482 THE HUMAN BODY, 



remainder is transmitted directly as heat to the exterior, and 

 is lost to the engine for all useful purposes. 



The Maintenance of an Average Temperature. This is 

 necessary for the continuance of the life of a warm-blooded 

 animal; should the temperature rise above certain limits 

 chemical changes, incompatible with life, occur in the tissues ; 

 for example at about 49 C. (120 F.) the muscles begin to 

 become rigid. On the other hand, death ensues if the Body 

 be cooled down to about 19 0. (66 F.). Hence the need 

 of means for getting rid of excess heat, and of protection 

 from excessive cooling. Either end may be gained in two 

 ways: by altering the rate at which heat is lost or that at 

 which it is produced. As regards heat-loss, by far the most 

 important regulating organ is the skin : under ordinary cir- 

 cumstances nearly 90 per cent of the total heat given off from 

 the Body in 24 hours goes by the skin (73 by radiation and 

 conduction, 14.5 by evaporation). This loss may be con- 

 trolled 



1. By clothing ; we naturally wear more in cold and less 

 in warm weather; the effect of clothes being, of course, not 

 to warm the Body but to dimmish the rate at which the heat 

 produced in it is lost. 



2. Increased temperature of the surrounding medium in- 

 creases the activity of the heart and lungs. A hastened cir- 

 culation by itself does not, as already pointed out (Chap. 

 XXVI), increase the general tissue activity of the Body, or 

 the oxidations occurring in it, and so, apart from the harder- 

 working heart itself, does not influence the amount of heat 

 liberated in the Body during a given time: but the more rapid 

 blood-flow through the skin carries more of that fluid through 

 this cool surface in each minute and in that w r ay increases 

 the loss of heat. The quickened respirations, too, increase the 

 evaporation of water from the lungs and, thus, the loss of heat. 



3. Warmth, mainly through reflex vaso-moter actions leads 

 to dilatation of the skin-vessels and cold to contraction. In 

 a warm room the vessels on the surface dilate as shown by its 

 redness, while in a cold atmosphere they contract and the 

 skin becomes pale. But the more blood that flows through 

 the skin the greater will be the heat lost from the surface 

 and vice versa. 



4. Heat induces sweating and cold checks it; the heat 

 appears to act, partly, reflexly through afferent cutaneous 



