ORIGIN AND SIGNIFICANCE OF THE CEREBRAL CORTEX 233 



In the organic circuit as defined by Dewey the process is 

 considered as a whole so that the response is conceived as logi- 

 cally implicit in the stimulus. The motor reaction, he says; is 

 not merely to the stimulus ; it is into the stimulus. " It occurs 

 to change the sound, to get rid of it." 'What we have is a 

 circuit, not an arc, or broken segment of a circle. This circuit 

 is more truly termed organic than reflex, because the motor 

 response determines the stimulus, just as truly as sensory stim- 

 ulus determines movement." This notion, which is difficult for 

 the practical scientific mind to understand, is considerably clari- 

 fied by some neurological considerations. 



From the standpoint of the cerebral cortex considered as 

 an essential part of the mechanism of higher conscious acts, 

 every afferent stimulus, as we have seen, is to some extent 

 affected by its passage through various subcortical association 

 centers (i.e., it carries a quale of central origin). But this same 

 afferent impulse in its passage through the spinal cord and brain 

 stem may, before reaching the cortex, discharge collateral im- 

 pulses into the lower centers of reflex coordination, from which 

 incipient (or even actually consummated) motor responses are 

 discharged previous to the cortical reaction. These motor dis- 

 charges may, through the "back-stroke" action, in turn exert 

 an influence upon the slower cortical reaction. Thus the lower 

 reflex response may in a literal physiological sense act into the 

 cortical stimulus complex and become an integral part of it. 



But there is another aspect of the problem which has recently 

 been brought to our notice by Kappers. 4 It is a well known 

 fact, which is not often taken account of in this connection, that 

 the descending cortical paths (central motor bund es) do not 

 typically end directly upon the peripheral motor neurones whose 

 functions they excite, but rather upon intercalary neurones 

 which lie in the reticular formation or even in the adjacent 

 sensory centers. These intercalary neurones in turn excite the 

 peripheral motor neurones. The same intercalary neurone which 

 receives the terminals of the pyramidal tract also receives collat- 

 erals from the peripheral sensory neurones of its own segment 

 (Fig. 4). This arrangement is the explanation of the fact that 



* Kappers, C. U. Ariens. Ueber die Bildung von Faserverbindungen auf Grund 

 von simultanen und sukzessiven Reizen. Bericht iiber den III. Kongress fur ex- 

 perimentelle Psychologie in Frankfurt a. Main, 1908. Also, Weitere Mitteilungen 

 iiber Neurobiotaxis. Folia Neuro-Biologica, Bd. I, No. 4, April, 1908, pp. 507-532. 



