62 HOMOIOTHERMISM 



tion is approached (at about 32 to 33 deg. C.) vaso-dila- 

 tation has reached its maximuin e:ffect and beyond that 

 impulses from the heat center can only accomplish an 

 increased heat loss from the body by augmenting sweat 

 secretion. All the skin changes resulting from excita- 

 tion of the heat center operate harmoniously to favor or 

 impede heat dissipation under normal conditions. 



Water is very well fitted for the requirements of tem- 

 perature equalization and regulation in animals. Its 

 mobility permits it to circulate freely and rapidly in the 

 organism. Henderson (1913) attributes the fitness of 

 water for temperature equalization and regulation to 

 three qualities: (1) great specific heat, (2) large amount 

 of heat required for evaporation, and (3) good thermal 

 conductivity. The first promotes the storage of heat ; the 

 second permits very rapid elimination of heat when the 

 environmental temperature exceeds that of the body; 

 the third contributes to the rapid equalization of heat 

 within the immobile tissues of the body and minimizes 

 the possibility of injury from local overheating. The 

 cooling power of water is great. One liter of water re- 

 quires 580 kilogram calories for its vaporization. This 

 factor makes water of extreme importance in heat loss. 



Water plays a significant role in the maintenance of 

 constant temperature in homoiotherms. It constitutes 

 about 70 per cent of the substance of an animal's body 

 and is one of the most abundant and available constitu- 

 ents of the environment. Aside from its function in the 

 regulation of temperature, it is fundamentally important 

 in the other physiological processes of the body. Water, 

 in consequence of its peculiar properties and abundance 

 in nature, is important in the regulation of the tempera- 

 ture on all parts of the earth, in both air and water, as 

 well as in organisms. 



