VISUAL OEGAN 0'^ VERTEBRATA. 



529 



line can be made out on the lower and inner side of tlie rudimentary 

 bulb ; this extends from the optic nerve to the free anterior edge 

 of the choroid. It corresponds to the fissure (choroidal fissure) 

 which was formed when the rudiment of the vitreous body grew 

 into the secondary optic vesicle (s), and which must therefore affect 

 the retina and the pigmented layer of the choroid (tapetum nigrum). 

 A large number of changes subsequently affect this rudiment of 

 the eye. The anterior edge of the secondary optic vesicle grows 

 out, together with the tissue that forms the rudimentary choroid, 

 and gives rise to the iris, which bounds the pupil. When the pro- 

 cess of the cutis pushes its way into the secondary optic vesicle, 

 blood-vessels pass (in the Mammalia) into the cavity ; these are dis- 

 tributed in the periphery of the rudiment of the vitreous body, so 

 that they must have a large share in the nutrition and growth of 

 this structure. The lens, also, of the Mammaha, is invested by a 

 vascular capsule of connective tissue, which disappears again before 

 birth ; in some, however, it does not disappear so early. 



MuLLEE,^ W., Die Stammesentwickelung des Auges der Wirbelthiere. Leipzig, 

 18V5. — Kesslee, L., Zur Entwickelung des Auges der Wirbelthiere. 

 Leipzig, 1877. 



§ 398. 



As to the form of the bulb, its anterior segment is much flat- 

 tened in Fishes (Fig. 296). The aquatic Amphibia have the bulb 

 flattened anteriorly ; the Ophidii and Crocodilini, among the Eeptilia, 

 are characterised by a more considerable curvature of the cornea. 



In most Birds (Fig. 298) the bulb is divided into an anterior and 

 a posterior segment; the former carries the very convex cornea. 

 This form of eye is most marked in the Raptores, but the cornea is 

 flattened in the Natatores and Grallatores. Among Mammals, also, 

 the spherical form of bulb may undergo great variation in form . 



The Sclerotic may be formed of various kinds of connective 

 substances ; in fact, it may be made up of connective tissue, of bony 

 parts, or of cartilage. This latter is found in the Selachii, Chimseree, 



Fig. 296. Eye of Esox 

 lucius. Horizontal sec- 

 tion, c Cornea, p Pro- 

 cessus f alciformis. s' s' Os- 

 sifications in the sclerotic. 



Fig. 297. Eye of Moni- 

 tor. Horizontal section. 

 c Cornea. p Processus 

 falciformis. 



Fig. 298. Eye of Falco 



chrysaetos. Horizontal 



section, p Pecten (after 



W. Sommering). 



and Ganoidei, and also in the Amphibia. These arrangements are 

 the most varied in the Osseous Fishes. 



2 M 



