THE VISUAL APPARATUS. 



149 



ments more prompt than those which are directed by the more 

 complex brain tracts. 



In cyclostomes all the optic tract fibers end in the tectum opticum 



Fig. 74. — A section of the optic lobe of a bird. From Cajal (Textura, etc.). 

 The letters refer to extended descriptions in Cajal's text. 



but in selachians a part of the tract ends in the diencephalon 

 and in all higher forms an increasingly large part of the tract 

 has this ending. The chief nucleus in which the fibers end is 



