THE PRODUCTION AND DISSIPATION OF HEAT 1101 



ism which allows us to endure temperatures above that of the blood 

 for days and even permits us to expose ourselves for a brief period of 

 time to temperatures above that of boiling water. 



While the amount of latent heat discharged by way of the respira- 

 tory tract, is inconsiderable in man, this channel serves practically as 

 the only means of thermolysis in many animals. This is especially 

 true of those whose bodies are covered with thick fur which in itself 

 hinders radiation. Thus, we note that the dog pants whenever a 

 more copious loss of heat is made necessary. The respiratory air is 

 then made to oscillate back and forth across the moistened mucous sur- 

 faces, and some bound heat is also lost in the fluid which dripples out of 

 the corners of his mouth. At the approach of winter, these animals most 

 generally acquire an even thicker coat of fur as well as more considerable 

 amounts of subcutaneous fat. The value of the latter as a conserver 

 of heat is well illustrated by the fact that water-fowls and especially 

 those inhabiting very cold waters, are abundantly supplied with it. 

 Moreover, since women are usually more copiously equipped with 

 adipose tissue, they are in a better position to withstand cold than men. 

 Heat may also be conserved by bringing the legs and arms in apposition 

 with the trunk, because in this position a smaller area of the body is 

 exposed to the surrounding medium. The opposite effect is produced 

 by exposing the flanks more fully to the medium, a common practice 

 among rabbits and dogs on warm summer days. 



Among the voluntary factors controlling the loss of heat, might be 

 mentioned the selecting and fitting out of the winter quarters of the 

 hibernating animals, the building of nests, the adaptation of the dwell- 

 ings of man to outside conditions, the wearing of clothing, and many 

 others. The value of clothes lies in the fact that they hinder the free 

 circulation of the air. Inasmuch as this medium is by no means 

 a good conductor of heat, they retard the escape of radiant heat from 

 the skin and become warm by absorption. This process is repeated 

 at every successive layer of clothing, because each layer acts as a 

 concentric air-jacket which tends to conserve the heat stored up in 

 the water vapor right next to the skin. But the thickness of the 

 clothing is not everything and attention must also be paid to its 

 quality, inclusive of its porosity, weight, color and conducting power. 

 Cotton and linen are good conductors and, therefore, allow the heat 

 to escape more readily. Wool possesses the opposite qualities and 

 is better adapted for cold weather. Besides, it is markedly hygro- 

 scopic and prevents a too rapid evaporation of the moisture and chilling 

 of the body. The coarser the material, the greater its radiating power, 

 and the cooler the clothing made from it. Furthermore, black cloth- 

 ing is warmer than white clothing, because it possesses a greater heat 

 absorbing power. During sleep, when the metabolism and ther- 

 mogenetic function of the tissues is at low ebb, extra covers are needed 

 to prevent an undue loss of heat. 



