EVOLUTION AND SIGNIFICANCE OF CEREBRAL CORTEX 311 



pyramidal tract also receives collaterals from the peripheral sensory neurons 

 of its own segment (Fig. 137) . This arrangement is the explanation of the 

 fact that the pyramidal tract fibers descend through the human spinal 

 cord for the most part in the dorso-lateral region, not in the ventral funic- 

 ulus like most other motor tracts. In most lower mammals the pyramidal 

 tract actually descends within the dorsal funiculus in the closest possible 

 association with the peripheral sensory fibers, and this arrangement is 

 clearly the primitive relation of the descending cortical pathway. 



Accordingly, stimulation of the skin of the body excites a dorsal spinal 

 root fiber which ascends toward the cortex within the spinal cord and also 

 gives collateral branches to intercalary neurons of the spinal cord itself. 

 The latter neurons may excite motor elements of the spinal cord to an im- 

 mediate reflex response which is well under way before the cortical return 

 motor impulse gets back to the spinal cord and discharges into these same 

 intercalary neurons which are already under sensory stimulation directly 

 from the periphery. The effect of this arrangement is that the central 

 motor path during function is under the influence of sensory stimulation at 

 both ends, and is not, as commonly described, under simple sensory stimula- 

 tion at the cortical end and purely emissive in function at the spinal end. 



Viewed from the standpoint of cerebral dynamics, the exact physiological 

 effect of the discharge of a central motor bundle such as the pyramidal tract 

 will be dependent upon the combined action of the sensory stimulation at 

 the cortical end and the state of sensory excitation at the spinal end, as well 

 as upon the resistance of the motor apparatus itself. 



We saw in a previous paragraph how the simple reflexes of the spinal 

 cord may become factors in the stimulus complex of the cortex. Here we 

 find, conversely, that the efferent cortical discharge may become a factor in 

 the local reflex stimulation of a motor spinal neuron. Fropi both stand- 

 points Dewey's conception of the unitary nature of the organic circuit, as 

 contrasted with the classical reflex arc concept, receives strong support. 



The thalamic correlation centers probably serve as the organs par excel- 

 lence where are elaborated those organic circuits which give to the higher 

 apperceptive processes of the cortex that quale to which Dewey refers. 

 The origin of this quale is to be sought partly in the subcortical assimilation 

 of a present stimulus complex to the pre-existing organic circuits structur- 

 ally laid down in the reflex mechanism, and partly in an affective quality 

 pertaining to the several organic circuits involved in the reaction. This 

 affective quality may be innate or it may have been acquired by experience 

 of the results of previous reactions of the sort in question. 



Head and Holmes have brought forward some very interesting evidence 

 that not only the affective quale of sensations but also the emotional life 

 in general is functionally related to the primitive intrinsic nuclei of the 

 thalamus, rather than to cortical activity (see p. 253). And certainly 

 there is much evidence in the behavior of lower animals, especially birds, 

 that a high degree of emotional activity is possible where the basal centers 

 are highly elaborated but the cerebral cortex is small and very simply organ- 

 ized. 



From all of these considerations it seems probable that the functions of 

 the higher association centers of the cerebral cortex do not consist of the 

 elaboration of crude sensory data or of any similar elements, but rather of 

 the assembling and integration of highly elaborated subcortical organic cir- 

 cuits which in the aggregate make up the greater part of the reflex and in- 

 stinctive life of the species. 



