HISTOLOGICAL STRUCTURE 31 



paratus of local reflexes and their organization into the larger, innate 

 patterns of behavior. 



4. The superficial neuropil is a subpial sheet of dendritic and axonal 

 terminals, in some places absent, in others very elaborately organ- 

 ized. Here are some of the most highly specialized mechanisms of 

 correlation in the amphibian brain, from which specific nuclei of 

 higher brains have been developed. Notable examples are seen in the 

 interpeduncular neuropil (chap, xiv) and the ventrolateral neuropil 

 of the cerebral peduncle described in the next section. This neuropil 

 seems to be a more sensitive medium for strictly individual adjust- 

 ments (conditioning) than the deeper neuropil, but of this there is no 

 experimental evidence. This hypothesis is supported by the fact that 

 in higher animals cerebral cortex develops within this layer and 

 apparently by neurobiotactic influence emanating from it. 



In the first synapses observed in embryogenesis numerous axonic 

 terminals converge to activate a single final common path (Coghill, 

 '29, p. 13), This is the first step in the elaboration of neuropil. As 

 differentiation advances, neurons are segregated to serve the several 

 modalities of sense and the several systems of synergic muscles, and 

 these systems are interconnected by central correlating elements. In 

 no case are these connections made by an isolated «hain of neurons in 

 one-to-one contact between receptor and effector. The central ter- 

 minals of afferent fibers from different sense organs are widely spread 

 and intermingled. Dendrites of the correlating cells branch widely in 

 this common receptive field, and the axons of some of them again 

 branch widely in a motor field, thus activating neurons of the several 

 motor systems. This arrangement is perfectly adapted to evoke mass 

 movement of the entire musculature from any kind of sensory stimu- 

 lation, and this is, indeed, the only activity observed in early 

 embryonic stages. 



It is the rare exception rather than the rule for a peripheral sensory 

 fiber to effect functional connection directly with a peripheral motor 

 neuron. One or more correlating elements are interpolated; and, as 

 differentiation advances, the number of these correlating neurons is 

 enormously increased in both sensory and motor zones and also in 

 the intervening intermediate zone. The axons of these intrinsic ele- 

 ments ramify widely in all directions, and, as a rule, they or col- 

 laterals from them interweave to form the closely knit fabric which 

 pervades both gray and white substance everywhere. This is the 



