THE VITREOUS BODY. 839 



synapsis occurs between the processes of the inner granules and the rod and cone 

 elements. 



The arteria centralis retinae and its accompanying vein pierce the optic nerve, 

 and enter the globe of the eye through the porus opticus. It immediately bifur- 

 cates into an upper and a lower branch, and each of these again divides into an 

 inner, or nasal, and an outer, or temporal, branch, which at first run between the 

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

 forward, dividing dichotomously. From these branches a minute capillary plexus 

 is given off, which does not extend beyond the inner nuclear layer. The macula 

 receives small twigs from the temporal branches and others directly from the central 

 artery ; these do not, however, reach as far as the fovea centralis, which has no blood- 

 vessels. The branches of the arteria centralis retinae do not anastomose with each 

 other in other words, they are " terminal arteries." In the foetus, a small vessel 

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

 of the lens. 



REFRACTING MEDIA. 



The Refracting media are three, viz. : 



Aqueous humor. Vitreous body. Crystalline lens. 



I. Aqueous Humor. 



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 the space bounded in front by the cornea ; behind, by 

 the front of the iris. The posterior chamber is a narrow chin,k between the 

 peripheral part of the iris, the suspensory ligament of the lens, and the ciliary 

 processes. 



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

 foetus of the seventh month, when the pupil is closed by the membrana pupillaris, 

 the two chambers are quite separate. 



II. Vitreous Body. 



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

 concavity of the retina, and is hollowed in front, forming a deep concavity, the 

 fossa patellaris, for the reception of the lens. It is perfectly transparent, of the 

 consistence of thin jelly, and is composed of an albuminous fluid enclosed in a 

 delicate transparent membrane, the membrana hyaloidea. 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 examina- 

 tion in the fresh state, but in preparations hardened in weak chromic acid it is 

 possible to make out a distinct lamellation at the periphery of the body ; and 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 entrance of the optic nerve 

 to the posterior surface of the lens, is a canal filled with fluid and lined by a 

 prolongation of the. hyaloid membrane. This is the canal of Stilling, which in 

 the embrvonic vitreous humor conveyed the minute vessel 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 albumin. 



The hyaloid membrane encloses the whole of the vitreous humor. In front 

 of the ora serrata it is thickened by the accession of radial fibres and is termed 



