THE VITREOUS BODY. 903 



into four or five branches, which at first run between the hyaloid membrane and 

 the nervous layer, but they soon enter the latter membrane, and pass forward, 

 dividing dichotomously. From these bi'anches a minute capillary plexus is given 

 off. which does not extend beyond the inner nuclear layer. In the foetus a small 

 vessel passes forward, through the vitreous humor, to the posterior surface of the 

 capsule of the lens. 



Humors of the Eye. 



The aqueous humor completely fills the anterior and posterior chambers of the 

 eyeball. It is small in quantity (scarcely exceeding, according to Petit, four or 

 five grains in weight), has an alkaline reaction, in composition is little more than 

 water, less than one-fiftieth of its weight being solid matter, chiefly chloride of 

 sodium. 



The anterior chamber is a space bounded in front by the cornea, behind by the 

 front of the iris. The posterior chamber was the name formerly given to a space 

 which was believed to exist between the iris in front and the capsule of the lens, 

 its suspensory ligament, and the ciliary processes behind. It is now known that 

 the posterior surface of the iris is in immediate contact with the lens throughout 

 the greater part of its extent. The only space which remains to represent the 

 posterior chamber is a narrow chink between the peripheral part of the iris, the 

 suspensory ligament, and the ciliary processes. 



In the adult these two chambers communicate through the pupil ; but in the 

 foetus in the seventh month, Avhen the pupil is closed by the membrana pupillaris, 

 the two chambers are quite separate. 



The Vitreous Body. 



The vitreous body forms about four-fifths of the entire globe. It fills the con- 

 cavity of the retina, and is hollowed in front for the reception of the lens and its 

 capsule. It is perfectly transparent, of the consistence of thin jelly, and is com- 

 posed of an albuminous fluid enclosed in a delicate transparent membrane, the 

 hyaloid. This membrane invests the surface of the vitreous body ; at the pars ciliaris 

 rctince it splits into two layers, an anterior, the suspensory ligament of the lens, and 

 a posterior, which passes over the front of the vitreous body. It has been supposed, 

 by Hannover, that from its inner surface numerous thin lamellae are prolonged 

 inward in a radiating manner, forming spaces in which the fluid is contained. In 

 the adult these lamellae cannot be detected even after careful microscopic exami- 

 nation ; but in the foetus a peculiar fibrous texture pervades the mass, the fibres 

 joining at numerous points, and presenting minute nuclear granules at their 

 point of junction. In the centre of the vitreous humor, running from the posi- 

 tion of the entrance of the optic nerve on the retina to the posterior surface of the 

 lens, is a canal filled with fluid and lined by a prolongation of the hyaloid mem- 

 brane. This is the canal of Stilling, and is the canal which in the embryonic 

 vitreous humor conveyed the minute artery from the central artery of the retina 

 to the back of the lens. The fluid from the vitreous body resembles nearly pure 

 water ; it contains, however, some salts and a little albumen. 



The hyaloid membrane encloses the whole of the vitreous humor, that portion 

 on its anterior surface, which is hollowed out for the reception of the lens, being 

 the posterior layer just mentioned ; while the anterior layer is the suspensory 

 ligament. It is a delicate structureless membrane, except where it forms the sus- 

 pensory ligament, where it contains longitudinal elastic fibres. Immediately 

 beneath the hyaloid membrane are found small, granular, nucleated cells which 

 are said to be possessed of amoeboid movements. 



In the foetus the centre of the vitreous humor presents a tubular canal, through 

 which a minute artery passes along the vitreous body to the capsule of the lens. 

 In the adult no vessels penetrate its substance, so that its nutrition must be 

 carried on by the vessels of the retina and ciliary processes situated upon its 

 exterior. 



