262 THE ORIGIN OF THE NERVOUS SYSTEM 



adjuster. This, however, does not mean that it is physio- 

 logically dominant in the more or less autocratic manner 

 in which the head or the apical end of the physiological 

 gradient is dominant. The cortex resembles rather a 

 deliberative assembly to which reports of matters 

 requiring consideration come in from the various con- 

 stituent groups or bureaus and in which they are 

 considered and action finally taken through the proper 

 channels (cf. Herrick, 191 3, p. 230). Not every afferent 

 impulse reaches the cortex. In many, and particularly 

 postcephalic arcs, as long as the reflex mechanism of 

 lower centers gives satisfactory results the cortex does 

 not become involved ; only when the stimulus continues 

 and summation, or some other effect, makes it possible 

 is the impulse able to reach the cortex. 

 As Herrick has so clearly pointed out: 



There are no afferent tracts leading to the cerebral cortex directly 

 from any peripheral sense organ or from any center within the 

 brain which is "pure," i.e., devoted to a single sensory function. 

 In other words, no simple sensory impulses ordinarily reach the 

 cortex, but only nervous impulses arising from lower correlation 

 centers, where complex reflex combinations of various sensory 

 systems are possible. The optic impulses reach the cortex most 

 nearly pure, i.e., with less subcortical associational relation with 

 other sensory systems (it is no accident that the visual sense 

 plays a dominant role in human cortical function) ; but even here 

 the optic centers in the thalamus from which the optic projection 

 fibres arise are intimately related with acoustic, tactile and other 

 important sensory centers. And in the case of all the other 

 sensory systems, the projection fibers which enter the cortex 

 come from centers which are separated from their respective sense 

 organs by two or more association centers of a high order of com- 

 plexity. Each of these subcortical associational centers may be 

 dominated physiologically by a single sensory system, but it is 

 structurally adapted for bringing that system into relation with 



