EYES. 



2OI 



and mesenchymatous cells enter the optic cup through the chorioid 

 fissure before its closure. Some suppose that the vitreous body arises 

 from a modification of these cells, some regard it as an exudate from the 

 blood-vessels, and others think it a retinal secretion. The fact that 

 the blood-vessels mentioned do not occur in birds is of interest in this 

 connection. In mammals, when the chorioid fissure closes, the vessels 

 appear to enter through the centre of the optic nerve (central retinal 

 artery and vein fig. 207). In the early stages the retinal artery 



FIG. 207. Diagrammatic section of half a mammalian eye. ac, anterior chamber; 

 ca, ciliary arteries; c, eyelash (cilium); cj, conjunctiva; co, cornea; cp, ciliary process; cr, 

 central retinal artery and vein; cs, conjunctival sac; ct, chorioid tunic; d, dura of optic 

 nerve; i, iris; on, optic nerve; os, ora serrata; pc, posterior chamber; pe, pigmented epithel- 

 ium; r, retina; sc, sclera; tg, tarsal gland; w, vorticose vein; cc, zonula zinii. 



divides inside the cup, one branch (hyaloid artery) going through the 

 vitreous body to the neighborhood of the lens, the other being distrib- 

 uted over the inner surface of the retina. Later the hyaloid artery 

 disappears, while retinal arteries are rare except in mammals. 



The outer wall of the optic cup forms the pigmented epithelium 

 of the eye, developing a large amount of black pigment which eventu- 

 ally surrounds and isolates the rods and cones, so that each can be 

 affected only by the light which falls directly upon it. As will readily be 

 understood the side of the pigment layer away from the retina corre- 

 sponds to the deeper surface of the skin and so comes into relation with 

 the connective tissue. From this is developed the envelopes of the 

 eye tunica vasculosa, sclera, etc. 



Surrounding the retina and pigmented epithelium and extending 

 forward over the lateral parts of the lens is the tunica vasculosa, 

 in which two parts are recognized, the iris and the chorioid. The 



