TEMPERATURE REGULATION IN HOMOIOTHERMS 61 



stimnlation of the meclianism for sweating. Currents of 

 air over tlie surface of an animal remove the water 

 vapor, bring dry air in contact with the skin, and thus 

 decrease the partial pressure of the gas next to the skin. 

 In a strong wind the partial pressure is at its minimum 

 in the layer of air next to the body. The drying effect 

 of winds is more or less independent of their tempera- 

 ture but is directly related to their water saturation 

 deficit. 



The effective area of surface exposed to cooling de- 

 pends in great measure on the state of the superficial 

 blood vessels. Dilatation greatly increases the effective 

 surface ; constriction, the converse. 



The effect of color is merely a matter of differential 

 absorption of radiant energy which produces local heat- 

 ing in proportion to the amount of energy absorbed and 

 so causes a more or less rapid evaporation. The black 

 moist nose of a dog is much cooler than a similar dry, 

 white area. 



There are about 2 to 3 million sweat glands in the 

 entire skin of a man. The number apparently varies 

 considerably in different species of mammals; the dog, 

 for example, has none. The volume of secretion among 

 species which possess sweat glands is also variable and 

 there is a marked variation among the individuals of a 

 single species. In man it may vary from a minimum of 

 700 ml. per twenty-four hours to three times that amount. 

 Whether volume changes are due to changes in number 

 of active glands or to variations in the output of individ- 

 ual glands is not known. Adolph (1923) has shoi\Ti that 

 when an active body is exposed to high external temper- 

 atures the quantity of sweat is a linear function of the 

 '^effective" skin temperature. It appears therefore that 

 as the limit of heat dissipation by conduction and radia- 



