272 FUNCTIONS OF NUTRITION. 



It is not necessary to assume that the food, thus required for heat- 

 production, furnishes directly the necessary warmth by its consump- 

 tion. The heat is no doubt generated from the nutritive changes in 

 the bodily tissues, and these changes are continued only by a supply of 

 food sufficient to provide for the demands of the system. 



Resistance of the Living Body to High External Temperatures. 

 It has been seen that, in man and warm-blooded animals generally, 

 a rise in the bodily temperature of 6 or t is certainly fatal ; and yet 

 the body may be exposed, as shown by repeated observations, to much 

 higher degrees of heat without injurious result. According to Car- 

 penter,* the temperature of the air, in many parts of the tropical zone, 

 rises daily, through a large portion of the year, to 43 C. In southern 

 Arizona, the temperature at midsummer, as observed by Pumpelly,f 

 ranges, in the shade, from 41 to 52 ; and it is well known that the 

 air of manufactory drying-rooms and of the Turkish bath may easily 

 be endured when considerably above 45. Either of these tempera- 

 tures would be fatal to man, if they indicated the actual warmth of 

 the internal organs. The body therefore must either possess some 

 means of diminishing its own heat-production, or else of compensating, 

 to a certain extent, external temperatures which are above the normal 

 standard. 



The most direct means of moderating the temperature of the body is 

 the cutaneous perspiration. This secretion, derived from the perspi- 

 ratory glands of the skin, is clear, colorless, and watery, with an acid 

 reaction, and a specific gravity of 1003 or 1004. 



It is a fluid of very simple composition, containing over 99 J per 

 cent, of water, and more than half its solid ingredients consisting of 

 inorganic salts. There are also traces of an organic substance similar 

 to albumen, and a free volatile acid, which gives to the secretion its 

 reaction and odor. 



The perspiration is a continuous secretion. In a condition of repose 

 or moderate bodily activity, it is exuded so gradually that it is at once 

 carried off by evaporation, and has received the name, under these 

 circumstances, of the insensible transpiration. The quantity of fluid 

 discharged in this way, according to Lavoisier and Seguin, amounts 

 on the average to 900 grammes per day. In addition to f his, about 

 500 grammes are discharged from the lungs, making 140u grammes 

 of daily exhalation from the whole body. The vaporization of this 

 quantity of water will consume 750 heat units; or about oj 3-fifth of 

 all the heat produced in the body during twenty-four hourt- 



The cutaneous perspiration may be increased by temporary causes. 

 An elevated external temperature or unusual muscular exertion, will 

 accelerate the circulation through the skin, and largely augment the 

 amount of fluid discharged. It may then exude more rapidly than 



* Principles of Human Physiology. London, 1869, p. 483. 

 f Across America and Asia. New York, 1871, pp. 41, 57, 59. 



