536 Comparative Morphology of Chordates 



cerebellum. A complex system of nervous pathways connects the 

 cerebellum with these dominating centers. The conspicuously great 

 size of the bird's cerebellum is therefore referable to several factors — 

 the richness of the incoming stream of impulses having origin in the 

 highly developed sense-organs; the vast number of neurons entering 

 the cerebellum along the pathways connecting it with the higher 

 dominating centers; the number, also "vast," of intrinsic neurons 

 required for the correlation of the diverse incoming impulses and their 

 translation into a set of outgoing impulses appropriate for the effecting 

 of a precisely coordinated set of muscular responses. 



It would not be of advantage to an animal that its activities should 

 at any time be wholly dominated by a single sense-organ. Proper adjust- 

 ment to the external situation requires that all of the animal's senses 

 shall participate in the determination of behavior. This favorable 

 arrangement is achieved by the existence of numerous correlation 

 centers which intervene between the primary sensory centers and those 

 ultimate motor centers which send out impulses causing muscular con- 

 traction or other responses. A correlation center is a localized mass of 

 "gray substance" having nervous connection with two or more pri- 

 mary centers, with other correlation centers, and, directly or indirectly, 

 with motor centers. Such correlation centers exist in all of the main 

 regions of the brain. They are least well developed in fishes, better 

 developed in amphibians and reptiles, and very highly developed in 

 birds. It is the function of the higher correlation centers to "analyze" 

 and "evaluate" (figuratively speaking) sensory stimuli and then to 

 synthesize them into a resultant which shall determine the type of 

 response best adapted to the whole external situation. Reports from 

 different sense-organs may be contradictory. The eyes may report that 

 a certain external object appears to be a desirable morsel of food. The 

 olfactory organs may report that it has a bad odor. Whether or not 

 it is to be eaten is "decided" by correlation centers. 



In birds it is the forebrain in which correlation centers are most 

 numerous and most highly developed. They occur in the lateral walls 

 (thalami) of the diencephalon, but most especially in the cerebral 

 hemispheres, which are accordingly much enlarged. The increase in 

 volume, however, is entirely due to thickening of the ventrolateral 

 wall, the corpus striatum. The dorsal wall, pallium, remains thin 

 and, in fact, relative to the corpus striatum, the pallium is thinner in 

 birds than in reptiles and its superficial gray layer is less strongly 

 developed. 



In the account of the reptilian brain (pp. 479-481) were set forth 

 reasons for regarding the pallium as the particular seat of the nervous 

 mechanisms concerned with behavior which at least approaches the 

 level of intelligence. Behavior controlled solely by the mechanisms of 



