720 Comparative Morphology of Chordates 



in conjunction with the cerebral cortex, and perhaps the same should 

 be said of some higher centers in the thalami. Therefore the neenceph- 

 alon may be conceived as consisting of not only the cortex-bearing 

 pallium but also the cerebellar and thalamic structures which are 

 most directly concerned with it. Even the ventral pons, which consists 

 mainly of fiber-tracts extending between cerebral and cerebellar 

 cortices, can be regarded as "new." In fact, between "old" and "new" 

 brains no sharp demarcation can be recognized, either structurally or 

 functionally. 



The lower parts of the brain — i.e., the regions exclusive of 

 pallium and cerebellum — appear as a more or less modified forward 

 continuation of the spinal cord, and consisting, within themselves, 

 mainly of reflex mechanisms, they resemble the cord functionally. 

 They are often referred to as constituting collectively the "brain- 

 stem" (Fig. 528), while the nervous structures of the pallium and the 

 cerebellum, superimposed upon the brain-stem, constitute a "super- 

 brain." The brain has a certain transverse segmentation of its own, 

 and the cranial nerves, exclusive of I and II, have such similarities 

 to the segmentally arranged spinal nerves as to make it possible to 

 interpret them as "modified spinal nerves." Therefore the brain-stern 

 is sometimes called the "segmental brain" and the pallial structures 

 and cerebellum are then "suprasegmental" structures. 



Reptiles, in relation to birds and mammals, stand — or once stood — 

 at the parting of the ways in more respects than one. Birds adopted 

 aerial life; mammals remained primarily and mostly terrestrial. Their 

 common reptilian ancestors must have made at least a good beginning 

 at elaborating a pallial cortex. Birds did not, to any important extent 

 if at all, go on with it. Instead, they developed the mechanism of the 

 palaeencephalon with the corpora striata as the especially im- 

 portant seat of complex nervous centers of the "old" type. Elaborate 

 as this mechanism came to be, it acquired nothing of the flexibility 

 which characterizes the action of a pallial cortex. The cerebellum is 

 highly developed, as it must be to coordinate the highly complex 

 muscular activities of the bird, but the cerebellum, both in bird and 

 mammal, is as rigidly automatic as any simple reflex center. Therefore 

 birds have only a relatively low degree of modifiabiiity of behavior. 

 Their lives, dominated by instinct, resemble those of fishes and insects. 

 Reverting to the f actor y-and-office analogy, in the case of birds it is an 

 old and established business which has settled down into deeply worn 

 ruts and is carried on automatically by subordinates supervised by 

 habit-bound executives, with little constructive and progressive direc- 

 tion by any higher officials and with small capacity for meeting emer- 



