88 THE BRAIN OF THE TIGER SALAMANDER 



pattern in terms of sensory correlation superficially and of motor 

 analysis in the deeper layers. The cerebral cortex of mammals also 

 exhibits both mosaic and laminated patterns of localization and in 

 far more complex designs (Huber and Crosby, '33, '34). 



Specific structure of this analytic type, with well-defined localiza- 

 tion in both gray and white substance, increases in amount as we 

 pass from lower to higher animals in the phyletic series; and this 

 increment progresses from the sensory and motor periphery inward 

 toward the upper cerebral levels, where the apparatus of integration 

 and synthesis is most elaborately developed. In submammalian 

 brains the amount of myelin present at successive levels of the brain 

 stem is a rough indicator of the relative mass of tissue of this ana- 

 lytic type. 



GENERALIZED STRUCTURE 



This type of tissue, as pointed out above, predominates in the most 

 primitive vertebrate brains. With advancing differentiation, we ob- 

 serve specialization of this tissue in three directions. (1) Some of it 

 retains its primitive generalized structure with little change. In uro- 

 deles this is true of a large proportion of it; in mammals it survives 

 in the periventricular system of cells and fibers of the diencephalon 

 and mesencephalon and in some other regions. (2) A progressively 

 larger proportion of this tissue is transformed into specifically local- 

 ized structures, as just described. (3) Another large proportion of it 

 is transformed into the relatively unspecialized tissues of the inter- 

 mediate zone of correlation and its highly elaborated derivatives in 

 the suprasegmental apparatus of the cerebellar and cerebral cortex. 



Doubtless all parts of the body participate in the total integration 

 and the determination of general attitudes and types of response, 

 but the brain exercises dominant control over overt behavior and 

 orders it in the interest of the welfare of the body as a whole. The ap- 

 paratus of these totalizing functions evidently includes many diverse 

 components, of which one of the most obvious is the neuropil, which 

 in primitive vertebrates pervades the entire brain, so that activity in 

 any part of it may affect the whole fabric, as elsewhere described. 

 This dispersed tissue 's not homogeneous, and it is not equipotential. 

 It is doubtless always active and in diverse ways in different places 

 at different times. Such localization of function as it exhibits can best 

 be conceived in dynamic terms, that is, in terms of what intercur- 

 rent nervous volleys act upon it in momentarily changing places, 

 rhythms, and intensities. We are dealing here with an equilibrated 

 dynamic system comprising many activated fields in interaction, and 



