REGIONAL ANALYSIS 41 



other hand, except for the olfactory component, is a hiter acquisition. 

 This is suggested by what is seen in Amphioxus and by the retai-ded 

 development of the cerebrum in all vertebrate embryos, as illustrated 

 especially clearly in the early fetal development of the opossum. 



At an early (and unknown) period of vertebrate ancestry a pair of 

 eyes was differentiated. These and the olfactory organs are the lead- 

 ing distance receptors, and as such they gave to the vertebrate 

 ancestors more information about their surroundings and hence 

 greater safety in moving about freely. The nose and eyes, with the 

 associated oculomotor apparatus, early assumed the dominant role 

 in the recognition of food, mates, and enemies, and their cerebral 

 adjustors were enlarged accordingly. The contact receptors are ade- 

 quate for sedentary, crawling, or burrowing ancestors, and here the 

 response to stimulation follows immediately. But, as Sherrington 

 long ago pointed out, in a free-swimming animal there is a time lag 

 between reception of the stimulus from a distant object and the con- 

 summation of the response. The pregnant interval between the antic- 

 ipatory and consummatory phases of the reaction gives the clue to an 

 understanding of the entire history of forebrain evolution. During 

 this interval there is a central resolution of forces, which eventuates 

 in appropriate behavior; and, with increasing complication of pat- 

 terns of behavior, this central apparatus of adjustment assumes more 

 and more structural complexity and physiological dominance over 

 the entire bodily economy (chap. vii). 



The details of these internal connections are not relevant here. It 

 suffices to present two summaries, one in this chapter in topographic 

 arrangement by regions from spinal cord to olfactory bulbs as con- 

 ventionally described and one in the next chapter on a different plan, 

 i.e., an arrangement in longitudinal zones which are functionally de- 

 fined. For the present purpose it is convenient to recognize seventeen 

 subdivisions of the central nervous system, each of which is char- 

 acterized by special physiological activities, though these activities 

 are not localized here exclusively. This subdivision might be carried 

 further into detail indefinitely. Numbers 2-6 in the following para- 

 graphs are in the rhombic brain; the others are in the cerebrum. 



THE SUBDIVISIONS, SPINAL CORD TO PALLIUM 

 1. THE SPINAL CORD 



The spinal cord is not described in this report except for some fea- 

 tures closely related to the brain, to which reference is made in the 

 next paragraph. The cord segments are organized for the regulation 



