PHYSIOLOGICAL INTERPRETATIONS 85 



period which cidminates at metamorphosis; and during this period 

 the texture of the brain undergoes two divergent Hues of differenti- 

 ation of tissue in correlation with the expansion of two types of ac- 

 tivity, the analytic and the synthetic, as described in the pi-eceding 

 section. The structural arrangement of the analytic apparatus is, in 

 its main features, predetermined in the hereditary organization; it is 

 stable and approximately the same in all members of the species. The 

 intervening synthetic and integrating apparatus, on the other hand, 

 is less rigid and is more labile in function. The pattern of its perform- 

 ance will vary from moment to moment in adjustment to every 

 change in sensory and motor activity and every fluctuation of cen- 

 tral excitatory state. But in even the most primitive vertebrates 

 some cross-connections between sensory and motor zones, which are 

 interrupted in the intermediate zone of correlation, are laid down in 

 the stable, heritable structure. These serve the standardized ("in- 

 stinctive") patterns of behavior, and the arrangement of these con- 

 nections is determined more by the motor equipment of the animal 

 than by the sensory equipment (Crosby and Woodburne, '38; Wood- 

 burne, '39). 



From these considerations it follows that the concept of localiza- 

 tion of function must not be formulated in static terms. It is localiza- 

 tion of action, and the spatial pattern of this localization reflects 

 every change in the character of the action. The structural pattern 

 of this localization is more stable at the afferent and efferent end- 

 points of the system, and it becomes less so as we pass inward from 

 these fixed points. The apparatus of standardized behavior like re- 

 flex is more rigidly localized than is that of more labile individually 

 modifiable behavior. 



Two types of structure which have just been contrasted may be 

 characterized as unspecialized or generalized, and locally differenti- 

 ated in specific stable and heritable patterns. The second was prob- 

 ably derived phylogenetically from the first, and in higher animals 

 both of them exhibit progressive differentiation of structure in diver- 

 gent directions. Some examples of these two types of cerebral archi- 

 tecture will next be cited, beginning with the second, which has been 

 investigated in more detail. 



SPECIFIC LOCALIZED STRUCTURE 



An early stage in the evolution of localized conductors of specific 

 sensory systems is illustrated by the connections of the lemniscus 

 systems described in chapter xi. These tracts of Ambly stoma are not 



