456 TEXT-BOOK OF PHYSIOLOGY 



The thermo-accelerator and thermo-inhibitor centers it is assumed are 

 both connected with the cutaneous surface through the intermediation of 

 afferent nerves and, therefore, in a position to be influenced by changes in 

 the external temperature. Evidence is also at hand that they may be in- 

 fluenced by changes in the temperature of the blood passing to and around 

 them. A fall of the external temperature develops nerve impulses which 

 transmitted to the thermo-accelerator center excite it to action and thus 

 increase heat-production; a rise in external temperature, on the other hand, 

 has an opposite effect by stimulation of the inhibitor center. 



The regulation and maintenance of the mean temperature or thermotaxis 

 is, therefore, the result of an adjustment between heat-production and heat- 

 dissipation, both of which are variable factors, and is accomplished by a 

 complex, self-regulating mechanism consisting of muscle, vascular and secretor 

 elements coordinated by the nerve system. Given this mechanism and the 

 variations in the external temperature the metabolizing power of the living 

 material is caused to vary in one direction or another as the external tem- 

 perature rises and falls. 



