240 DR KNOX on the Philosophical Anatomy 



one very delicate branch of a small artery has been consigned 

 the task of secreting and nourishing the lens, and perhaps even 

 the vitreous humor ; though, relative to this last, anatomists and 



the aqueous humor ; and the assertion, of its not being present in the anterior cham- 

 ber of the aqueous humor in the foetus, appears to me extremely doubtful. 



In examining the eyes of the foetus in the mammalia, with a view to the determi- 

 nation of the vascular structure of the pupillary membrane, several appearances 

 have presented themselves to me, which I shall here briefly state, since they may 

 be useful to those who may not have opportunities for making very minute vascular 

 preparations of the eye. It has always appeared to me (and I have sacrificed a great 

 number of very delicate preparations, in order to determine the anatomy of the pu- 

 pillary membrane), that, by means of this membrane, the anterior and posterior 

 chambers of the aqueous humor, form in the foetus two distinct shut sacs, each 

 enclosed in a proper capsule. With regard to the anterior of these, it is very evi- 

 dent from several preparations now lying before me, that it is formed by the inner 

 membrane of the cornea, (or at least of a membrane covering the inner surface of the 

 cornea) ; which is reflected over the whole anterior surface of the iris and pupillary 

 foramen ; but I cannot speak so decisively of the formation of the posterior sac, or 

 that situated behind the iris, i. e. I am unable to say, whether it terminates at the 

 equatorial margin of the lens, after investing, though loosely, the floating termina- 

 tions of the ciliary processes, or whether it invests the anterior surface of the lens. It 

 is very evidently connected with the membrane forming the canal of Petit. Betwixt 

 the portion of membrane extending from the equatorial margin of the lens to the 

 pupillary edge of the iris, (in the foetus), and which being continued forward, con- 

 stitutes one layer of the pupillary membrane, and the iris itself; there is a trian- 

 gular space, very distinct in several preparations I have now before me. This space 

 is occasioned by the membrane proceeding from the lens to the iris, in a straight 

 line, like the string of a bow; whereas the iris is arched or concave. It is pro- 

 bable that the (diagrammatic figure (Plate X.) may explain, much better than any 

 words could do, the distribution of these membranes in the foetus or very young 

 animal. 



The great share the hyaloid membrane of the vitreous humor has in the forma- 

 tion of these membranes, as well by its intimate connection with the capsule of the 

 lens, as by its most evident and distinct connection with the posterior layer of the pu- 

 pillary membrane, is very remarkable, when contrasted with the extreme delicacy of 

 the vascular membrane of the retina, which scarcely contributes any thing towards 

 the formation of this admirable and very singular structure. Viewing the interior of 



