Integrative Systems 157 



The primary somatic function is performance of gross mechani- 

 cal work. It is mass action or molar activity, whereas characteristic 

 primary visceral function consists in molecular activity, chemical 

 change, or some nonmotor cellular activity. 



Somatic activity is, for the most part, immediately related to 

 the external environment; visceral activity, to the internal 

 environment. 



Contraction of somatic striated muscle is essentially voluntary; 

 visceral activities, including contraction of the usually nonstriated 

 muscles which may be concerned in them, are involuntary. 



The body-wall is primarily "somatic" in the nature of its function, 

 but its blood-vessels and glands perform various essential accessory 

 functions which are of the visceral type. The coelomic visceral terri- 

 tory is not invaded by anything typically somatic. 



Peripheral Nervous Organs 



functional classes of nerves 



In comparative descriptions of nerves and their associated receptors 

 and effectors, the more important characteristics which are usually 

 considered are the following: position in the animal; intrinsic struc- 

 ture (segmentation and striation of muscles) ; relation to central 

 nervous parts; nature of function — whether of the "somatic " or the 

 "visceral" type, in the sense of the foregoing discussion; embryonic 

 origin; and evolutionary origin. There is no complete correlation 

 among these several characteristics. Certain nerves may possess some 

 of them in common and differ as to the others. In the attempt to 

 classify nerves and their related structures, it is the general practice to 

 give most weight to their functional nature (recognition of "func- 

 tional systems"). Mere position in body is considered least impor- 

 tant. Embryonic origin is likely to be regarded as more important 

 than definitive intrinsic structure, and relations to central nervous 

 parts more important than peripheral connections. 



Application of these principles results in recognition of four main 

 classes of nerves. In the following table are set forth the characteristics 

 of a typical nerve of each class. Certain of the data given refer specifi- 

 cally to a spinal nerve, but the cranial nerves, except the olfac- 

 tory and optic, may be resolved into components whose characteristics 

 are essentially like those of corresponding parts of a spinal nerve. In 

 the brain (the medulla) the centers related to the roots of the nerves 

 are located at four dorsoventral levels as they are in the cord. Named 

 in order from above downward, they are somatic sensory, visceral 

 sensory, visceral motor, and somatic motor (Fig. 145). It is signifi- 

 cant to note that, in this arrangement of centers, the brain and cord 



