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Basic Structure of Vertebrates 



springs so that they press upon the part in opposite directions. Their 

 "antagonistic" action conduces to proper working of the machine. 

 The two autonomic divisions are "antagonistic" only in the sense 

 that they cooperate toward the welfare of the animal as a whole by 

 inducing reactions of opposite nature. 



The autonomic system is essentially an action-controlling mech- 

 anism. Its afferent constituents are relatively few and the sensations 

 which they mediate are mostly rather vague and not capable of precise 

 localization. The effectors of the autonomic system are muscle-cells 

 (usually nonstriated, but striated in the heart), secretory cells, and 

 various large integumentary pigment-cells (chromatophores) which 

 are capable of alternately dispersing and concentrating their con- 

 tained pigment-granules so as to make the cells more effective or less 

 so in determining the color of the skin. The "antagonistic" actions of 

 the autonomic system consist in starting and stopping or accelerating 

 and retarding processes; in causing contraction and relaxation of 

 muscles or increase and decrease in quantity of secretion; and concen- 

 tration and dispersing of granules in a chromatophore. It might be 

 expected that, in general, the starting and accelerating of activities 

 might be the function of one autonomic division while the other would 

 attend to the retarding and stopping. But that is not the fact, as may 

 be appreciated by inspection of the following list of some typical 

 autonomic controls. 



