of the Canal of PE TIT. 24 1 



physiologists, with the exception of HALLER, have preserved the 

 profoundest silence. 



When we consider the vast comparative size of the vitreous 

 humor, we may feel assured, that one small branch of an ar- 



the eye-ball philosophically, we might say, that it is composed originally of a 

 or suite, of colourless capsules, forming generally shut sacs, in which are deposited 

 perfectly transparent substances, having various refractive powers. Posteriorly we 

 have the hyaloid sac, and the contained vitreous body : %dly, The crystalline capsule 

 and lens : 3dly, The posterior chamber of the aqueous humour, formed in the fetus 

 by its own capsule : lastly, The anterior chamber, like the rest, a shut sac. The 

 membranes forming these various capsules appear in the foetus to be continuous. 



In the foetus, or very young animal, the quantity of fluid in either of the aqueous 

 chambers of the eye, must be very trifling, if it actually exist ; for the pupillary mem- 

 brane is in contact with the anterior surface of the capsule of the lens. On the 

 other hand, the crystalline humor in the foetus is very large, and has the posterior 

 segment of its capsule entirely covered with bloodvessels. The branch of the cen- 

 tral artery of the retina which passes through the vitreous humor in a young animal, 

 apparently quite disproportionate to the other branches of the arteria centralis, is for 

 the purpose, undoubtedly, of furnishing the lens with the means of rapid growth. 

 In the adult animal it is altogether obliterated, seemingly because no longer required, 

 the lens having, at an early period of life, acquired its full growth, and being, like 

 the enamel of the teeth, subject neither to decay nor renovation. 



The vessels of the pupillary membrane come to it chiefly from those of the iris : 

 it may receive a few from the terminating branches of the artery distributed to the 

 capsule of the lens, but assuredly these must be very few. They were demon- 

 strated by Dr W. HUNTER, who also describes, very accurately, the distribution 

 of the posterior layer of the pupillary membrane : " Where the membrana pupillse 

 exists, there is a fine vascular membrane all around, which passes in the posterior 

 aqueous chamber, from near the edge of the lens to the edge of the pupilla *." Now, 

 it is extremely easy to demonstrate, that the membrana pupillaris consists of two 

 layers ; the posterior of which is a continuation of the one just described by Dr W. 

 HUKTEB. From the peculiar form of this membrane, (I mean the portion proceed- 

 ing from the margin of the lens and hyaloid membranes to the pupillary margin of 

 the iris), I should imagine it to be immediately ruptured and destroyed by the con- 

 traction of the iris, on the first admission of light through the pupil. 



* Medical Commentaries, p. 63. 



nh 2 



