140 



ANATOMY OP THE CENTRAL NERVOUS SYSTEM. 



majority of mammals the decussation is only partial. That probably de- 

 pends upon the position of the eyes which, in the lateral position, as in 



Fig. 88. — Sagittal section some distance from the median line of the brain 

 of Taranus sriseus, showing the course of the Deeussatio transversa from the 

 chiasma to its terminus. 



fishes, reptiles, etc., have no part of the field of vision in common, while, 

 in the position found in the owls, the ape, and man, the visual field is quite 



T93tiiui mesencephal, 



ecussEition of 

 lated stratum 



/ DecussEition of the deep medul- 



Comm. post. 



-y « Tr. strio-thalam. 



Decuss. transversa 



Fig. 89. — ^Frontal section through the midbrain of Scyllium canioula. 



separate only in the lateral portions, the images of the median portion being 

 coincident. So it is explained why careful investigations on the chiasma 



