THE THALAMUS 131 



behavior to exteroceptive (optic, auditory, somesthet- 

 ic) stimuli here he in the midbrain. 



The forms of forebrain exhibited by various groups 

 of fishes are exceedingly diverse and most of these 

 occupy terminal branches of a widely spread gene- 

 alogical tree; they do not directly lead up to any still 

 higher vertebrate types. Into the details of the rami- 

 fications of this ancient phylum we need not enter 

 here.^ 



As we pass from fishes to higher animals the thala- 

 mic centers are enlarged and complicated and the side 

 walls of the primitive endbrain evaginate or buckle 

 outward to form the cerebral hemispheres which un- 

 dergo parallel enlargement and differentiation. The 

 more complex life of land animals demands better ma- 

 chinery of exteroceptive adjustment than can be ac- 

 commodated in the midbrain. 



In all of these cases the configuration and internal 

 structure of the forebrain have been molded to a large 

 extent by the interaction within this territory of de- 

 scending olfactory and ascending non-olfactory sys- 

 tems of nervous conduction and the elaboration of the 

 nervous apparatus of correlation among these diverse 

 functional systems in the fabrication of the behavior 

 patterns characteristic of the different species. 



Broadly speaking, in lower vertebrates fibers from 



' The phylogenetic relationships of fishes and of their forebrains in 

 particular have recently been discussed by several authors, e.g., Holm- 

 gren (1922), Johnston (1923), Herrick (1921, 1922^, 1924). 



