EFFERENT PATHS AND REFLEX ARCS 



Even after cerebral control has been entirely eliminated in the dog by de- 

 cerebration, many reflex functions remain, which represent the unguided activity 

 of the lower elements in the motor mechanism ; and we now know that a similar 

 independent reflex activity may occur in the spinal cord of man after total trans- 

 verse lesions (Riddoch, 1917). 



THE GREAT MOTOR PATH 



The great motor path from the cerebral cortex to the skeletal musculature, 

 through which the bodily activities are placed directly under voluntary control, 

 is in man and mammals the dominant factor in the motor mechanism. We 

 have seen that afferent channels from the various exteroceptors reach the cere- 



Fig. 236. Cortical localization upon the lateral aspect of the human cerebral hemisphere. (Starr.) 



bral cortex; and that through the correlation of the olfactory, auditory, visual, 

 tactile, thermal, and painful afferent impulses which pour into it, there is built 

 up within the cortex a representation of the outer world and its constantly chang- 

 ing conditions. The responses appropriate to meet the entire situation in which 

 the individual finds himself from moment to moment are in large part at least 

 initiated in the cerebral cortex and are executed through the motor mechanism. 

 In these responses the great motor path is the dominant factor, although other 

 parts of the mechanism are secondarily called into action, especially the pro- 

 prioceptive reflex arcs, including the coordinating and tonic mechanism of the 

 cerebellum. 



This great motor path consists of two-unit chains. The so-called upper 

 motor neurons conduct impulses from the motor cortex to the motor nuclei of the 



