48 HOMOIOTHERMISM 



oxygen to cells. Lack of water also slows down oxidative 

 processes. When about half of an animal's vital limit 

 is exceeded, there is a relative increase in carbon dioxide 

 production (Caldwell, 1925). In land animals which 

 have well developed mechanisms for temperature regula- 

 tion, climate influences the type of regulation which is 

 used and this indirectly influences metabolism. 



Nervous Control of Metabolism. — In all vertebrates 

 the nervous system exerts more or less influence on 

 metabolic processes, and on body temperatures. The 

 ability which nervous domination gives to control tem- 

 perature is, of course, most important for land animals, 

 which live in a thermally variable environment, but it is 

 present also in vertebrates which never leave the water. 

 The diminution of the oxygen supply has little effect on 

 the rate and strength of gill movements of a Necturus. 

 They are controlled reflexly from the brain, and condi- 

 tions in the brain centers are the effective agents (Stew- 

 art, 1923). There are also **heat centers" in a turtle's 

 brain. "The respiratory rhythm is more powerfully 

 affected by variations in the temperature of the cells 

 composing the respiratory centers than by variations in 

 the activity of the general metabolic processes or in the 

 gases inspired." When temperature of the environment 

 is constant, the amount of carbon dioxide in the blood 

 appears to be the chief factor in regulation (Lumsden, 

 1924). In homoiotherms, various factors maintain a 

 constant body temperature — insulation, surface cooling 

 through vaso-dilation and evaporation, increased oxida- 

 tion, etc. The nervous system plays an increasingly im- 

 portant role in passing from more primitive to more 

 specialized vertebrates and brain centers take on more 

 complicated relations to temperature regulation. In a 

 mammal the result of the removal of cerebral lobes is a 



