58 THE BRAIN OF THE TIGER SALAMANDER 



a number of diagrams, some in this contribution and some in previous 

 papers. 



This analysis of the more obvious structural features of an am- 

 phibian brain in terms of physiological criteria is an artificial sche- 

 matization of a complicated fabric, the several parts of which are so 

 intimately connected that there is an over-all integration of their 

 activities. The main highways of through traffic have been mapped, 

 with signboards pointing the way at the crossroads. Selected ex- 

 amples of some of the lines of fore-and-aft through traffic are illus- 

 trated in the diagrams ; but this kind of analysis does not take us to 

 our destination. It does not tell us how mixed traffic is actually 

 sorted out and so reorganized as to give the body as a whole efficient 

 adjustment to the momentarily changing exigencies of common ex- 

 perience. These problems are discussed in subsequent chapters, but, 

 first, the schematic outline will be summarized here. 



Each zone is structurally diversified, and many of these local dif- 

 ferentiations are directly correlated on the sensory side with the 

 modalities of sense represented in the end-organs with which they 

 are connected and on the motor side in a similar way with synergic 

 systems of muscles. Each sensory and motor system of nerves has a 

 local primary central station in direct connection with the periphery, 

 and each of these stations has widely spread connections within its 

 own zone and with other zones, thus insuring efficient correlation of 

 sensory data, co-ordination of motor responses, and integration of 

 the action system as a whole. In this summary the sensory systems 

 are given special attention because these are the most useful guides 

 in the analysis of the structure of this brain. 



THE SENSORY ZONE 



The sensory zone is defined as those parts of the brain that receive 

 fibers from peripheral organs of sense. In some fields the number of 

 such fibers is large, in others it is very small; and some parts of the 

 brain, like the cerebral peduncle, have both sensory and motor 

 peripheral connections. Within the sensory zone there is a compli- 

 cated apparatus of correlation, and in lower vertebrates the receptive 

 areas and surrounding tissues dominated by them are so large that 

 most of the mass of the brain can be blocked into fields appropriately 

 designated "nosebrain," "eyebrain," and so on ('31a, p. 129). This 

 feature is due to the fact that in these animals the sense organs are 

 well developed and highly specialized, but the motor apparatus is 



