488 AN AMERICAN TEXT-HOOK OF PHYSIOLOGY. 



the lo>> of heat by this channel. The heat-loss occurs both in warming the 

 air and in the evaporation of water from the lungs, so that the cooler and 

 drier the air inspired the larger relatively is the heat-loss. The importance 

 of respiration as a heat-dissipating factor is illustrated by the fact that about 

 10.7 per cent, of the total heat-dissipation occurs in this way (see p. 477). 



Next in importance to radiation is the amount of water evaporated from 

 the sldn. Each gram of water requires 582 calories to vaporize it, and it is 

 estimated (p. 477) that 364,120 calories are dissipated in this way, or 14.5 

 percent, of the total heat-dissipation. An increase of external temperature 

 increases the irritability of the sudoriparous glands, thus favoring secretion and 

 heat-dissipation. The value of sweat, however, as a means of carrying off 

 heat, is materially affected by the temperature of the air as well as by the 

 amount of moisture present. The higher the temperature and the less the 

 moisture the more rapidly evaporation occurs, and consequently the greater 

 the loss of heat ; when air i> moist and of high temperature evaporation takes 

 place relatively slowly, if at all. Therefore, individuals can withstand sub- 

 jection to dry air of a higher temperature and for a longer period than when 

 the atmosphere is moist. In the former case sweat is rapidly secreted and 

 vaporized, and thus a marked rise of internal temperature may be prevented. 

 James found that a vapor bath at 44.5° C. (112° F.) was insufferable, while 

 dry air at 80° C. (176° F.) caused little inconvenience. When air is of high 

 temperature and loaded with moisture we say that it is "sultry," but dry air 

 of the same temperature is not unpleasant. 



Muscular activity increases heat-production, excites the circulation and 

 respiration, and increases the secretion of sweat, all of which directly or indi- 

 rect Iv increase heat-dissipation. 



The surface of //" body as a radiating surface cannot be regarded in the 

 same light as an indifferent, inanimate surfape, such as metal or wood. The 

 coefficient of r<i<Iiafi<>n (the quantity of heat emitted during a unit of time at a 

 standard temperature from a given area) in an inanimate body remains fixed, 

 because the surface itself is virtually unchangeable; but the coefficient for the 

 living organism is subject to material alterations. These alterations depend 

 chiefly (1) upon the actions of the pilo-motor mechanism whereby the relation 

 of the natural covering (hair or feathers in the lower animals) of the body to 

 the skin is effected ; (2) upon changes in the conductivity of the skin owing to 

 variations of the blood-supply ; (3) upon the varying thickness of the skin in 

 different species, in different individuals, and in different parts of the body; 

 (4) upon the temperature of the surroundings; (5) upon the extent of the 

 body-surface exposed; Hi) upon the character of the clothing. When the 

 arrector pili muscles contract the skin is made tense and the cutaneous blood- 

 vessels .ire pressed upon and rendered anaemic, thus lessening the quantity of 

 fluid in the skin and as a consequence lowering the coefficient of dissipation; 

 moreover, in animals whose natural covering is fur or leathers, these fibres 

 cause an erection of one or the other, a- the case may be, and in this way 

 affect the radiating coefficient. The coefficient is enormously increased by 



