FUNCTIONAL DIVISIONS OF THE NERVOUS SYSTEM. 



97 



of the animal in its surroundings are arranged on a common plan 

 in all segments of the body in all vertebrates, and constitute a 

 distinct portion of the nervous system both structurally and func- 

 tionally. These structures are best called collectively the somatic 

 afferent division of the nervous system. 



The stimuli having reference to food and the visceral activities 

 include those arising in the viscera and those produced by chem- 

 ical changes in the surrounding medium. Chemical changes 

 in the surroundings do not stimulate the somatic sense organs. 

 The special organs of the sense of taste, which in fishes lie in 



R. dors 



R. ven 



Fig. 45. — A diagram of the component elements in the spinal cord and the nerve 

 roots in a trunk segment, to illustrate the four functional divisions of the nervous 

 system, s. s., somatic sensory; v. s., visceral sensory; v. m , visceral motor; s. m., 

 somatic motor. 



the skin as well as in the branchial and mouth cavities, and the 

 olfactory organs are affected by chemical stimuli and are used 

 in finding food. All the nervous structures concerned with 

 impulses arising in the viscera, in the taste organs and in the 

 olfactory organ are closely related and constitute the visceral 

 afferent division of the nervous system. 



The movements of the soma are aroused chiefly by somatic 

 afferent impulses and have to do with the relation of the animal 

 to its surroundings. Thus, all the usual movements of locomotion, 

 of offense and defense, and so forth, are directed ordinarily in 

 response to stimuli from without. Somatic movements are also 

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