58 HOMOIOTHERMISM 



Supplemental to the production of heat by oxidation 

 during periods of falling temperatures, either in the 

 environment or in the blood itself, are certain physiolog- 

 ical adjustments on the part of the organism, some re- 

 tard heat loss and others hasten the dissipation of heat. 

 Among these are: (1) cutaneous vaso-constriction, which 

 decreases loss of heat through the skin; (2) reduction 

 in blood volume, which apparently permits the constric- 

 tion of peripheral blood vessels without excessive en- 

 gorgement of those on the interior; (3) behavior which 

 results in the aggregation of groups of individuals or 

 ** huddling'' reflexes which in cold surroundings reduce 

 the area of exposed body surface, and thus check loss 

 of heat by radiation and conduction. 



Thermolysis. — The mechanisms for regulation of heat 

 dissipation are mainly employed when there is danger 

 of overheating an animal's body. The dangers from 

 supraoptimum temperatures usually arise from within 

 the body rather than from the environment. A rise in 

 temperature either in the blood or skin results in the 

 heat center in the central nervous system stimulating 

 an increased heat dissipation which may be brought 

 about by the following physiological means: (1) hyper- 

 pnea, or panting, (2) cutaneous vaso-dilatation, (3) 

 evaporation of moisture from exposed moist surfaces, 

 (4) decreased muscular exertion, (5) maximum exposure 

 of body surfaces, as in outstretched animals during hot 

 weather. 



Physical thermolysis is accomplished by radiation, 

 conduction, and water loss. It has been estimated that 

 at ordinary room temperatures about 75 per cent of the 

 heat produced by the human body is lost by radiation 

 and conduction and 25 per cent through the evaporation 

 of water from the surfaces of lung and skin. At higher 



