2l6 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



The fact that the blood-vessels mentioned do not occur in birds is of 

 interest in this connexion. 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. 231). In the early 

 stages the retinal artery divides inside the cup, one branch (hyaloid 

 artery) going through the vitreous body to the neighborhood of the 

 lens, the other being distributed 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 



Fig. 231. — Diagrammatic section of half a mammalian eye. ac, anterior chamber; 

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

 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 epi- 

 thelium; r, retina; sc, sclera; tg, tarsal gland; vv, vorticose vein; zz, zonula zinii. 



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 

 corresponds to the deeper surface of the skin and so comes into rela- 

 tion with the connective tissue. From this is developed the envel- 

 opes 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 

 whole is richly vascular, and the chorioid, suppHed by the ciliary 

 arteries which enter at the side, is the chief source of nourishment 

 for the rod and cone cells. To the vascular part certain other por- 

 tions are added in some groups. Thus just outside the blood-vessels 



