156 Basic Structure of Vertebrates 



Somatic and visceral activities differ in their environmental re- 

 lations. Somatic activity is, for the most part, directly concerned with 

 the external environment ; visceral activities, with the internal environ- 

 ment. The outer and inner tubes are vitally related to each other. The 

 food which the visceral tube digests must be supplied by somatic ac- 

 tivity, and the body-wall is wholly dependent on the visceral organs. 

 But in the business of getting the food, the somatic activity is directly 

 concerned with the external situation, while the visceral activities in- 

 volved in digesting the food and distributing its nutriment are wholly 

 directed toward maintaining a normal internal economy. 



A most important difference between somatic and visceral activities 

 involves an intangible and undefinable factor which can be stated only 

 in terms of human experience. Actions resulting from contraction of 

 striated somatic muscles are, with few exceptions, voluntary or 

 potentially voluntary. Visceral activities are, with rare exceptions, 

 involuntary and commonly do not rise into consciousness at all. The 

 " I " that wills to do things cannot make himself sweat by willing to do 

 so. But he can will to run or to saw wood and may thus produce in- 

 creased output of perspiration. To assert that somatic activities are 

 voluntary in all vertebrates involves some assumptions. We can only 

 say that there are such similarities in structure and behavior as would 

 seem to justify the assumption that all vertebrates, even fishes, possess 

 in some degree the capacity for voluntary action. In such vertebrates 

 as fishes and salamanders, the range of voluntary action is, no doubt, 

 narrowly limited, and the volition may not rise far above the level of 

 instinctive action. 



To summarize: The activities of the vertebrate body are of two 

 types, which may be designated as "somatic" and "visceral," each 

 name referring to the division of the body of which the type is espe- 

 cially, but not exclusively, characteristic. 



The primary somatic function is movement, produced by con- 

 traction of muscles which are (at least primitively) usually segmented 

 and always constituted of striated fibers. 



The primary or essential function of a visceral organ is com- 

 monly some such process as secretion, excretion, absorption, or 

 diffusion. In a lymph-node or gonad, it is proliferation of cells. In a 

 blood-vessel or ureter, it is mere passive conduction of a fluid. Mus- 

 cular contraction plays a secondary or accessory part in visceral activi- 

 ties, and the muscles, if present, are usually nonstriated and are 

 always developed from unsegmented mesoderm. 



Somatic muscles are relatively massive, powerful, and quick-act- 

 ing. Visceral muscles commonly occur in thin sheets and are rela- 

 tively weak and slow in contracting. 



