THE NERVOUS SYSTEM OF MAN l6l 



system to environmental changes barriers of 

 which we are unconscious have been raised. There 

 are entirely different barriers to freedom of action 

 in the hereditary features of the nervous system 

 that direct attention to an entirely different set of 

 limitations. These latter have to do particularly 

 with establishing the routes over which the several 

 classes of stimuli must travel. 



The sense organs and nerves just described re- 

 late man to his external environment and such 

 grouping of the nervous system leads to the ex- 

 pression so common nowadays, the somatic sen- 

 sory and motor divisions. But this is to overlook 

 what was for a long time denominated the sym- 

 pathetic or autonomic part of the nervous system. 

 In this grouping are to be found the nerves that 

 control muscular reactions in the visceral organs 

 and the receptors that carry stimuli from these 

 same organs to the spinal cord and brain. The 

 most recent term for this division is the visceral 

 sensory and motor nerves. 



When one attempts to make a clear classifica- 

 tion of the stimuli that excite the visceral re- 

 ceptors, one meets with a great dearth of accurate 

 information. It Is probable that pressure, chemi- 

 cal changes in the form of hormones, toxicity from 

 the digestive tract and muscular fatigue are 

 among the more important of the visceral sensory 

 stimuli. Just what the stimulus Is that excites the 



