160 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



Chiasm. — There is great diversity in the mode of 

 crossing of the optic nerves. In fishes they cross bod- 

 ily (Fig. 99, a), or else one pierces 

 the other (/>). In reptiles they form 

 a kind of basket-work (c and d). 

 Thus far there is a complete cross- 

 ing of fibers in a more or less com- 

 plex way. Each side of the brain 

 controls the opposite eye. But in 

 birds, and especially in mammals, 

 half of the fibers cross and half do 

 not (i?), as shown more fully in Fig. 

 93, page 148. By this arrange- 

 ment each side of the brain sup- 

 plies both eyes, and each eye is 

 controlled by both sides of the 

 brain ; and therefore the two eyes 

 co-operate as one instrument. This 

 arrangement is necessary to bin- 

 ocular vision. This, therefore, is 

 the only true chiasm. It is prob- 

 able, therefore, that no animals 

 below birds have binocular vi- 

 sion. This is confirmed by the po- 

 sition of the eyes, which is our 

 next point. 



Position of the Eyes. — The position of the axes of 

 the eyes has an evident relation to binocular vision. In 

 man the two eyes are directly in front, with the axes 

 parallel in a passive state. From this state of parallelism 

 they may be easily converged on a near object. They 

 are therefore in the best possible position for binocular 

 vision. The same is true, and perhaps in equal degree, 

 in apes. But below this the eyes are wider and wider 

 apart, and set more and more on the side of the head. 



Fig. 99. — Different modes 

 of crossing of the optic 

 nerves : a and b, fishes ; 

 c and d, reptiles ; e, 

 mammals. 



