THE LIVING BIRD 21 



switch-board, the pons varolii (pontal fibres exist in 

 birds, but they are relatively few), giving it added 

 importance but also transferring its previously 

 autocratic and automatic control to the more or less 

 voluntary hemispheres. 



We are now in a position to understand the brain 

 of a bird. Once again, we are constantly reminded 

 of the reptile, not in its macroscopic appearance, 

 but in its constitution. The first thing that strikes 

 us when looking at the exposed brain (Fig. 2) is 

 the size of the cerebral hemispheres. We might 

 readily conclude that here we have a brain that 

 compares favorably with the mammalian, but the 

 illusion is dissipated when we examine its make-up. 

 In place of a thick cortex, the roof is thin and the 

 cortical cells are but meagrely represented, (they 

 are even scarcer than in some reptile brains) the 

 substance of the hemisphere consisting of an enor- 

 mously developed corpus striatum, a much older 

 part of the brain than the cortex. There is no 

 corpus callosum to unify the two halves : the pons 

 varolii is poorly represented: the pyramids are en- 

 tirely wanting. The corpora striata, large aggrega- 

 tions of nerve cells, differ not only in structure from 

 the cortex, but likewise in function. They are not 

 much more than enlarged, elaborated and somewhat 

 improved editions of the old primitive brain -stem 



