454 THE HUMAN BODY. 



simultaneously work equivalent to 502 calories. So of all 

 the energy lost from it in that time about may take the 

 form of mechanical work; this is a very large proportion 

 of the total energy expended, being a much higher per- 

 centage than that given by ordinary machines. The best 

 steam-engines can utilize as mechanical work only about 

 ^Q of the total energy liberated in them and lost from 

 them in a given time; the 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 tis- 

 sues ; for example at about 49 C. (120 F.) the muscles be- 

 gin to become rigid. On the other hand death ensues if the 

 Body be cooled down to about 19 C. (66 F.). Hence the 

 need for means of getting rid of excess heat, and of protec- 

 tion 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 

 mo'st important regulating organ is the skin: under ordi- 

 nary circumstances nearly 90 per cent of the total heat given 

 off from the Body in 24 hours goes by the skin (73 by radi- 

 ation and conduction, 14. 5 by evaporation; see above table). 

 This loss may be controlled 



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 diminish 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 

 circulation by itself does not as already pointed out (p. 388) 

 increase the general tissue activity of the Body, or the oxida- 

 tions 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 and increases the loss of heat in that way. The 



