THE VISUAL APPARATUS. 



147 



of the optic tracts giving off thick collaterals which enter the 

 thalamus of the same side. The number of uncrossed fibers in 

 any animal seems to depend upon the position of the eye. When 

 the eyes are laterally placed as in most submammalian orders the 

 uncrossed fibers are at a minimum. Among mammals, those 

 whose eyes are lateral, so that the fields of vision do not overlap, 

 have few uncrossed fibers; while those (apes, man) whose fields 

 of vision largely overlap have a large part of the optic tracts un- 

 crossed. The uncrossed fibers in man arise chiefly from the tem- 

 poral part of the retina and run in a fairly well isolated bundle in 

 the optic tracts and chiasma. Beyond the chiasma the optic tracts 



r. opticus 



Nuc. magn. tecti' 



Decussatio dorsalis 



FIG. 73. Part of a section of the tectum opticum of the sturgeon, schematic. 



run up in the lateral part of the thalamus and end in the dorsal 

 part of the thalamus and in the tectum opticum. 



STRUCTURE OF THE TECTUM OPTICUM. In the lower fishes the 

 tectum contains a large number of cells of several forms, most 

 of which lie near the ventricle. The outer portion of the tectum 

 is composed of fiber layers. Cells of both type I and type II are 

 present. The cells of type II (Figs. 39 and 73) are slender 

 spindle-shaped elements vertically placed near the ventricle, 

 whose single thick dendrites rise toward the surface and branch 

 profusely in the outer layers of the tectum. The neurite arises 

 from some point of this dendrite far removed from the cell-body 



