An account of the structure of the eye :

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WILLIAM H. WELCH MEDICAL LIBRARY

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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,

TH 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.

TH 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 deferTT°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.

PREVIOUS 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. i7
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-LID5. 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

28 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 ;

3o 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.

TH3 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

4o 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

5o 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-

S2 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

56 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

SB 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-

[ 84 ]

CONCLUSION.

HAVING 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