KYK. 



177 



.!> >|iredily recovers its transparency upon the 

 removal of tlic pressure. Tli<: chemical com- 

 puMtioii of the cornea is similar to tli.it of the 

 fibrous membranes in general and the sclerotic 

 tn particular : like the latter structure, it is con- 

 verted into gelatine by boiling; but Berzelius 

 states that it contains also a small quantity of 

 fibrine or coagulated albumen, as proved by 

 the formation of a precipitate upon adding the 

 cyanuret of ferro-prussiate of potass to acetic 

 arid, in which the membrane has been digc.itrd. 

 The rornea possesses great strength, being 

 seldom or never ruptured by blows on the eye- 

 ball, which frequently tear the sclerotic exten- 

 sively. It does not yield to distension from 

 increased secretion, effusion, or suppuration 

 within the eyeball in consequence of inflam- 

 mation, but it becomes extended and altered 

 by growth both in shape and dimensions, as 

 may be observed in prominent staphyloma, 

 hydrophthalmia, and that peculiar alteration 

 called stiii>/n/liiiii petlucidum, in which the 

 spherical form of the membrane degenerates 

 into a cone, but retains its transparency. 



The cornea is destitute of red vessels, yet it 

 affords a signal example of colourless and 

 transparent texture possessing vital powers 

 inferior to no other. No structure in the body 

 appears more capable of uniting by the first 

 intention. The wound inflicted in extracting a 

 cataract is often healed in forty-eight hours, yet 

 the lips are bathed internally with the aqueous 

 humour, and externally with the tears. Ulcers 

 fill up and cicatrize upon its surface ; and al- 

 though the vessels, under such circumstances, 

 frequently become so much enlarged as to 

 admit red blood, yet there can be no doubt 

 that ulcers do heal without a single red vessel 

 making its appearance. Abscesses form in the 

 cornea, and contain purulent matter of the 

 same appearance as elsewhere ; they are gene- 

 rally said to be between the layers of the 

 cornea, but they are evidently distinct cavities 

 circumscribed by the inflammatory process as 

 in other cases; occasionally, however, the 

 whole texture of the cornea becomes infil- 

 trated with purulent matter, as the cellular 

 membrane in erysipelas. The rapidity with 

 which this membrane is destroyed by the ul- 

 cerative process is another proof of its superior 

 vitality. In a few days a mere speck of ulce- 

 ratiun, the consequence of a pustule, extends 

 through the entire thickness, and permits the 

 iris to protrude; and in gonorrhceal and infantile 

 purulent ophthalmia, the process is much more 

 rapid and extensive. It is true that in the 

 latter case the destruction is attributed to gan- 

 grene or sloughing, and to a certain extent 

 correctly; but an accurate observer must admit 

 that the two processes co-operate in the pro- 

 duction of the lamentable consequences which 

 result from these diseases. Ulcers of the cornea 

 fill up by granulation and cicatrize as in other 

 parts of the body, but the repaired part does 

 not possess the original organization, and is 

 consequently destitute of that transparency and 

 regularity of surface so essential for its func- 

 tions ; hence the various forms and degrees of 



VOL. II. 



opacity enumerated under the tiihnicul titles 



of uttillflii, li'iifii/iiu, lH<ili!iiritii, nilintit, &C. 



which are probably never remedied, however 

 minute they may be, notwithstanding the ge- 

 neral reliance placed in the various stimulating 

 applications made for this purpose. Slight 

 opacities, or nebula: as they are called, if con- 

 fined to the conjunctiva! covering of the cornea, 

 gradually disappear after the inflammation sub- 

 sides, as does also diffused o|>acity of the 

 cornea itself, the consequence of scrofulous 

 inflammation; but I believe opacities from 

 ulceration and cicatrix are seldom if ever re- 

 moved. The effect of acute inflammation is 

 to render this, and perhaps all transparent and 

 colourless membranes, white and opaque with- 

 out producing redness ; this may be seen in 

 wounds, where the edges speedily become 

 gray ; and in the white circle which frequently 

 occupies the margin of the cornea in the in- 

 flammations of the eyeball commonly called 

 iritis. 



The cornea in a state of health is destitute 

 of sensibility. Of this I have frequently sa- 

 tisfied myself by actual experiment in cases of 

 injury of the eye, where the texture of the part 

 is exposed. When foreign bodies, such as 

 specks of steel or other metals, are lodged in 

 its structure, the surgeon experiences much dif- 

 ficulty in his attempts to remove them, from 

 the extremely painful sensibility of the con- 

 junctiva as he touches it with his needle ; but 

 the moment he strikes the point of the instru- 

 ment beneath the foreign body into the cornea 

 itself, the eye becomes steady, and he may 

 touch, scrape, or cut any part of the membrane 

 uncovered by conjunctiva without complaint. 



It has already been stated that the cornea, 

 as it constitutes the transparent medium for 

 the passage of the rays of light, is composed of 

 three distinct forms of structure altogether dif- 

 ferent from each other, the conjunctiva, the 

 cornea proper, and the elastic cornea. The 

 latter membrane is now to be described. In 

 many of our books this membrane is vaguely 

 alluded to as the membrane of the aqueous 

 humour ; but with this it must not for a mo- 

 ment be confounded. It is a distinct provision 

 for a specific purpose, totally different from 

 that for which the other is provided. It was 

 known to and described by Duddell, Decemet, 

 Demours, and latterly by Mr. Sawrey ; but all 

 these authors having unfortunately published 

 their accounts in separate and probably small 

 treatises, not preserved in any journal, I have 

 not been able to consult them. It is, however, 

 distinctly recognized by Clemens, D. W. 

 Sommerring, Dlainville, and Hegar ; and in a 

 paper on the anatomy of the eye in the Me- 

 dicc-Chirurgical Transactions, I endeavoured to 

 direct attention to it without effect. The struc- 

 ture here alluded to is a firm, elastic, exqui- 

 sitely transparent membrane, exactly applied to 

 the inner surface of the cornea proper, and se- 

 parating it from the aqueous humour. \\ hen 

 the eye has been macerated for a week or ten 

 days in water, by which the cornea proper is 

 rendered completely opaque, this membrane re- 



N 



