TEMPERATURE REGULATION IN HOMOIOTHERMS 59 



surrounding temperatures this proportion changes, rela- 

 tively more heat escaping by evaporation and less by 

 radiation and conduction. 



High temperatures usually cause animals to breathe 

 more deeply. Animals, such as the fur bearing mammals 

 which lack sweat glands, show the most marked hyper- 

 pnea. When a dog extends its tongue and pants, the 

 loss of water from its body is greatly increased. Ani- 

 mals subjected to sources of radiation from which they 

 absorb heat often show pronounced hyperpnea. 



Blood volume is increased when the body tempera- 

 ture rises and this acts as a means of preventing vaso- 

 dilatation and also maintaining a sufficient supply of 

 water for sweat secretion. 



When the external temperature is high or when 

 thermogenesis is great, as in muscular work, the cutane- 

 ous blood vessels dilate, those within the abdomen con- 

 strict, and a larger volume of warm blood is thus sent 

 to the surface to be cooled. The cooled blood is returned 

 to the interior to take up more heat and is again returned 

 to the skin. Thus a great flow of blood through the skin 

 is obtained and this results in a maximum discharge of 

 heat by radiation and conduction. On a cold day the 

 reverse takes place, so that the warm blood is restrained 

 from circulating too freely through the skin where more 

 heat would be lost. A rabbit depends chiefly on its vaso- 

 motor activity for temperature regulation, as it does 

 not possess a satisfactory water reserve. This is likely 

 true with other herbivores although no evidence seems 

 to be available. Certain drugs may paralyze the vaso- 

 motor mechanism. For example, an animal under the in- 

 fluence of alcohol feels warmer and loses more heat than 

 a normal individual. 



Thermolysis is voluntary to a certain extent, for by 



