208 



COMPARATIVE ANATOMY 



fore-brain known as the primary optic vesicle. It, therefore, Uke the 



olfactory lobe, represents a part of the brain. 



In the adult brain, the optic nerve is seen to arise from the 



thalamencephalon, and three more or less sharply-differentiated 



portions of it may in most cases be distinguished ; these are 

 spoken of, from the proximal to the distal end 

 respectively, as the optic tract, chiasma, and 



The chiasma, that is, the crossing of the 

 two optic nerves, is always present, though 

 not always freely exposed, for it may retain 

 a primitive position deeply embedded in the 

 base of the brain, (e.g., Cyclostomi, Dipnoi). 



In most Teleosts the optic nerves simply 

 overlie one another (Fig. 166, a), but in some 

 of these Fishes (Clupea, Engraulis, Fig. 

 166, b), one nerve passes through a slit in the 

 other, and this condition of things is gradu- 

 ally carried still further in Reptiles, until 

 finally the fibres of the two nerves intercross 

 in a very complicated manner (Fig. 166, C, D), 

 giving rise to a sort of basket-work ; this is 

 finest and most delicate in Mammals, where 

 its structure can only be analysed by compar- 

 ing a series of sections. A more or less 

 complete crossing of the fibres of each optic 

 nerve may also take place more peripherally 

 before they spread out in the retina. 



In contrast to the eyes of Invertebrates, 

 which arise by a differentiation of the cells 

 of the superficial epiblast, the sensitive ele- 

 ments of the Vertebrate eye correspond to a 

 peripheral portion of the brain (Figs. 167, A 

 and b). 



As the primary optic vesicle otows out- 

 wards towards the outer skin of the embryo, 

 the portion which connects it with the brain 

 becomes constricted and by degrees loses its 

 cavity, giving rise to a solid cord, from which 

 the optic nerve is formed. 



At the point where the vesicle touches 

 the epiblast, the latter becomes thickened and the outer 

 wall of the vesicle invaginated to form a double-walled cup, 

 the secondary optic vesicle (Fig. 167, b). The inner and 

 outer walls of the cup then become fused together, the 

 former giving rise to the serisory epithelium of the retina, and 

 the latter to the piigment epithelium. The fibres of the optic 

 nerve are first differentiated in its retinal portion, and grow 



Pig. 166. — Chiasm.iof 

 THE Optic Nekves. 

 ( yemidiagrammatic . ) 



A, chiasma charac- 

 teristic of the greater 

 number of Teleostei ; 



B, Herring; C,Lacer- 

 ta agilis ; I), an Ag- 

 ama ; E, a higher 

 Mammal. 



Ohi, chiasma of the 

 bundle of nerves ly- 

 ing centrally ; Ce, 

 Ce\ 8, S\ lateral 

 fibres; Co, commis- 

 sure. 



