles adt together, 
 the brows are drawn nearer to one another at 
 the fame time that they are depreffed. In this 
 adtion, the fkin that covers the lower part of 
 the forehead between the eye-brows, and that 
 which covers the upper part of the nofe, is 
 drawn into wrinkles of different diredtions, as 
 is particularly feen in frowning. The eye-lids 
 are likewife brought clofer by this adtion, and 
 the eyes, it is fuppofed, are thereby in fome 
 degree occafionally defended from dufb or other 
 fmall bodies floating in the air, as well as 
 from the glare of a very ftrong light. 
 
 The elevation of the eye-brows is performed 
 by the frontal mufcles, whofe flefhy fibres co- 
 
 ver 
 
10 THE EYE-BROWS, 
 ver the anterior portion of the frontal bone, and 
 are partly inferted into the Ikin of the brows. 
 When thefe mufcles contract, they draw up 
 the eye-brows, and at the fame time wrinkle 
 the Ikin of the forehead and move the fcalp. 
 
 The eye-brows receive blood-velTels from 
 the temporal and frontal arteries; and nerves 
 from a branch of the fifth pair of nerves. This 
 branch, accompanied with an artery proceed- 
 ing from the internal carotid artery, runs along 
 the periofteum lining the focket of the eye, 
 and pafting through the fuperciliary hole or 
 notch of the frontal bone, is partly bellowed 
 upon the eye-brow and its mufcles. 
 
 In relpect to the general ufes of the eye- 
 brows, they are faid to intercept the rays of 
 flrong light falling in fome oblique directions 
 from above ; but though probably they are fel- 
 dom of much fervice on this account, they are 
 certainly very ufeful in preventing rain, fweat, 
 or other moiflure from running immediately oft 
 the forehead into the eyes : this they do, by di- 
 recting the courfe of fuch moiture either along 
 the fides of the nofe, or down the temples. 
 
 It might feem proper likewife in this p^ce, 
 
 a£ 
 
THE EYE-LIDS. u 
 as a preliminary article to the following de- 
 fcription, to give fome account of the orbit or 
 bony focket, in which the greateft part of the 
 organ of fight is contained : but as the gene- 
 ral form and the general ufe of this bony cavity 
 are well known, and as feveral bones contri- 
 bute to the formation of it, which are fubfer- 
 vient likewife to other purpofes, a regular de- 
 fcription of it here will be omitted. In the 
 courfe of the following account, different parts 
 of the focket will neceffarily be mentioned, and 
 whatever relates in this refpedt effentially to 
 explaining the prefent fubjedt, (hall be occafion- 
 ally obferv ed. The next parts to be confidered 
 therefore are the eye-lids. 
 
 THE EYE - LIDS. 
 
 f i ' H E eye-lids are connedted to the cir- 
 -I- cumference of the focket by the tunica 
 conjundtiva, which will be particularly de- 
 fcribed hereafter with the coats of the eye. It 
 is neceffary, however, to obferve here in gene- 
 ral of the tunica conjundtiva, that it is a very 
 fmooth and fine membrane, which lines the in- 
 ternal furface of the eye-lids, and from the edge 
 
 of 
 
12 THE EYE-LIDS, 
 of the focket, to which it adheres all round, is 
 refledted upon and covers the anterior half of 
 the globe of the eye. 
 
 The eye-lids are compofed of the common 
 teguments, a cartilage called tarfus, and an in- 
 ternal membrane. They have two angles or cor- 
 ners, one the fmall or external, the other the 
 large or internal angle. The form of each eye- 
 lid is that of a fegment or portion of a circle, 
 and in regard to the eye is fuch, that when both 
 eye-lids are fhut they make a uniform arch, 
 adapted to the convexity of the eye, and in con- 
 tact with it : but in regard to one another 
 when fhut, their edges are fo contrived, that 
 they leave a fort of groove or channel be- 
 tween them, which is narrow towards the 
 outer angle, and wider towards the inner, and 
 ferves to condudt the tears as they come from 
 the upper part of the eye, to what are called 
 the lacrymal points. The margin of each eye- 
 lid being of confiderable thicknefs, is divided 
 into the outer and inner edge. It is the outer 
 edge only of each lid that is fuppofed to join 
 when the eye-lids are fhut; the inner edge 
 being formed oblique or flanting, makes the 
 
 groove 
 
THE E Y E - L I D S. 13 
 groove or channel juft mentioned for the paflage 
 of the tears to the lacrymal points. 
 
 The outer edge of each eye-lid is furnifti- 
 ed with a row of hairs. Thefe hairs are 
 deftgned to prevent duft or other fmall bo- 
 dies flying in the air, from being admitted 
 to the ball of the eye, by the immediate 
 warning which they give to fhut the eye-lids 
 when any fuch bodies touch them. They 
 ferye likewife in fome meafure to defend the 
 eyes againft the impreflions of very ftrong light. 
 The lofs of thefe hairs, which happens fomc- 
 time from diforders in the lids, fhews the 
 utility of them in their natural ftate and fltua- 
 tion. The eye-lids, in fuch cafes, are often 
 difagreeably irritated, even by the common 
 light, and are kept almoft in conftant motion, 
 either on that account, or to prevent duft or 
 other fmall fubftances from being admitted and 
 lodging upon the eye. 
 
 On the internal edge of each lid is a row of 
 fmall holes, which are the excretory dudts of 
 what are termed the ciliary glands. Thefe 
 glands, which are likewife diftinguifhed by the 
 name of Meibomius’s glands, appear of a whi- 
 tifh colour, and are fituated on the internal 
 
 furface 
 
14 THE EYE-LIDS, 
 furface of the eye-lids, immediately under the 
 tunica conjunctiva. Their duCts, which are 
 extremely fhort, open at the internal edge of 
 each lid, and when fqueezed, yield a kind of 
 oily or unCtuous matter. This matter is lup- 
 pofed to be of ufe in preventing the attrition 
 of the eye-lids from their frequent motion ; 
 and by keeping their edges moift and foft, it 
 ferves in a great degree to hinder them from 
 being inflamed and excoriated, and from ad- 
 hering to one another. 
 
 Under the external teguments of the eye-lids 
 is the thin cartilage, called tarfus, which forms 
 the chief part of the edge of each lid. The 
 tarfus of each lid is different in refpeCl to its 
 breadth and figure. In the lower lid it is nar- 
 row, and nearly of an equal breadth all along ; 
 in the upper lid it is broader in general than in 
 
 t 
 
 the lower, and riling in its middle and fuperior 
 part, is of a femilunar fhape. From the fupe- 
 rior edge of the upper tarfus, and the inferior 
 of the lower, is continued a membranous cx- 
 panfion to the neighbouring edge of the orbit. 
 Each ofthefe membranes together with its refpec- 
 tive tarfus, has the form of the eye-lid to which 
 it belongs, and is called ligamentum tarfi. The 
 
 tarfi 
 
THE EYE-LIDS. 15 
 tarfi are concave towards the globe of the eye, 
 and convex on the other fide; and upon 
 their internal furface are tranfverfe channels 
 for the reception of the ciliary glands before- 
 mentioned. By the firm fubftance of the 
 tarfi, and by their form, they ferve to ftrengthen 
 and to keep the lids fmooth, and equally ex- 
 tended, fo that they may be freely moved 
 alike in every part; and they make that arch 
 by which the internal furface of the eye-lids is 
 adapted to the convexity of the eye. 
 
 The infide of the lids is lined with a fine 
 and very fenfible membrane, which is a por- 
 tion of the tunica conjundtiva, as has been alrea- 
 dy obferved and as will appear more particu- 
 larly hereafter. 
 
 The mufcles fubfervient to the motions- of 
 the eye-lids, are the mufculus orbicularis, and 
 the levator palpebrs fuperioris. 
 
 The mufculus orbicularis furrounds and co- 
 vers both lids. The origin of this mufcle 
 is at the great or internal angle of the 
 eye; and the fibres of it Ipreading from 
 thence and covering the lower lid, proceed on 
 to the outward or lefier angle: pafling round 
 this angle they are continued over, and cover 
 
 tne 
 
16 THE EYE-LIDS. 
 
 the upper lid, and then terminate at the great 
 or internal angle where they began. This 
 mufcle has a flight ligamentary tendon, which 
 extending tranfverfely from the nafal procefs of 
 the maxillary bone towards the internal angle 
 of the eye, and appearing like a white line, ter- 
 minates at the union of the lacrymal points. 
 
 The ufe of this mufcle is by its contraction, 
 to bring the upper lid downwards, and pull the 
 lower lid upwards, by which means both lids 
 are drawn together fo as to Shut upon the eye. 
 
 The levator palpebras fuperioris arifes from the 
 bottom of the orbit by a fmall tendon; and as 
 the flefliy fibres of this mufcle pafs over the 
 globe of the eye, they gradually fpread, and after- 
 wards terminate by a broad tendinous expan- 
 sion in the fuperior part of the tarfus belonging 
 to the upper lid. The ufe of this mufcle is to 
 open the eye, by drawing the eye-lid upwards. 
 
 The integuments of the eye-lids, and the 
 orbicular mufcle, are furniflied with blood by 
 ramifications from the angular, temporal, and 
 frontal arteries; and thefe communicate with 
 branches that are fent to the internal mem- 
 brane of the eye-lids. The levator palpebras 
 fuperioris receives a ramification from a con- 
 siderable 
 
THE EYE-LIDS. i 7 
 fiderable branch, which is fent through the in- 
 ferior orbitary fifiiire from the internal maxil- 
 lary artery, and is diilributed to this and feve- 
 ral other parts of the eye. 
 
 The veins of the eye -lids correfpond pretty 
 nearly with the courfe of the arteries, and 
 emptying themfelves into the neighbouring 
 veins of the temples and face, have their 
 blood carried by them into the external ju- 
 gulars. 
 
 The eye -lids are fupplied with nerves from 
 the opthalmic branch of the fifth pair. The 
 orbicular mufcle of the eye-lids receives nerves 
 from the fame branch ; likewife from another 
 branch of the fifth pair, called the fuperior 
 maxillary branch, and from the portio dura of 
 the feventh pair. 
 
 The levator palpebral fuperioris receives a 
 branch from the third pair. 
 
 The eye- lids ferve as a veil or curtain, which 
 may be clofed or opened at pleafure by the ac- 
 tion of their mufcles. When they cover the 
 eyes intirely, as in deep, they exclude the 
 light, which otherwife would hinder reft. They 
 prevent the eyes from growing dry and uneafy, 
 and defend them from being injured by a 
 
 C variety 
 
18 THE EYE-LIDS, 
 variety of fmall bodies which might fall upon 
 them from the common air. Even the fofteft, 
 pureft air, and the mildeft light, would create 
 very painful fenfations in the globe of the eye, 
 if it was intirely diverted of this moft ufeful co- 
 vering. In the day, the eye-lids form a fort of 
 fhade, and occartonally moderate the influx of 
 light into the eye. The frequent motion of 
 them ferves alfo to fpread the tears or lacrymal 
 fluid over the anterior furface of the globe of 
 the eye, and by that means wafhes off and 
 cleanfes it from any foulnefs which may have 
 been collected there. By the fame means the 
 tranfparency of the cornea is preferved. Nor 
 is this frequent motion of the eye-lids, which 
 is fo neceflary for the purpofes jurt mentioned, 
 at all inconvenient in refpedt to vifion ; as the 
 quicknefs of the motion prevents its being any 
 continued and perceptible obftrudtion to the 
 free paffage of the rays of light into the eye. 
 
 The eye-lids are fubjedt to little tumours 
 either in their fubrtance or on the furface 
 of them, which fanciful writers have di- 
 vided into feveral kinds, and have diftinguifh- 
 ed by particular names from a refemblance 
 which they conceived they had to fome other 
 
 things. 
 
THE EYE-LIDS. 19 
 things, as hail-fiones, barley-corns, &c. Of 
 thefe it is fufficient to fay, that a perfon who is 
 well acquainted with the firuCture of the parts 
 and with the plain rules of furgery, will find 
 very little difficulty in the treatment of them, 
 further than the common difficulty of handling 
 or applying dreffings to parts which are con- 
 fiantly in motion, as well as contiguous to fo 
 tender an organ as the eye. But there is one 
 complaint of the eye-lids, which may de- 
 ferve more particular mention in this place. 
 This diforder chiefly affeCts the ciliary glands, 
 and is the occafion of that rednefs which 
 is fometimes obfervable along the edges of the 
 lids, attended with excoriation, and a difcharge 
 of humour. Different methods may be pro- 
 perly directed for the cure or relief of this dif- 
 order in different circumfiances of it ; but 
 there is one which is not fo generally attended 
 to as others, and which in fome infiances has 
 been found fuccefsful, after other means, both 
 internal and external, have proved ineffectual. 
 A prudent ufe of the lunar cauftic to the parts 
 has anfwered this purpofe ; probably by very 
 powerfully confiringing and ftrengthening the 
 excretory duCts of the ciliary glands, which 
 C 2 a long 
 
20 THE EYE-LIDS, 
 a long determination of humours to them 
 has relaxed and enlarged ; and by drying up and 
 healing thofe excoriations of the orifices of 
 thefe du£ts and of the neighbouring fkin, 
 which have been produced by the difcharge. 
 
 The parts next to be confidered are the la- 
 cry mal gland, the lacrymal points, and the la- 
 crymal bag; the firft ferving to fecrete the 
 lacrymal fluid to be diffufed upon the anterior 
 furface of the eye, the two laft ferving to car- 
 ry off this fluid after it has anfwered the pur- 
 pofes for which it was feparated. To thefe 
 may be added, the lacrymal caruncle. 
 
 The Glandula lacrymalis, Puncta 
 
 LACRYMAL I A, SACCULUS LACRYMALIS, 
 
 and Caruncula lacrymalis. 
 
 Glandula lacrymalis. In the upper 
 part of the focket, a little above the external 
 angle, is a deprefiion which receives the fupe- 
 rior part of tne glandula lacrymalis. This 
 gland, which was formerly termed glandula 
 innominata, is fituated behind the tunica con- 
 junctiva, and is confiderably large. It is of a 
 whitiih colour, and is compofed of feveral 
 2 - fmall 
 
THE EYE-LIDS. 21 
 fmall lobes, each of which, it has been faid, fends 
 out an excretory duCt. But though the num- 
 ber of thefe duCts is not eafily determinable, 
 it is certain there are fuch duCts, which pierc- 
 ing obliquely the tunica conjunctiva of the up- 
 per lid, open on the infide of that membrane, 
 near the fuperior part of the tarfus. 
 
 The office of the lacrymal gland is to fepa- 
 rate a lymph or fluid, which paffes through 
 the excretory duCts, to be diffufed over, and to 
 moiften the fore-part of the eye-ball ; which 
 might otherwife grow dry, and become pain- 
 ful from the friction of the lids, and the aCtion 
 of the air upon it. And in order that this con- 
 ftant fupply of fluid may be properly diffufed, 
 there is a frequent motion of the lids which 
 fpreads the fluid equally. By this moiffiire 
 likewife, any foulnefs between the eye-ball and 
 lids may be waffied off. 
 
 It is obfervable, that when any extraneous, 
 irritating fubftance is accidentally lodged upon 
 the eye and excites pain, an unufual quantity 
 of this lacrymal fluid is immediately feparat- 
 ed, and is diffufed over the fore-part of the 
 globe, in order to carry off the caufe of the irri- 
 tation. The affiftance which nature furnifhes 
 
 C 3 thus 
 
22 THE EYE-LIDS, 
 thus for relief of fuch accidents, and like- 
 wife the profufion of the lacrymal fluid, which 
 is inftantaneoufly feparated and flows from the 
 eyes upon ludden and violent fits of crying, are 
 circumftances very extraordinary, confidering 
 there is not any collection or vifible referve of 
 lymph to anfwer occafionally thefe purpofes. 
 And here, though it is not a remark effentially 
 relative to practice, it may not be amifs to take 
 notice, that the generality of brute animals are 
 furnished with nearly the fame apparatus for 
 the fecretion and conveyance of the lacrymal 
 fluid, and the fame purpofes feem to be an- 
 fwered by this fluid in them as have been ob- 
 ferved to be in man, except in the laft infiance. 
 It is a quality peculiar to the human Ipe- 
 cies, to fhew the pafiion of grief by a fudden 
 gnd involuntary difcharge of tears; unlel's we 
 admit of a few exceptions, which are imperfeCt 
 in their kind, and are but barely allowable at 
 all. 
 
 Puncta lacrymalia. Upon the inner 
 edge of each eye-lid, near the internal angle, is 
 a fmall eminence, which is perforated obliquely 
 by a fine orifice. Thefe orifices, which are 
 fituated oppofite to one another, are termed 
 
 lachry- 
 
THE EYE-LID 5 . 23 
 kcrymal points ; and they ferve as wafte-pipes 
 to carry off the lacrymal fluid after it has an- 
 fwered the purpofes already mentioned, upon 
 the anterior furface of the eye. The manner 
 in which this fluid is diredted along the groove, 
 formed by the eye-lids to the lacrymal points, 
 has been before taken notice of; but in order 
 that the courfe of the lacrymal fluid may not 
 be obftrudted after it arrives at the lacrymal 
 points, it is neceffary that thefe orifices fhould 
 be conffantly kept open, or at leafl: that they 
 fhould never be totally clofed. To this end, 
 each of the lacrymal points is incircled by a white 
 cartilaginous ring, the direction of which inclin- 
 ing a little obliquely towards the globe of the 
 eye, they never come into contadf with one ano- 
 ther intirely; but when the eye-lids are flightly 
 fhut, the lacrymal points touch each other only at 
 that part next the outer edge or external fkin ; 
 and when the eye-lids are quite fhut, they are 
 ffill kept at nearly the fame diftance from one 
 another, by the interpofition of a fmall round 
 fubftance in the corner of the eye, called the 
 lacrymal caruncle. 
 
 It muff be further obferved, in regard to 
 th efe lacrymal points, that they are the orifices 
 C q or 
 
24 THE EYE-LIDS, 
 of two fhort duCts, which run behind the ten- 
 don of the orbicular mufcle, and which uniting 
 near the lacrymal fac, form one duCt that im- 
 mediately opens, beyond the angle of the eye- 
 lids, into the upper part of the fac. 
 
 The courfe of thefe dudts, as they run be- 
 hind the tendon of the orbicular mufcle, may 
 deferve confideration in refpedt to the operation 
 for a fiftula lacrymalis. It was long, we know, 
 fuppofed, that a tranfverfe incifion of this ten- 
 don might occafion an inverlion or diftortion of 
 the eye-lid ; and therefore, in performing that 
 operation, particular directions were given to 
 avoid this circumftance. This precaution has 
 indeed of late been almoft intirely difregarded. 
 Since, however, it is fuppofed by fome, that 
 from fuch a divifion of the tendon, thefe dudts 
 of the lacrymal points may happen to be 
 wounded ; and as the divided parts of the 
 dudts may poffibly not heal afterwards fo 
 #ptly as to admit a free paffage of the 
 lacrymal fluid through them, it fhould feein 
 right to prevent this accident where it can be 
 done with propriety. For this purpofe, one of 
 the methods which was formerly propofed of 
 beginning the incifion immediately below the 
 
 ten- 
 
THE EYE-LIDS. 2$ 
 tendon, in order to avoid wounding the ten- 
 don, particularly in thofe cafes where there is 
 little or no external fwelling, appears a reafon- 
 able practice in one refpedt, as it will cer- 
 tainly prevent a divilion of the lacrymal dudts. 
 
 The Sacculus racrymalis. The lacry- 
 mal lac is defcribed to be an oblong membra- 
 nous bag, and, if feen in a ftate of diftention, 
 it has that appearance. The prints which we 
 have of it give it likewife the fame figure, re? 
 prefenting it uniformly diftended. It ought 
 however to be obferved, that in a natural ftate 
 of the lacrymal fac, in which the tears have a 
 free paffage through it, the anterior lide of it 
 fuffering no diftention, is nearly flat. The 
 upper part of the fac lies under the tendon 
 of the orbicular mufcle, and receives the rears 
 from the lacrymal points by the dudts juft 
 taken notice of. Near the middle, which is 
 the wideft part of the bag, it is fomewhat lefs 
 than a quarter of an inch in diameter; but de- 
 fending a little lower it gradually leffens, and 
 palling obliquely backwards, terminates by a 
 very fmall opening, under the os turbinatum in- 
 ferius, in the cavity of the noftril. The lacry- 
 mal bag is iiluated juft within the orbit, in a 
 
 bony 
 
26 THE EYE-LIDS. 
 
 bony paflage, formed by the os unguis and the 
 os maxillare. The length of this paflage is about 
 three quarters of an inch, the greateft part of 
 which is to be confidered as a groove, the an- 
 terior furface of the bag for about the fpace of 
 half an inch, not being covered with bone. 
 The remaining narrower part of the paflage is 
 formed into a complete canal, which inclofes 
 the contracted and fhorter part of the bag, as it 
 paffes into the noftril. 
 
 Whoever duly confiders the fituation of the 
 lacrymal fac, as it lies in the paflage juft de- 
 fcribed within the edge of the focket, will per- 
 ceive, I imagine, that a proper compreffion 
 of the fac (however ufeful fuch a compreflion 
 might be, could it be eafily made) is not gene- 
 rally fo practicable as fome have thought it to 
 be in the cure of a fiftula lacrymalis. Lefs ftill 
 will be expended from this method of com- 
 prefiion, when it is confidered, that the difeafe 
 generally beginning with an obftrudtion in 
 that part of the palfage which forms the conir- 
 plete bony canal above-mentioned, this ob- 
 ft ruction muft be firft removed before any 
 effectual progrefs can be made by other means 
 towards a real cure of the diforejer. 
 
 The 
 
THE EYE-LIDS. 27 
 
 The ftrudture and ufes of the parts defcribed 
 in this and the preceding article, ought, in ge- 
 neral, to be ftridly attended to, in order to 
 conceive rightly of the operation for the fiftula 
 lacrymalis. Many other circumftances are ne- 
 cefiary to be likewife attended to in regard to 
 the management of that complaint ; but as thefe 
 have been treated of by the heft of our modem 
 writers on the operations of furgery, and as the 
 fubjed has been elaborately confidered very late- 
 ly, I am precluded from faying more upon it 
 at prefent. 
 
 Caruncula lacrymalis. The lacry- 
 mal caruncle is fituated between the internal 
 angle of the eye-lids, and the ball of the eye. 
 It is a fmall reddifh fubftance, and has the ap- 
 pearance of being flefhy, though it is thought 
 to be glandular. It is faid to have a great num- 
 ber of very fine hairs upon its furface, and to 
 be covered with an oily, yellowifti matter, not 
 unlike that which is difcharged from the ciliary 
 glands. Thefe hairs, it is fuppofed, fierve to 
 ftop and prevent any feculencies or extraneous 
 fubftances from entering into, and obftruding 
 the lacrymal points ; a ufe that is ingenioufly 
 enough applied, if the exiftence of thefe hairs 
 
 was 
 
2 8 THE EYE-LIDS, 
 was really afcertained. The caruncle itfelf, as 
 was intimated in the preceding article, by its 
 fituation in the comer of the eye, ferves to pre- 
 vent the internal edges of the lids at that part 
 from ever coming quite into contact with one 
 another; and thereby the orifices of the lacry- 
 mal points being to a certain degree kept open, 
 the tears or lacrymal fluid pafs freely through 
 thefe points into the fac. 
 
 The lacrymal gland, points, bag and ca- 
 runcle, receive arteries from the maxillary and 
 temporal branches, and nerves from the opthal- 
 mic branch of the fifth pair. 
 
 THE 
 
[ 29 ] 
 
 THE 
 
 SECOND LECTURE. 
 
 Of the form and coats of the Eye, and of 
 the Optic Nerves. 
 
 HE figure or natural form of the eye is 
 
 that of two unequal portions of unequal 
 fpheres. The portion of the fmaller fphere is 
 fituated anteriorly? the portion of the larger 
 fphere makes the pofterior part of the eye. If 
 the eye had been one fphere, the diftance from 
 the cornea through the center of the eye to the 
 retina would have been fo fhort, that the 
 focus of the rays of light would have fallen 
 beyond the eye, and vifion would have been 
 confufed. 
 
 In confidering the particular parts of the 
 eye, they may be divided into the coats and 
 the humours ; to which may be added, the 
 mufcles. 
 
 Of the Coats of the Eye. 
 
 The coats of the eye are the tunica con- 
 junctiva, or adnata j the fclerotica and cor- 
 
 nea ; 
 
3 o THE TUNICA CONJUNCTIVA, 
 nea; the choroides with the uvea or iris ; and 
 the retina. To thefe may be added, the cap- 
 fular tunics of the cryftalline and vitreous hu- 
 mours, which will be taken notice of in the 
 defcription of thofe humours. 
 
 The Tunica conjunctiva. 
 
 Some anatomifts divide the tunica conjunc- 
 tiva, adnata, or as it has been fometimes term- 
 ed, though improperly, albuginea, into diffe- 
 rent membranes. One of thele is faid to be a 
 continuation of the periofteum lining the 
 focket, the other a continuation of the internal 
 membrane of the eye-lids ; both of them are 
 faid to pafs from the edge of the focket over the 
 anterior part of the eye to the termination of 
 the fcleroticain the cornea; and likewife to be 
 covered with an extreme fine membrane, pro- 
 ceeding from the fcarfskin of the eye-lids. But 
 without purfuing any further account of thefe, 
 it may be fufficient to obferve, that though 
 the conjunCliva may fometimes admit of 
 being divided into diftinCt laminae, as may 
 be done in other membranes where no inch 
 diftinCtion is observed, yet as the divifion 
 4 here 
 
THE TUNICA CONJUNCTIVA. 3? 
 here is far from being eafily practicable, and 
 ,as the diftinCtion does not appear to have any 
 real ufe, it may be properly ranked amongft 
 the many other articles which can ferve only to 
 burthen the memory. 
 
 The tunica conjunCtiva, therefore, the origin 
 of which is not well determined, may be 
 confidered as one membrane, which lines the 
 internal furface of the eye-lids, and which, from 
 the edge of the focket, to which it adheres, 
 is reflected upon, and covers the anterior half 
 of the globe of the eye ; the part of this mem- 
 brane which lines the lids being diftinguifhed 
 as the conjunctiva of the eye-lids, the portion 
 of it that is fpread upon the globe, as the con- 
 junctiva of the eye. The conjunctiva of the 
 eye-lids adheres clofely to the internal furface 
 of the tarfus. It is full of capillary blood- 
 vefiels, which appear very plainly, and it is 
 faid to be perforated with numerous fmall ori- 
 fices, through which a lymph conflantly 
 pafles, ferving with the tears to moiften the in- 
 fide of the eye-lids, and the anterior furface of 
 the eye. This portion of the tunica conjunc- 
 tiva is endued with a remarkable degree of fen- 
 fibility, as is known by the extreme pain which 
 
 fome- 
 
32 THE TUNICA CONJUNCTIVA, 
 fometimes enfues from dull or other irritating 
 fubftances accidentally lodging upon this part. 
 The conjunctiva of the eye, which adheres all 
 round to the edge of the focket, and from 
 which circumftance probably it has been fup- 
 pofed to have its origin from the periofteum 
 lining the focket, is loofely connected to the 
 eye by a reticular fubftance, as far as where the 
 fclerotica is faid to terminate in the cornea. 
 Here it has been thought that the conjunctiva 
 ends, or changes its nature and becomes one 
 with the cornea; but by a careful dilfeCtion it 
 may be feparated from the whole furface of the 
 cornea. 
 
 It may be proper to add here, that immedi- 
 ately under and adherent to the conjunctiva of 
 the globe, is lituated the tendinous expanlion of 
 the four firait mufcles of the eye ; which ex- 
 panlion by its internal furface adheres clofe- 
 ly to the forepart of the fclerotica, and cover- 
 ing all that part of the globe which appears 
 white, terminates at the edge of the fclerotica. 
 Some have given the name of tunica tendinea 
 to this expanlion. It is the colour of this ten- 
 dinous fubftance, and of the anterior part of 
 the fclerotica, in which at laft this expanlion is 
 
 loft. 
 
TH 3 TUNICA CONJUNCTIVA. 33 
 loft, that chiefly forms, what is commonly 
 called the white of the eye ; for though the 
 conjunctiva has been fuppofed to produce this 
 colour, and from thence probably received one 
 of its names, albuginea, yet in reality the con- 
 junctiva, though whitilh, does not afford much 
 colour of itfelf, but being tranfparent, the co- 
 lour of thefe fubjacent parts is feen ftrongly 
 through it. 
 
 The temporal and frontal arteries fend 
 branches to the tunica conjunctiva, and thefe 
 branches communicate with thofe which are 
 fent to the integuments of the eye-lids, and to 
 the orbicular mufcle. The fame membrane 
 is likewife fupplied with blood-veffels, by 
 means of the internal maxillary artery, which 
 fends a confiderable branch into the focket, 
 where it divides, and fpreading into various 
 ramifications, is diftributed to feveral parts of 
 the eye, and amongft others, to the tunica con- 
 junctiva. The veffels of that portion of this 
 membrane, which have been diftinguifhed by 
 the name of conjunctiva of the eye, are 
 chiefly fuch as in their natural ftate carry on- 
 ly ftmph or ferum, and are not vifible unlefs 
 made fo by anatomical injections ; or elfe, 
 D when 
 
34 the tunica conjunctiva. 
 
 when in the living fubjeCt, they are accident- 
 ally diftended with blood in inflammations of 
 the eye ; or when the blood is admitted into 
 and Magnates in them, by a mere weaknefs 
 or relaxation of thefe veffels. 
 
 The admiffion of blood into thefe veffels, 
 is a circumfcance which frequently deferves 
 attention in diforders of the eyes. The 
 eyes we know are fubjeCt to inflammations 
 from a variety of caufes, in moil of which, not 
 only the fanguinary veflels of the conjunctiva 
 are fuller and more diftended than is common, 
 but the blood, as was before intimated, is 
 likewise thrown into the numerous lymphatic 
 veflels, and occaftons chiefly that general ap- 
 pearance of rednefs which attends thefe dif- 
 orders. The method of treating thefe in- 
 flammations in their firft ftate, is fufficiently 
 obvious ; but there is a time in many of thefe 
 cafes, (to be judged of by the general indica- 
 tions, and by the means of cure that have been 
 already made ufe of,) when the rednefs and 
 fullnefs of the veflels will in a great degree re- 
 main, and yet to purfue the fame method of 
 cure as was at firfl diredted, would be very 
 improper. The truth is, the fine lymphatic 
 
 veffels. 
 
THE TUNICA CONJUNCTIVA. 35 
 velTels, which either from fullnefs, heat, or 
 other general caufe in the conflitution, have 
 had the blood thrown into them, will for 
 a conliderable time admit the fame after the 
 original caufe of the diforder is removed : 
 They are now fo relaxed and weakened by 
 the diffention which they have differed, that 
 they dill admit the blood, for want of 
 power to contradi themfelves to their natural 
 diameter, and thereby to refill the influx of it 
 into them. 
 
 Upon this occaiion it may not be improper 
 to take notice of thofe external applications, 
 which are known by the names of eye waters, 
 liniments, or ointments for the eyes. 
 
 Among the variety of medicines which are 
 diftinguifhed, as being good for particular com- 
 plaints, there are few that have been more 
 celebrated, than certain compofitions for the 
 diforders affefting the eyes and eye-lids. Mofc 
 of thefe are recommended indifcriminately for 
 almoil every complaint of thefe parts ; in 
 which kind of practice it is not tp be fuppofed, 
 that any jud idea of the medicine directs the ufe 
 of it. On the other hand, the real properties 
 of thefe medicines are not, I conceive, fufli- 
 D -2 ciently 
 
36 THE TUNICA CONJUNCTIVA, 
 ciently explained, where a more intelligent 
 method might have been expedted. Moft 
 writers upon the diftempers of the eyes, re- 
 commend applications of this fort, but they 
 either fpeak of them in too general terms, 
 without attempting to fhew wherein their 
 proper ufe confifls, or they give a par- 
 tial and undeferved preference to fome par- 
 ticular compofition. In fadt, thefe medicines 
 are almofi: all capable of doing fervice, under 
 certain circumflances of the diftemper but 
 as the advantages arifing from a promifcuous 
 ufe of them, muft be owing rather to accident 
 than judgment, the manner in which they may 
 be fuppofed to adt ought to be attended to. If 
 we examine into the ingredients of thefe com- 
 pofitions, we Ihall find the bafis of them is 
 more or lefs of a reftringent nature, and it 
 is from this property that they are capable of 
 doing fervice by contradling and thereby 
 flrengthening the vefiels, which have been too 
 much diftended. In fome cafes a different 
 form of thefe medicines may agree better than 
 others; a liniment may be preferable to a wa- 
 ter, or a water to a liniment, according to the 
 nature or quantity of the difcharge, with other 
 
 circum- 
 
THE TUNICA CONJUNCTIVA. 37 
 circumftances : befides that there is a peculiari- 
 ty in fome conftitutions, which will not admit 
 of the fame means of cure, either external or 
 internal, as are effectual in other people, and 
 this is particularly true in the prefent cafe, 
 in regard to unctuous applications — So likewife 
 the ftrength of the medicine, with refpect to 
 its degree of reftringency, is to be obferved ; 
 and what is ftill of great confequence, the 
 preparing or compounding thefe medicines 
 ought to be done with the utmoft exactnefs 
 and neatnefs, that no rough particles may 
 come in contact with the eye, which 
 may create any unneceflary irritation there. 
 Thefe precautions being attended to, the ap- 
 plications juft mentioned may be often em- 
 ployed with the greateft advantage ; and when 
 the principle upon which they are directed is 
 properly confidered, it is eafy to fee that many 
 forms or compoiitions of thefe medicines will 
 equally anfwer the purpofe, without attributing 
 any fpecific property to them. 
 
 The nerves of the tunica conjunctiva pro- 
 ceed from the opthalmic branch of the fifth 
 pair. 
 
 In regard to the ufes of the tunica conjunctiva, 
 D 3 as 
 
38 THE OPTIC NERVES, 
 as a ligament to connedt the eye to the 
 edge of the focket, and, being of a texture ad- 
 mitting fome extenfion, it allows, notwith- 
 ftanding this connexion, of a free movement 
 of the eye in different directions without pain 
 or inconvenience. Alfo by the adhefion of this 
 membrane all round to the edge of the focket, 
 any foulnefs collected on the eye, or any ir- 
 ritating fubfcance accidentally lodging upon it, 
 are prevented from palling to the back part 
 of the globe ; an accident which might pro- 
 duce extreme pain, and could not without 
 great difficulty be relieved. This membrane 
 likewife gives a fmoothnefs to the parts which 
 it covers, and thereby makes the fridtion lefs 
 confiderable between the eye and the eye-lids. 
 
 Previous to the defcription of the other coats 
 of the eye, it may be neceffary to give fome 
 account of the optic nerves. 
 
 OF THE OPTIC NERVES. 
 
 HE optic nerves are two thick, round, 
 
 JL and whitiffi bodies or chords, which 
 arife pretty near each other, from two con- 
 fiderable eminences in the brain, termed 
 
 thalami 
 
THE OPTIC NERVES. 39 
 thalatni nervorum opticorum. From thence 
 they pafs in an incurvated courfe outwards and 
 forwards, but approach each other afterwards, 
 and feem to unite before that part of the brain 
 called infundibulum. After this feeming union 
 of the optic nerves, they divide, and are fent 
 feparately through their proper orifices in the 
 fockets, to the pofterior part of the eye-balls, 
 where their coats and nervous fubftance are 
 fuppofed to be expanded and continued on, as 
 will be more particularly expreffed hereafter, 
 to form the different coats of the eye. 
 
 The infertion of the nerve is not diredtly 
 oppofite to the center of the pupil, but a little 
 on the inf de towards the nofe ; the reafon of 
 which will be explained in the defcription 
 of the retina. 
 
 The fubflance of the optic nerves connfls 
 of medullary fibres, which are productions of 
 the medullary fubftance of the brain. They are 
 covered only with the pia mater, till they reach 
 the orifices through which they pafs into 
 the focket : there the dura mater gives them 
 another covering, which is continued on with 
 the firft. 
 
 Different opinions have arifen about the 
 D 4 feeming 
 
4 o THE SCLEROTICA 
 
 Teeming union of thefe two nerves before the 
 infundibulum. Some think that they crofs one 
 another here ; others, that their fubftance 
 is mixed and blended together ; and others ima- 
 gine that there is only a ftridt cohelion of the 
 two nerves, without any mixture of their fub- 
 ftance, an opinion which feems to have been 
 warranted by obfervations related by wri- 
 ters. 
 
 To return now to the defcription of the 
 remaining coats. 
 
 OF THE SCLEROTICA AND CORNEA. 
 
 H E fclerotica and cornea are mentioned 
 
 by fome anatomical writers as two 
 different coats of the eye, though they are 
 now generally allowed to be only two dif- 
 ferent names to fignify different parts of the 
 fame coat ; the fclerotica being the external 
 coat of the pofterior part of the eye ; the 
 cornea, the external coat of the anterior part : 
 others diffinguifh thefe two portions of this 
 membrane, into cornea opaca and cornea lu- 
 cida ; by the firft, underftanding the poflerior 
 portion ; by the latter, the anterior. 
 
 Till 
 
AND CORNEA. 41 
 Till of late, it was the general opini- 
 on of anatomifls, that the three different 
 principal coats of the eye, were continuations 
 and expanfions of the different fubffances of 
 the optic nerve; that is, the fclerotica was 
 thought to be a production or continuation of 
 the covering which this nerve receives from 
 the dura mater ; the choroides was fuppofed to 
 proceed from the fecond coat of the optic 
 nerve, arifing from the pia mater ; and the 
 retina was faid to be an expanfion of the me- 
 dullary fubffance of this nerve. Thefe con- 
 tinuations or expanfions are not now ge- 
 nerally allowed, as they cannot be fufficiently 
 and perfectly afcertained by laying open and 
 examining the courfe of the optic nerve : 
 on the contrary, by examining this nerve, and 
 comparing the ftruCture of it with the fclero- 
 tica, the choroides, and the retina, there ap- 
 pears to be a confiderable difference between the 
 texture and thicknefs of thefe membranes 
 and the texture and thicknefs of the parts 
 whence they are fuppofed to be derived and 
 continued. But though the force of this 
 objection may have fome effeCt againff the 
 common theory, yet, as other reafons are 
 
 brought 
 
42 THE SCLEROTICA 
 brought to invalidate this objection, and as the 
 origin of thefe coverings to the humours of 
 the eye is not othervvife well accounted for, it 
 may be proper to retain the old opinion till a 
 more certain one is eftablifhed. 
 
 The fclerotica then, or the external coat of 
 the pofterior part of the eye, may be faid to be 
 a production of the dura-matral coat of the 
 optic nerve. In man it is inelaftic. This dif- 
 pofition in the fclerotica appears to be par- 
 ticularly ufeful ; for as it is necelfary to alter 
 occafionally the convexity of the cornea, in or- 
 der to adapt the eye to the different diftance 
 and magnitude of the objeCt to be feen ; and 
 as in one inftance which will be mentioned 
 hereafter with the mufcles, this alteration 
 is fuppofed to be effected by drawing the eye 
 backward into the focket, and prefting it 
 againft the fat contained there, by which means 
 the humours of the eye being protruded for- 
 wards, the cornea, which is elaftic, is rendered 
 more or lefs convex ; if the fclerotica had been 
 as capable of being diftended as the cornea 
 is by its elafticity, this prehure would have 
 been loft : the convexity of the cornea would 
 confequently have remained unaltered, as the 
 2 coats 
 
ANDCORNEA. 43 
 
 coats of the eye would have given way in every 
 part alike. The fclerotica is made up of feveral 
 lamina? clofely connected, whofe fibres run 
 in different diredtions, and form a denfe, 
 compadt fubftance. It is opa-ke, and alfb 
 of confiderable thicknefs, particularly in 
 the pofterior part where the optic nerve 
 enters ; but it gradually grows thinner towards 
 its anterior part : here we fee a change of 
 appearance, and the external coat of the an- 
 terior part of the eye, from a fancied refem- 
 blance to tranfparent horn, takes the name of 
 cornea. 
 
 The cornea confifls of two principal laminse, 
 an external and an internal, each of which 
 is compofed of thinner lamina?. The fubftance 
 of the cornea is, as was before obferved, in 
 fome degree elaftic, that, in order to fit the eye 
 to the different magnitudes and diflances of 
 objects, its convexity might be rendered greater 
 or lefs, as the humours of the eye are more or 
 lefs protruded : it is likewife perforated with a 
 great number of exceeding fmall orifices, 
 thro’ which a very fine fluid is fuppofed to be 
 conftantly difcharged, but which foon evaporates. 
 
 The 
 
44 THE SCLEROTICA 
 
 The fclerotica and cornea are furnifhed with 
 arteries chiefly from a branch of the internal ca- 
 rotid, which entering the focket with the optic 
 nerve, gives, amongft other ramifications, feveral 
 which run into the posterior portion of the fcle- 
 rotica : of thefe, fome are diftributed into the 
 different parts of this membrane, whilfl: others, 
 after running a little way obliquely in the fub- 
 ftance of it, penetrate through it, and pafs on 
 to the next coat, the choroides. 
 
 The nerves proceed chiefly from the op- 
 thalmic branch of the fifth pair, which 
 running into the fubflance of this membrane, 
 fome, like the blood-veflels, are fent into the 
 different parts of it ; whilfl: others, after paffing 
 a little way in the fubfliance of the fclero- 
 tica, penetrate through it, and are continued 
 on between the fclerotica and choroides towards 
 the iris, having the appearance of fine liga- 
 ments. 
 
 The fclerotica and cornea ferve in general 
 to enclofe and cover the humours of the eye : 
 the cornea, in particular, ferves to tranfmit the 
 rays of light into the eye, and produce the firft 
 refradtion of thofe rays neceflary to vifion. 
 
 The 
 
 / 
 
AND CORNEA. 45 
 
 The natural tranfparency of the cornea, fo 
 neceflary for the admiffion of light into the 
 eye, is liable to be altered and obfcured by in- 
 flammations or humours affedting the fore- 
 part of the eye, the frequent confequences of 
 which, where they remain any time, are a 
 thickening of particular parts of the cornea, 
 impofthumations in this membrane, or ulce- 
 rations : thefe are apt to leave opake marks or 
 ipots in the cornea, which according to their 
 flze or fltuation, intercept more or lefs the 
 rays of light, and are with much difficulty, if 
 at all removed, elpecially in grown people. 
 A very large number of thofe unhappy per- 
 fons, who, from a defedt or total lofs of fight, 
 live uncomfortably to themfelves, and burthen- 
 forne to their friends or the community, owe 
 their misfortune in this refpedt, to the want 
 of early application for affiftance, or to that 
 affiffance being adminiffered lefs fpeedily than 
 the nature of fuch cafes generally requires, 
 whence the fpots or marks before -mentioned. 
 It cannot therefore be too often and too ear- 
 neffly inculcated, that every fuitable method 
 for relief fhould be employed here with the 
 utmoff: expedition. 
 
 THE 
 
4.6 THE CH OROIDES. 
 
 THE CHOROIDES.. 
 MONG the variety of inftances in ana- 
 
 tomy, where the, fubjed is perplexed by 
 different diflindions, this membrane is one. 
 Fromitsrefemblance to thechorion, in refpedof 
 its numerous blood- veffels, it appears originally 
 to have had the name of choroides. From fome 
 fancied fimilitude to a grape in regard to its 
 form and colour, the whole or part of this mem- 
 brane had likewife the name of uvea given 
 to it. Among the moderns, fome divide it into 
 two parts, calling the pofterior portion, choroi- 
 des, the anterior uvea, and the external furface 
 cf the latter, on account of the variety of 
 its colour, iris. Others call the whole anterior 
 portion iris, referving the name of choroides 
 to fignify as ufual the pofcerior portion of this 
 membrane ,- and this lafl diifindion will be ob- 
 ferved in the prefent account. 
 
 The choroides is f tuated within the fclero- 
 tica, lying contiguous to the inner furface of it, 
 and derives its origin, according to the moil; 
 generally received opinion, from the fecond or 
 pia-matral covering of the optic nerve. When 
 the fclerotica is removed, the choroides ap- 
 
 pears 
 
THE CHOROIDES. 4 7 
 pears of a dark-brown or blackifh colour. The 
 choroides is divided into two laminas, the ex- 
 ternal of which is {lightly connected to the 
 fclerotica, and is covered with a black fubftance 
 interpofed between it and the fclerotica. Both 
 laminas are extremely vafcular, but the internal 
 is furnifhed with a great number of inofculat- 
 ing blood-veftels, whofe almoft innumerable 
 extremities projed from its inner furface, and 
 have been termed by lome writers villi and pa- 
 pillae ; and this lamina being firft diftinguifhed 
 by Ruyfch, has the name of tunica Ruyfchiana. 
 The fame black fubftance which, as was juft 
 now obferved, covers the external furface of the 
 choroides, being interpofed between that and 
 the fclerotica, appears alfo on the internal fur- 
 face of this membrane between it and the re- 
 tina. Some reckon this black fubftance, which 
 has been confidered by writers as a pigment, to 
 be an exceeding fine membrane analogous in 
 its texture, to what is termed in anatomy the 
 rete mucofum : it eafily feparates, and, when 
 the choroides is put into water, dilTolves into a 
 kind of black paint. This black fubftance 
 feems to anfwer different purpofes : thus in the 
 choroides, properly fo called, it abforbs the rays 
 
 of 
 
48 THE CHOROIDES. 
 of light, and prevents the reflection of them 
 again into the eye ; on the back of the iris and 
 between the ciliary procefles, it prevents the 
 paflage of the rays through them, it being 
 neceflary to diftinCt vifion that no rays fhould 
 pafs to the retina, but fuch as, entering at the 
 pupil, pafs through the cryftalline humour. 
 
 Near where the fclerotica and cornea join, 
 the choroides is connected and adheres ftrongly 
 all round to the fclerotica. At this circle of 
 adhefion the choroides feems to change its co- 
 lour and texture, appearing as a whitifh kind 
 of ring of a compaCt fubftance, which ferves 
 to form this connection between the fclerotica 
 and choroides, and is termed ligamentum ci- 
 liare. Here the internal lamina of the cho- 
 roides dips inwards to make what are termed 
 the procefl'us ciliares, a name which is given 
 to thefe procefles, from their fuppofed refem- 
 blance to the cilia or eye-lafhes. The ciliary 
 procefles, which are very numerous and lie 
 clofe to one another, appear as radiated folds 
 or wrinkles of the internal lamina from the 
 part before-mentioned, and the external ex- 
 tremity of each fold refpe&s the circle of the 
 ligamentum ciliare. From this point the pro- 
 cefles 
 
THE CHOROIDES. 49 
 ceffes run upon the fore-part of the vitreous 
 humour to the edge of the cryftalline hu- 
 mour in which they are inferted and termi- 
 nate, like lines drawn from a circumference to 
 a center : the whole radiated ring made by the 
 ciliary procelfes is fometimes diftinguifhed by 
 the name of corona ciliaris ; and it may be 
 proper likewife, in order to prevent confufion, 
 to obferve that many writers defcribe thefe pro- 
 ceffes by the term of ciliary ligament. After 
 the black pigment, which lies thick in thefe 
 ciliary folds, is walhed off, numerous blood- 
 veffels, that are continued from the cho- 
 roides, appear plainly in an injedted eye ; 
 and fome of thefe enter into the cryftalline 
 humour. It is afferted likewife that between 
 the duplicatures of the ciliary procelfes 
 flelhy fibres may be feen lying in fmall grooves 
 of the membrana vitrea ; but modern enqui- 
 ries do not confirm the existence of thefe fibres. 
 Upon cutting the coats of the eye, and fepa- 
 rating the cryftalline and vitreous humours 
 from their adhefions to the ciliary procelfes, 
 part of the pigment before-mentioned is left 
 lying in black radiated lines upon thefe hu- 
 
 E mours : 
 
 I 
 
5 o THE CH OROIDES, 
 mours : thefe lines, of which further mention 
 will be made, are called duCtus nigri ; and be- 
 tween them it is faid the mufcular fibres are 
 fituated. 
 
 In regard to the ufes of the ciliary procefles, 
 they ferve, by being covered with the black 
 pigment, to prevent any rays of light 
 from falling on the retina, which have not 
 pafled firft through the cryftalline humour. 
 They likewife fend veflels both to the cap- 
 fula and body of the cryftalline humour, and 
 others probably to the vitreous humour. There 
 is alfo reafon to believe that by their nume- 
 rous villi they contribute very confiderably to 
 the fecretion of the aqueous humour. One 
 opinion of their ufe has been more generally 
 received, namely, that by their contraction, 
 the cryftalline and vitreous humours are brought 
 forward, making the cornea more prominent, 
 as when we would view fmall and near ob- 
 jects diftinCtly ; but as the exiftence of mui- 
 cular fibres in the ciliary procefles is by no 
 means proved, it may be reafonably doubted 
 whether the aCtions of thefe procefles can 
 have any power in the motion of thofe hu- 
 
 Kaving 
 
 mours. 
 
THE CHOROIDES. 51 
 
 Having defcribed thus the ciliary proceffes 
 as radiated folds or wrinkles, proceeding from 
 that part of the internal lamina of the cho- 
 roides which adheres to the ciliary liga- 
 ment, it remains next to be obferved, that the 
 external lamina of the choroides, after it has 
 formed the ciliary ligament, leaves the fcle- 
 rotica ; and being continued in nearly a tranf- 
 verfe direction, makes the anterior lamina 
 of the perforated partition, which is feen 
 through the cornea lying acrofs the eye, and 
 is called the iris : the internal lamina 
 
 of the iris is formed by a continuation of 
 the internal lamina of the choroides, after it 
 has dipped inwards in the manner above de- 
 fcribed and made the ciliary proceffes. 
 
 The hole or opening in the iris called 
 the pupil, is obferved to have a different 
 form, in different animals, according to the 
 fituation or form of the eye itfelf, and ac- 
 cording to the peculiar occafions of the ani- 
 mal in its way of life. In man the pupil is 
 round, the circle of which allows of being 
 made greater or lefs upon different occafions 
 by means of the iris, which has a power of 
 
 E 2 fhut- 
 
S 2 THE CHOROIDES. 
 fhutting out too much light or admitting a fuf- 
 ficient quantity, according to the nearnefs or 
 diftance of the objeCt, or according to the de- 
 gree of light required: thefe motions are call- 
 ed contraction and expanfion. It has been a 
 difpute, whether thefe motions are the effeCt 
 of mufcular fibres or of a nervous tenfion and 
 relaxation. Thofe who feem to have examin- 
 ed this affair carefully, agree in general, that 
 there are between the laminas of the iris, two 
 orders of fibres, one circular or orbicular, and 
 lying round the margin of the pupil or inter- 
 nal edge of the iris ; the other radiated, one 
 extremity of which terminates at the circular 
 order, the other at the external edge of the 
 iris. It is indeed exceeding difficult to difcover 
 the fibres faid to produce the motions of the 
 iris; but as it is certain, that the iris is con- 
 tracted and dilated, it is reafonable to fuppofe, 
 from the analogy of other parts, that mufcular 
 fibres fubfift here and produce thefe adlions. 
 
 The ufe of the pupil is to tranfmit the rays 
 of light to the humours of the eye which are 
 fituated behind it : the ufe of the iris, as was 
 before intimated, is by its dilatation and con- 
 traction 
 
THE CHOROIDES. 53 
 tra&ion to determine the diameter of the pu- 
 pil in any given quantity of light, fo as to ad- 
 mit a fufficient number of rays for clear vifion, 
 and to exclude the reft. 
 
 The arteries, which in the defcription of the 
 fclerotica, were faid to penetrate through the 
 pofterior portion of that membrane, pafs after- 
 wards through the external lamina of the cho- 
 roides and are diftufed through this membrane 
 in numerous branches, fome of which, from 
 their direction, are called vafa vorticofa. 
 From the branches of the external lami- 
 na are continued the fine veflels of the in- 
 ternal or the membrana ruyfchiana ; and of 
 thefe, fome probably pafs on to the vitreous 
 humour as others do, by means at leaft 
 of the ciliary precedes, to the cryftalline 
 humour, contributing, it is to be believed, 
 to the formation of thofe humours. From 
 the veflels of the external lamina like- 
 wife are fent feveral ramifications to the cir- 
 cumference of the iris, where they produce a 
 vafcular circle called circulus arteriofus. From 
 this vafcular circle pafs off many fmaller vef- 
 fels, fome of which, after they have run about 
 
 E 3 two 
 
54 THE CHOROIDES. 
 
 two thirds of the breadth of the iris, inofcu- 
 late and form the appearance of arches : 
 from thefe arches are fent ftill finer veflfels, 
 terminating at the internal edge of the iris, 
 and ferving probably to fecrete a part of the 
 aqueous humour of the eye. 
 
 The veins of the choroides, and indeed of 
 the other internal parts of the eye, empty 
 their blood chiefly into the optic finufes, 
 to be conveyed to the internal jugulars : fome 
 of thefe veins however communicate with the 
 external veins of the eye, and their blood is 
 confequently carried to the external jugulars. 
 
 The nerves of the choroides proceed from 
 the ganglion, formed by fhort ramifications of 
 the opthalmic branch of the fifth pair of 
 nerves and by a branch of the third pair. 
 Thefe run along the optic nerve, and perfo- 
 rating in white filaments the fclerotica, they 
 pafs afterwards between the fclerotica and cho- 
 roides towards the iris ; and then dividing 
 again into feveral other very minute branches 
 they terminate in the fubftance of the iris. 
 
 As an appendix to this article, it may not 
 be improper to take notice here of the mem- 
 
 brana 
 
THE CHOROIDES. 55 
 brana pupillaris or velum pupillae, an ext ream 
 fine membrane, of which very little has been faid 
 by writers, and which has not been at all attend- 
 ed to till of late years. Both in the human and brute 
 fcetus a very fine membrane is fpread like a 
 film before the pupil : This membrane, which 
 is difcoverable only in a foetus, is termed the 
 membrana pupillaris or velum pupillae, and 
 feems to be a continuation of the external la- 
 mina of the iris. The ufe of the membrana 
 pupillaris is not fatisfadtorily determined : 
 fome have fuppofed it ferves to moderate 
 the impreffion of the rays of light, and 
 render them lefs troublefome to the tender or- 
 gans of animals newly born; a plaufible account 
 at lead:, if not a true one. Agreeable to this, 
 is the fuppofition, that after birth the mem- 
 brana pupillaris burfts or breaks, and con- 
 tracting itfelf, approaches to the edge of the 
 pupil ; but what really becomes of this mem- 
 brane after birth is ftill uncertain. 
 
 THE 
 
5 6 THE RETINA. 
 THE RETINA. 
 
 ITHIN the internal lamina of the cho 
 
 roides is a white foft membrane, which 
 from its fuppofed refemblance to a net, is call- 
 ed retina, and is the third coat of the eye. 
 
 As the fclerotica was faid to proceed from 
 the external or dura-matral covering of the 
 optic nerve, and the choroides to be an expan- 
 fion of the fecond or pia-matral coat, fo the re- 
 tina is fappofed to be a production of the me- 
 dullary fubftance of this nerve, expanded like a 
 membrane ; though it muft indeed be allow- 
 ed, that by laying open the coats of the optic 
 nerve, its medullary fubftance cannot be clear- 
 ly traced, as continuing on to form the retina. 
 
 At the bottom of the eye, juft where the 
 optic nerve is inferted, the choroides is defici- 
 ent, in order to admit the paffage of that 
 nerve : at the fame place there appears a fmall 
 white protuberance a little deprefied in its 
 middle ; and at this depreflion pafs out blood- 
 veflels, which in general are expanded, thro’ 
 the fubfiance of the retina. Thefe vefiels pro- 
 
 ceed 
 
THE RETINA. 57 
 ceed from an artery that runs in the middle of 
 the optic nerve, and which, entering with 
 that nerve into the globe of the eye, fpreads 
 afterwards in the manner juft mentioned: 
 Some of the blood-veftels appear plainly in 
 the retina without the affiftance of injections. 
 The retina is connected, to the choroides, and 
 extends from the infertion of the optic nerve, 
 over the furface of the vitreous humour, to the 
 external edge of the ciliary proceffes ; and 
 fome fay it may be traced between the ciliary 
 proceftes to the circumference of the cryftal- 
 line humour. 
 
 The retina is generally fuppofed to be 
 the immediate feat of vifion ; but this opi- 
 nion has been controverted, and the cho- 
 roides has been faid to be the part which re- 
 ceives the impreffion of the objeCt. ' It ftiould 
 feem however, by the impoffibility of rays of 
 light palling through the black pigment to the 
 choroides, that the retina is the part of the 
 eye upon which viftble objeCts are fenfibly im- 
 preffed •, and the rays of light ftriking upon 
 this membrane, the fenfation is conveyed by 
 the optic nerves to the common fenforium in 
 the brain. 
 
 It 
 
S B THE RETINA. ’ 
 
 It is obferved that the center of this expan- 
 fion,or the part of the eye where the optic nerve 
 is fuppofed to enter, either on account of fome 
 particular difpofition of the nerve at that place, 
 or becaufe the artery before-mentioned enters 
 the eye there, is infenfible. It appears there- 
 fore to be a wife contrivance of providence that 
 the optic nerve fhould not enter the eye, in the 
 point where the axis of vifion falls, but is in- 
 serted on the internal fide of the axis of vifion ; 
 by which means, though fome of the rays 
 from any point of an objedt may happen to fall 
 on the infenfible part of one eye, yet other rays 
 from the fame point will fall on the fenfible 
 part of the other eye; for it is proved by ex- 
 periment, that one eye being fhut, three ob- 
 jects may be fo placed as one of them may be 
 loft, though vifible when both eyes are open. 
 The reafon of this is, the objedt whofe rays 
 fall on the infenfible part of one eye, is difco- 
 verable by other rays falling on the fenfible 
 part of the other eye. 
 
 THE 
 
[ 59 ] 
 
 THE 
 
 THIRD LECTURE. 
 
 OF THE HUMOURS OF THE EYE. 
 
 H E cavity of the eye is filled by pel 
 
 lucid fubfiances called humours, which 
 are three, the aqueous, the cryfialline, and 
 the vitreous. 
 
 The Aqueous Humour. All that fpace 
 in the eye between the cornea and the anterior 
 part of the cryfialline humour, is filled with a 
 limpid water, called therefore the aqueous hu- 
 mour. The fpace in which this humour is 
 confined, is divided into two parts called cham- 
 bers ; that part between the cornea and the 
 iris, and which is much the larger of the 
 two, is called the anterior chamber; that 
 part between the iris and the cryfialline 
 humour, which is very fmall, elpecially 
 near the pupil where the cryfialline comes al- 
 mofi into contact with the iris, is called the 
 pofterior chamber : thefe two chambers com- 
 
 municate 
 
60 THE HUMOURS OF THE EYE. 
 municate by means of the pupil. The veflels 
 which immediately furnifh this fluid are fo ex- 
 tremely fine, as makes it difficult to ipeak of 
 them with certainty. It is however probable, 
 that part of the aqueous humour is fe- 
 creted from the blood, as was before obferved, 
 by the very fine veflels fent from the vafcular 
 arches defcribed in the account of the iris, 
 and which terminate at the internal edge of 
 the iris, and that a confiderable portion of it is 
 fupplied by the fine arterial villi of the ciliary 
 procefles. Thefe veflels, it is to be fuppofed, 
 by the fmallnefs of their diameters exclude all 
 particles of the animal fluids that are grofler 
 than the molt limpid water, at the fame time 
 that they fuffer this fine fluid to pafs into the 
 eye. It is obfervable, that when a wound is 
 made in the cornea, and the aqueous humour 
 is difcharged by this accident, it is renewed in a 
 few days and fometimes fooner. From this 
 obfervation, it is reafonable to fuppofe that the 
 aqueous humour is perpetually fecreted ; and 
 if it is thus conilantly feparated from the 
 blood, there muft neceflarily be fome means of 
 carrying it off, fince without this refource the 
 cornea would be too much diflended and pro- 
 truded. 
 
THE HUMOURS OF THE EYE. 61 
 truded. The manner in which this fuperfluous 
 humour is carried off is not clearly demonftra- 
 ble ; but it is probable that part of it is taken 
 up by abforbent veins, and part of it may tran- 
 lpire through the cornea. It is faid that the a- 
 queous humour is not perfectly clear in new- 
 born infants, in order that their tender eyes 
 may not at firff be too violently affedted by the 
 light : it is further obferved that in old age 
 this humour fometimes changes its confiff- 
 ence, becoming thicker ; and likewife alters 
 its colour, growing whitifh ; by which means 
 the rays of light do not pafs fo freely to the 
 retina as ufual : and this among other more 
 common caufes may be one reafon of imperfedt 
 fight in old people. 
 
 The ufe of the aqueous humour is to keep 
 the cornea diftended, and in fuch a Rate of 
 convexity as to refradc the rays of light in a 
 proper degree ; to preferve a due affiance be- 
 tween the cornea and the cryftalline humour ; 
 and to ferve as a proper medium for the iris to 
 float loofely in, whereby its different actions 
 may be performed more eafily. Some like- 
 wife fuppofe the aqueous humour is neceffary 
 
 for 
 
62 THE HUMOURS OF THE EYE. 
 for the cryftalline to move in, that objects may- 
 be feen more diftincdly at different diftances. 
 
 The Crystalline Humour. Next to 
 the aqueous is the cryftalline humour, which 
 is a transparent mafs refembling cryftal, 
 whence it takes its name. It is fituated be- 
 tween the other two humours, its anterior 
 part being oppofite to and very near 
 the pupil ; its pofterior portion being lodg- 
 ed in a cavity formed for its reception in 
 the middle and fore-part of the vitreous hu- 
 mour. The figure of the cryftalline is that 
 of a lens, convex on both fides but rather 
 more fo pofteriorly. This humour, which is 
 the lead: of the three, is of a much more firm 
 confidence, particularly in the middle of it, 
 than either the aqueous or vitreous ; for this 
 reafon it has fometimes been called the crydal- 
 line body. In the center or more compact 
 part, it feels like glue and may be eafily pref- 
 fed into different forms ; but it gradually be- 
 comes of a lefs firm confidence, fomewhat like 
 a thick jelly, towards the outer furface. In 
 time it is fubjedl to change both its confidence 
 and colour, growing dill more firm, efpecially 
 in the middle, and gradually taking a yellowifh 
 
 tinge. 
 
THE HUMOURS OF THE EYE. 63 
 tinge. The cryflaliine humour is mem- 
 branous, and confifts of a great num- 
 ber of coats inclofed in one another and 
 connected mutually one to another by very 
 fine fibres. The cryflaliine is contained in a 
 capfula formed by a continuation of the co- 
 vering- of the vitreous humour hei-eafter to be 
 defcribed. This capfula, which is fometimes 
 termed aranea, is a fine tranfparent mem- 
 brane, elaftic, and rather thicker in its ante- 
 rior than its poflerior part. It is furnifhed 
 with vefiels from the ciliary proceffes, and from 
 an artery which enters at the bottom of the 
 retina and runs through the vitreous humour, 
 as will be further taken notice of in the de- 
 fcription of that humour. If the capfula be 
 pricked with a needle, and a blow-pipe ap- 
 plyed to the orifice, the air may be introduced 
 between the capfula and the cryflaliine, and 
 will form a fmall tranfparent bladder. It has 
 been obferved like wife, that after pricking the 
 capfula in this manner, a little clear water will 
 fometimes be difcharged from the pundhire; 
 and it has been thought that the external por- 
 tion of the cryflaliine body is of an aqueous 
 nature, .and that the middle and more folid 
 
64 THE HUMOURS OF THE EYE. 
 part of it fwims as it were in this wa- 
 ter. It has been aflerted, in confequence 
 of this opinion, that the body of the cry- 
 ftalline humour has no connection or com- 
 munication with the capfula, nor is nou- 
 rithed in the ordinary way by a circulation of 
 fluids, but by abforption. This opinion, how- 
 ever, though it has been efpoufed by very emi- 
 nent writers, does not appear to be fufficiently 
 warranted ; as there is good authority to believe 
 that the veflels, which pafs from the ciliary pro- 
 cefles to the capfula, are continued from the 
 capfula to the body of the cryftalline. Ad- 
 mitting then fuch a continuation of veflels, 
 which we mud fuppofe to be extremely fine, 
 it is reafonable to conclude that the capfula 
 being elaftic, will if pun&ured be drawn 
 back in confequence of that elafticity, and 
 that the fine veflels juft mentioned, being ne- 
 cefiarily torn by this means, the body of the 
 cryftalline will be feparated from its proper coat, 
 and water will ouze out from thefe lacerated 
 lymphatics. Perhaps too the quantity of this 
 water may be increafed by the eye being exa- 
 mined after it has been kept fiome time, which 
 may have occaiioned part of the cryftalline to 
 
 ditfolve. 
 
THE HUMOURS OF THE EYE. 6^ 
 diflolve; The chief ufe of the capfula is to 
 confine the cryftalline humour in its pro- 
 per fituation, to which purpofe the con- 
 nection between the capfula and the vitre- 
 ous humour, the manner of which will be 
 explained in the next article, is fubfervient. 
 To explain the ufes of the cryftalline itfelf, it 
 has been fuppofed by fome that the ciliary 
 procefifes, which are partly inferred into the 
 circular rim or anterior portion of the vitreous 
 humour, will, when they contradt, deprefs the 
 vitreous humour ; that this depreffion of the 
 vitreous humour will pufh forwards the cavity 
 that receives the pofterior part of the cry- 
 ftalline, and of courfe the body of the cry-* 
 ftalline humour will be protruded nearer 
 to the cornea; but that when the ciliary pro- 
 ceftes are relaxed, the vitreous humour will 
 return to its former fituation, and the cryftal- 
 line again recede nearer to the bottom of the 
 eye. By this means it is imagined that the 
 the diftance of the cryftalline from the re- 
 tina is occafionally fitted to the different di- 
 ftances of objedts. The moft acknowledged 
 ufe of the cryftalline humour is to pro- 
 
 F duce 
 
66 THE HUMOURS OF THE EYE. 
 duce a fecond refraction of the rays of 
 light, the firft refraCtion, which is produced 
 by the cornea and the aqueous humour, not 
 being fufficient to bring them to a focus at the 
 retina. 
 
 Vitreous Humour. The third andlaftof 
 the humours of the eye is the vitreous, called 
 fo from its refemblance to melted glafs. It oc- 
 cupies all the pofterior, and indeed the great- 
 eft part of the globe of the eye : the confiftence 
 of it’is thicker than the aqueous, but lefs firm 
 than the cryftalline ; it is a very tranfparent 
 mafs, and has a good deal the appearance of a 
 fine clear jelly ; but if expofed fome time to 
 the air, it gradually collapfes and waftes, a 
 fine limpid water ouzing^ through the mem- 
 branes which give this humour its confiftence 
 for the fubftance of this mafs is compofed of 
 numerous little membranous cells or ve- 
 ficles filled with a pellucid humour. It has 
 already been hinted that the formation of this 
 humour may be partly owing to a continua- 
 tion of the velfels making the papillae of the 
 internal membrane of the choroides; but pro- 
 bably a confiderable fhare of it is derived from 
 
 the 
 
THE HUMOURS OF THE EYE. 67 
 the ramifications of a veffel, which proceeds 
 from the artery that has already] been defcrib- 
 ed as running through the center of the op- 
 tic nerve. The ramifications of this artery 
 are indeed in general diftributed on the retina ; 
 but there is one branch arifing from it which 
 paffes quite through the vitreous humour, and 
 fpreads itfelf in a beautiful manner on the back 
 of the capfula of the cryfcalline. This veffel 
 gives off very fine lateral branches as it paffes 
 through the vitreous humour, and from thefe 
 it is reafonable to fuppofe the pellucid humour 
 above-mentioned, may be in a great mea- 
 fure fecreted. In the middle and fore-part 
 of this humour is a depreffion or cavity, which 
 receives the pofterior portion of the cryftaliine, 
 and the margin of this cavity appears like 
 a circular protuberance. On this circular protu- 
 berance are feen the little radiated chan- 
 nels before-mentioned, which are made by the 
 ciliary proceffes as they pals to the circular 
 edge of the cryftaliine, and which appear 
 black at their bottom upon feparating the cry- 
 ftalline and vitreous humours from their adhe- 
 fions to the ciliary proceffes. This appearance 
 
 F 2 of 
 
68 THE HUMOURS OF THE EYE. 
 of blacknefs is owing to the fubftance before 
 defcribed under the name of black pigment, 
 which covers the ciliary procefles, and of 
 which part is now left at the bottom of thefe 
 channels after the procefles are removed. 
 
 The vitreous humour is covered with a coat 
 that is perfectly pellucid, and is termed the 
 tunica vitrea. It is contiguous to the retina, 
 and is a double membrane. The internal la- 
 mina of this membrane is faid to infinuate it- 
 felf into the fuhftance of the vitreous humour, 
 in order to form the little cells or veficles 
 before-mentioned ; but this humour having a 
 depreflion or cavity, as has been juft defcribed. 
 In which the pofterior portion of the cryflal- 
 line is lodged, the two laminae of the tunica 
 vitrea feparate when they arrive at the edge of 
 this cavity : here the internal of thefe two la- 
 minae pafles between the vitreous and cryftal- 
 line humours, ferving to line the cavity in 
 which the cryflalline lies, and to make the 
 poflerior part of its capfula; the other lamina 
 pafles over the cryflalline, and becoming 
 thicker and elaflic makes the anterior part of 
 the capfula. 
 
 The 
 
THE HUMOURS OF THE EYE. 69 
 The ufe of the vitreous humour is to fill up 
 the back part of the eye, and together with 
 the cryftalline and aqueous, to preferve the due 
 diflance between the cornea and the retina; 
 ferving alfo with the other humours, fo to re- 
 fraCt the rays of light as they may converge 
 to a point on the retina. 
 
 The different humours being defcribed, it 
 may not be improper here to add fome re- 
 marks on the depreflion and extraction of the 
 cryftalline, when it is become opake and 
 forms the difeafe called a cataraCt. 
 
 The difpute about the cataraCt, whether 
 it be an opacity of the cryftalline humour, 
 or whether it be a film or membranous fub- 
 ftance formed in the aqueous humour behind 
 the iris does now no longer prevail. It is the 
 general opinion at prefent that a cataraCt is an 
 opacity of the cryftalline. By what means was 
 this difpute determined ? by a method the moft 
 likely to deftroy an erroneous theory and to 
 eftablifh a true one, by examining into the 
 fcate of the parts after death. From this 
 examination it appeared, that the cryftalline 
 humour was become more or lefs opake, and 
 
JO THE HUMOURS OF THE EYE. 
 on the other hand, the fuppofed film or mem- 
 brane was not to be difcovered. The feat and 
 nature of the cataradt being in this manner 
 eftablifhed, the means of cure were more 
 plainly indicated : either the opacity of the 
 cryftaliine humour was to be cleared by me- 
 dicine, or the humour itfelf was to be removed 
 from its fituation by an operation. The firft 
 of thefe has not been found practicable : there 
 are no medicines yet difcovered, which either 
 internally or externally ufed, have the power 
 of reftoring the opake cryftaliine humour to its 
 natural tranfparency, ’Twas necelfary there- 
 fore, in order to have the light tranfmitted to 
 the retina, that this opake body fhould be 
 removed from its natural fituation by an 
 operation. Two ways have been pradtifed 
 for this purpofe ; the one to deprefs the hu- 
 mour, the other to extradl it intirely out of the 
 eye. The method by deprefllon has generally 
 been by palling an inftrument, the couching 
 needle, through the coats of the eye, into the 
 cryftalline humour, and pulhing the humour 
 gently down towards the lower and pofterior part 
 of the eye. The needle being then carefully with- 
 drawn. 
 
THE HUMOURS OF THE EYE. 71 
 drawn, and the cryftalline left in this new fitua- 
 tion, the rays of light are again tranfmitted 
 through the vitreous humour to the retina ; and 
 the defeCt in' the refraCfion of thefe rays, for 
 want of the cryftalline in its natural fituation, is 
 to be fupplyed by a proper optic glafs. 
 
 Whether the cryftalline humour is by this 
 operation always feparated and diflodged from 
 its cavity in the fore-part of the vitreous hu- 
 mour, or whether in attempting to do this, the 
 vitreous is not fometimes turned and moved 
 along with the cryftalline humour, (from the 
 ftrong connection between them by means 
 of the capfula of the cryftalline) is not eafy to 
 be determined. This is certain from experi- 
 ence, that it is much lefs difficult to deprefs the 
 humour than to keep it deprefted. The cryftalline 
 humour left at the lower part of the eye may 
 be conftdered there perhaps not improperly as 
 a foreign body, and having dn this place 
 no particular cavity for its reception as it 
 has in its natural fituation on the fore-part of 
 the vitreous humour, the common prefliire up- 
 on different parts of the globe in the feveral 
 motions of it, but particularly on its po- 
 F 4 fterior 
 
72 THE HUMOURS OF THE EYE. 
 fterior portion, by the contraction of the 
 mufcles when, the eye is drawn inwards, 
 would be very apt to protrude this foreign 
 body again into the bed or cavity which na- 
 ture originally prepared for it. So likewife 
 the force of coughing, vomiting, or put- 
 ting the head into fome particular pofition, have 
 been known to raife the cataract into view 
 again ; and if we allow that the vitreous 
 humour may be turned or moved along 
 with the cryftalline, in the depreffion of the 
 cryftalline, ’tis equally probable that nature, 
 ever follicitous to preferve the due order of her 
 neceffary functions, will by fome fuch means 
 as have been juft now recited, replace this or- 
 2an in its former fituation. 
 
 Upon the whole, the frequent return of the 
 opake cryftalline into its natural pofition after 
 it had been deprefted, the troublefome fymp- 
 toms which, though not often, yet fometimes 
 enfued either from wounding the coats of 
 the eye, lacerating the ciliary proceftes, or 
 doing other injury to thefe delicate parts 
 in the depreffion of the cryftalline, made the 
 fuccefs of the operation uncemin in general. 
 
THE HUMOURS OF THE EYE. 73 
 and the effe&s in fome inftances difagree- 
 able ; fo that any other method of operating 
 which carried with it a reafonable pro- 
 fpedt of anfwering the' purpofe well, had a 
 claim to confederation. Another method was 
 propofed, and at ftrft light feemed fo reafon- 
 able and proper, that, like many other dif- 
 coveries, ’tvvas rather furprifing it fhould not 
 be fooner brought into common ufe (for 
 the general fcheme of the operation ap- 
 pears to have been thought of .before, and 
 the operation had probably been pradtifed by 
 the remarkable, itinerant operator of our own 
 country) than that it fhould now be received 
 with approbation and eagernefs. It had been 
 obferved, that in the operation by depreffion, 
 the cryftalline had fometimes been diflodged 
 from its cavity in the vitreous and had come 
 forward into the anterior chamber of the aque- 
 ous humour, and that upon fuch occaftons an 
 opening had been made in the cornea, and the 
 cryftalline fuccefsfully extracted. From this 
 confideration, ’twas not unreafonable to try 
 whether the opake cryftalline might not gene- 
 jierally be extracted intirely out of the eye with 
 
 fafe- 
 
74 THE HUMOURS OF THE EYE. 
 
 fafety and fuccefs, through a proper opening 
 made for its paflage in the cornea. An open- 
 ing is made in the cornea in fuch a manner as 
 to give room for the cryftalline to pafs through 
 it ; the cryftalline either by the common moti- 
 ons of the eye or by a gentle prefture made up- 
 on the eye, is brought forward towards the 
 anterior chamber of the aqueous humour ; the 
 capfula of the cryftalline is divided in order to 
 free the body of the cryftalline from any 
 attachments which might keep it back, and 
 to prevent the vitreous humour from being 
 drawn along and difcharged with it ; the cry- 
 ftalline is protruded and pafies out through the 
 opening made in the cornea. Thus the obftacle 
 which hindered the tranfmiffion of light thro’ 
 the vitreous humour to the retina, is for 
 ever removed, and the want of a due refrac- 
 tion of the rays occaftoned by the difcharge of 
 the cryftalline is to be fupplyed afterwards, as 
 in the operation by depreftion, by an optic 
 glafs of a fuitable convexity. 
 
 What has been the fuccefs of this me- 
 thod ? not equal to what was wifhed and 
 reafonably expected. Whether in the ope- 
 i ration 
 
THE HUMOURS OF THE EYE. 75 
 ration by extraction, the iris was fome- 
 times wounded in paffing the needle thro’ 
 the cornea or in dividing the capfula of the 
 cryftalline ; whether it was over-diftended or 
 torn by the cryftalline’ s paffing through the 
 pupil ; whether in the expulfion of the cry- 
 ftalline the ciliary proceffes were lacerated, and 
 an extravafation of blood enfued from a rupture 
 of the fine veffels of thefe proceffes or of other 
 parts of this delicate organ ; or laftly, and what 
 appears to me very probable, whether the 
 quantity of air let into the eye by the large 
 wound made in the cornea, injures the hu- 
 mours and the membranes that inveft them, 
 as often happens in fome other cafes where 
 membranous parts are expofed to the air, is 
 difficult to determine. This is certain, that 
 tedious and painful inflammations are apt 
 to follow the operation by extraction, and 
 fometimes confiderable impofthumations en- 
 fue : in confequence of thefe, adhefions have 
 been formed ; the figure of the pupil 
 has been changed or the motion of it in- 
 tirely loft ; and fometimes the globe of the eye 
 has wafted and been deflroyed. Even the fear 
 
 from 
 
76 THE HUMOURS OF THE EYE. 
 from the wound in the cornea, or the thickening 
 of the cornea in confequence of the long- 
 continued inflammation upon it, are apt fome- 
 times to hinder the free admiffion of the light 
 through it. 
 
 Upon thefe confiderations the operation by 
 depreffion, hitherto at leaf!:. Teems preferable; and 
 it may be further faid, that the operation by 
 extraction may be afterwards pradtifed if the 
 other method fhould prove ineffectual. In 
 regard to the method by depreffion, as the 
 fymptoms attending it are feldom very bad, 
 and are free from danger; as Tome cafes admit 
 of a reafonable hope of fuccefs ; and even when 
 fuccefs is wanting, as the patient is left much 
 in the fame Rate as before the operation was 
 performed ; where the circumftances of the 
 cafe are fuch as are commonly judged proper 
 for the operation, it ought certainly to be re- 
 commended. 
 
 The next and the laft parts to be confider- 
 ed, are the mufcles. 
 
 OF 
 
OF THE MUSCLES OF THE EYE- 
 
 f | ' H E eye, thus curioufly conftruCted to 
 JL admit the rays of light and to receive 
 the impreffion of external objects on the bot- 
 tom of it ; placed likewife in a fituation the 
 moft favourable for commanding the view of 
 objects ; and lodged in a bony cavity ferving to 
 fecure it againft many accidental injuries ; 
 would with all thefe advantages, have been 
 much circumfcribed in its ufes, if it had 
 been fixed in the focket without the 
 power of being occafionally moved in dif- 
 ferent directions, according to the pofition 
 of the objeCt to be viewed. In order there- 
 fore to anfwer the important purpofes of 
 vifion moft effectually, the eye-ball is made 
 capable of various motions, and is furnifh- 
 ed with a proper apparatus of mufcles for the 
 performance of thofe feveral motions. Each 
 eye has fix mufcles ; four of which are called 
 right or ftrait mufcles ; and they receive like- 
 wife other denominations relative to their fitu- 
 ation, functions, or fome power which they are 
 fuppofed to have in expreffing particular af- 
 fections 
 
78 THE MUSCLES OF THE EYE. 
 fedtions of the mind. Thus one is called rec- 
 tus fuperior, attollensf or elevator, and fuper- 
 bus another is termed rectus inferior, depref- 
 for, and humilis ; a third has the names of 
 redtus exterior, abducens, and indignatorius ; 
 a fourth is Ityled redtus interior, adducens, and 
 bibitorius ; all thefe ftrait mufcles arife by Ihort 
 narrow tendons from the bottom of the focket 
 near the hole which gives palfage to the op- 
 tic nerve : they foon become flelhy, and run- 
 ning over the posterior part of the globe of the 
 eye, are inferted into the anterior parts of the 
 fclerotica by thin broad tendons, which by 
 their expanlions unite with one another, and 
 are continued on to form the white of the eye. 
 
 The mufculus redtus fuperior, palling over 
 the upper part of the globe of the eye, is in- 
 ferted into the fuperior and anterior part of the 
 -fclerotica ; the redtus inferior palfes along the 
 under part of the globe, and is inferted into 
 the fclerotica oppofite to the infertion of the 
 preceding mufcle ; the redtus exterior runs on 
 the fide of the globe next the temple, and is 
 inferted into the fclerotica on the fame fide ; 
 the redtus interior palfes on the fide of the 
 globe next the note, and is inferted into the 
 
 fclerotica 
 
THE MUSCLES OF THE EYE. 79 
 fclerotica on that fide, and oppofite the infer- 
 tion of the former. 
 
 When thefe mufcles aft fingly, theyferve ei- 
 ther to lift the eye-ball upwards, or turn it 
 downwards, to move it outwards or turn it in- 
 wards, according to the refpeftive aftion of 
 each mufcle. Thus the reftus fuperior railes 
 the anterior portion of the globe of the eye up- 
 wards when we lift up our eyes ; the reftus 
 inferior brings it downwards, when we turn 
 the eye in that direftion : the reftus exterior 
 ferves to move the globe fideways towards the 
 temple; and the reftus interior to turn it inwards 
 towards the nofe. When two neighbouring ftrait 
 mufcles aft together, they give the eye fome 
 degree of obliquity in its motion, as when the 
 reftus fuperior and the adduftor move toge- 
 ther, they turn the eye obliquely upwards and 
 towards the great angle ; and fo of the others : 
 Thefe laft motions have been fuppofed to be ef- 
 fected by the oblique mufcles. When all thefe 
 mufcles aftjucceffively, theygive the appearance 
 of a rotatory or rolling motion to the eye. If 
 all the ftrait mufcles aft with equal force and 
 at the fame time, they are faid to draw the 
 eye-ball in fome degree towards the bottom of 
 
 the 
 
80 THE MUSCLES OF THE EYE. 
 
 the focket, and to keep it as it were fixed and 
 directed right forward. It is thought likewife 
 that by drawing the eye towards the bottom 
 of the focket, the posterior part of the eye may 
 be fo comprelfed again!! the fat contained in the 
 focket, as to Ihorten the diftance between the 
 retina and cornea : at the fame time the hu- 
 mours of the eye are fuppofed to be protrud- 
 ed forwards, and to render the cornea more 
 convex, by which means the eye, it is imagin- 
 ed, is fitted to fee fmall or near objects di- 
 ftin&ly. 
 
 Befides thefe four ftrait mufcles there are 
 two others, which from their direction, are 
 called oblique. They are diftinguifhed by the 
 names of obliquus major or fuperior, and the 
 obliquus minor or inferior. The firfh of thefe 
 is likewife termed trochlearis, from the ten- 
 don of this mufcle palling through a cartila- 
 ginous pully that is fituated in the orbit near 
 the great or inner angle of the eye. The two 
 oblique mufcles together are by fome named 
 circumagentes and amatorii, from the power 
 which they were fuppofed to have of winding 
 or rolling about the eye in the manner of 
 looking that is commonly termed ogling. 
 
 The 
 
THE MUSCLES OF THE EYE. Si 
 The mufculus obliquus major or luperiof 
 arifes tendinous from the bottom of the focket, 
 near the entrance of the optic nerve, and 
 pafling towards the upper part of the focket 
 near the great angle of the eye, the found ten- 
 don of this mufcle runs through a cartilagi- 
 nous pully, which is there fixed to a depref- 
 fure in the os frontis. From thence it is re- 
 flected and runs obliquely backwards inclofed 
 in a ligamentous fheath, ' and is inferted tendi- 
 nous into the fclerotica upon the upper part 
 of the globe of the eye, a little more back- 
 ward than the infertion of the redtus fupe- 
 rior. 
 
 Themulculus obliquus minor or inferior arifes 
 chiefly flelhy, jufl: within the edge of the 
 lower and anterior part of the focket near the 
 nofe, and pafling obliquely backwards under 
 the globe is inferted into the fclerotica on the 
 external fide of the eye. 
 
 When the firfi: of thefe mufcles, the obli- 
 quus major or fuperior, aCls fingly, it rolls 
 the eye about its axis, drawing the globe for- 
 wards and turning the pupil downwards ; 
 when the fecond, the obliquus minor or infe- 
 
 G rior, 
 
82 THE MUSCLES OF THE EYE. 
 rior, aTts fingly, that too rolls the eye about 
 its axis, ferving likewife to draw the globe for- 
 wards ; but turns the pupil upwards. When 
 both the oblique mufcles adt together, they 
 draw the eye outward from the focket, ferv- 
 ing by this means as antagonifts to counteradt 
 the power of the flrait mufcles, which, 
 when they all adt together and with equal 
 force, draw the eye-ball backwards ; and 
 even when they adt fingly, in the performance 
 of their other motions, they have fome 
 tendency to draw the eys-ball backwards. The 
 oblique mufcles are thought likewife to have a 
 further ufe in bringing the eye-ball forwards 
 or outwards from the focket, as thereby they 
 take oft all prefllire from the back part of the 
 eye, and fit the form of the eye to view di- 
 ffant or large objedts diftindtly. 
 
 The mufcles of the eye-ball are furnifhed 
 with blood-vefiels from the external carotid ar- 
 tery, by means of the internal maxillary ar- 
 tery : The branch of the internal maxillary, 
 which has been deferibed to enter the focket 
 and to be divided there into feveral ramificati- 
 ons, gives fome to thefe mufcles. 
 
 The 
 
THE MUSCLES OF THE EYE. 83 
 The nerves which pafs to thele mufcles are 
 from the third, the fourth, and the fixth 
 pair. 
 
 The third pair of nerves, or as they are 
 likewife called, the motory pair or movers of 
 the eyes, give a branch to the fuperior, to the 
 inferior, and to the internal right mufcles ; 
 alfo another branch to the inferior oblique 
 mufcle. The fourth pair, or as they are fome- 
 times called, the pathetic pair, are fpent on 
 the fuperior oblique mufcles 5 and the fixth 
 pair of nerves are fent chiefly to the exterior 
 light mufcle or abducens. 
 
 G 
 
 C O N- 
 
[ 8 4 ] 
 
 CONCLUSION. 
 
 H AVING endeavoured thus to give a 
 plain account of the ftrudure and 
 ufes of the eye and eye-lids, I have pro- 
 ceeded in this fubjed as far as the duty of 
 my office requires. It might however have 
 been no unpleafing talk to have recapitulated 
 and given in one colleded view the various ufes 
 to which the feveral parts defcribed are moft 
 admirably fubfervient; but as I may already 
 have engaged more of your time than, has been 
 agreeable to your convenience or inclination, I 
 Ihall only beg leave to take notice in general 
 
 terms -of the lituation of the eye, as it is 
 
 placed in that part of the body which is moft 
 advantageous for commanding the view of 
 
 objeds- of the figure of the eye, which 
 
 with the contained humours, is belt calcu- 
 lated to receive and refrad the rays of light, fo 
 as to form a diftind pidure of external objeds 
 
 upon the bottom of it -of the manner in 
 
 which the eye is conneded to the focket, by 
 which means it is properly retained there, and 
 \ yet 
 
CONCLUSION. $ s 
 yet is capable occafionally of a free movement 
 in different directions without pain or inconve- 
 venience of the defence with which na- 
 
 ture has furnifhed this organ againft many ex- 
 ternal accidents, by inclofing the greateft part 
 
 of it in a firm, bony cavity or cafe of the 
 
 lodgement of it upon a foft yielding bed of fat, 
 which facilitates the motions neceffary to vi- 
 fion, prevents the injuries of preffure from 
 thefe motions, and keeps the parts in a due 
 Hate of foftnefs and flexibility of the vari- 
 
 ous movements which the eye is capable of, 
 by means of its feveral mufcles, according as 
 the different fituation of the objeCt to be view- 
 ed requires of the coats or coverings of the 
 
 eye, and the fubftances called humours con- 
 tained in thofe coats, for the production of the 
 neceffary refractions of the rays of light- — — 
 of the power which the eye has by the expan- 
 fion and contraction of the pupil to admit a 
 fufficient quantity of light or exclude too much, 
 according to the diflance or nearnefs of the 
 objeCt to be viewed, or according to the de- 
 gree of light it is feen in which is almoft per- 
 petually varying laftly of the number, the 
 
 courfe 
 
 I 
 
86 CONCLUSION, 
 courfe and the finenefs of the blood-vefiels and 
 nerves ferving for the conflant fupply of the 
 humours, and for the due nourifhment and fen- 
 
 fation of the other parts of the eye There 
 
 is one obfervation more, which arifes indeed fo 
 naturally from the fubjed as to make the men- 
 tion of it here hardly neceflary— - every fenfible 
 and feeling mind will refled: with pleafure, ad- 
 miration and reverence, on the infinite art and 
 wifdom, which are fo remarkably difplayed 
 in the ftrudure and ufes of this exquifite or-, 
 gan. 
 
 JL iiu 
 
 E N D. 
 

 fa 
 
 V 
 
 > Or. 
 
 L 
 
 i 
 
 Gat aker 
 1761