HISTOLOGICAL STRUCTURE 39 



visceral nucleus or in the peduncle. The excitatory state of the hypothalamus, in 

 turn, affects all activity in the peduncle. 



This sort of circular activity is everywhere present in these urodele brains, 

 working through the diffuse periventricular and deep neuropil and in many places 

 also through specially differentiated tracts (p. 76). This is the primordial appa- 

 ratus of integration, and in its more differentiated form it is part of the apparatus 

 by which individuated partial patterns of local reflexes are kept in appropriate 

 relationship with the larger total patterns of behavior. 



In a recent survey ('44o) of tlie optic and visceral nervous circuits here under 

 consideration, several different patterns of linkage of the component units were 

 listed. These include direct activation of eye muscles through the basal optic tract, 

 similar activation of the skeletal musculature, indirect activation of either or both 

 of these sj^stems of muscles by visual stimuli through the tectum and thalamus, 

 direct activation of either or both systems of muscles through the visceral-gustatory 

 path, indirect visceral -gustatory action by way of the superior visceral nucleus, 

 with or without intercurrent influence of the tectum upon this nucleus by way of 

 the tecto-bulbar tract, and the variable effects of concurrent discharge of many 

 other systems of fibers into each of the centers of adjustment involved. Since eacli 

 of these patterns of linkage is complex and the structural units themselves are not 

 simple, it is evident that in actual performance the number of ways in which the 

 known units may be combined and recombined is practically unlimited. The simplest 

 possible activity of stimulus-response type involves a central resolution of forces in 

 an equilibrated dynamic system of inconceivable complexity. Oversimplifica- 

 tion of the problem will not hasten its solution. 



The preceding description illustrates the relations of a specific field of specialized 

 neuropil to other parts of the brain with known functions. These visible connections 

 exhibit a structural arrangement which can be interpreted as putative pathways of 

 transmission of the several components of a complex action system, some con- 

 cerned with stable reflex patterns and some with more labile individually ac- 

 quired components. The relations of these two classes of components of the action 

 system to each other and of local units to the integrated whole present the major 

 problems of neurology. 



