ANIMAL HEAT. 595 



roundings, the greater is the loss of heat. The larger the animal the greater the 

 body-surface, and therefore the greater is heat-dissipation ; but in proportion 

 to body- weight smaller animals have larger body-surfaces, therefore heat-dissi- 

 pation is relatively greater, although not absolutely so (see p. 537). The area 

 of body-surface involved in heat-dissipation is affected by the position of the 

 individual. Thus, by bringing the arms arid legs in contact with* the body 

 the total surface exposed is lessened. On the other hand, animals which 

 habitually have their legs in apposition with the trunk have their radiating 

 surfaces increased when their legs are extended. For instance, in the rabbit 

 extension of the legs enormously increases heat-dissipation, so that the bodily 

 temperature is profoundly affected. 



The condition of the vascular system exercises an important influence. 

 Circumstances that excite the circulation affect heat-dissipation both directly 

 and indirectly. Thus, heat-loss is directly increased by the excitation of the 

 respiratory movements, by the increased secretion of sweat, and by the larger 

 supply and increased temperature of the blood to the skin. Increased activity 

 of the circulation also increases heat-production, and thus indirectly affects heat- 

 dissipation. Opposite conditions, of course, lessen heat-dissipation. 



The larger the quantity of air respired, other things being equal, the larger 

 the loss 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. 584). 



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

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

 estimated (p. 584) that 364,120 calories are dissipated in this way, or 14.5 

 per cent, 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 is 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- 

 rectly increase heat-dissipation. 



