DUTIES OF THE SKIN. 15 



of being utilised for the production of muscular activity ; for 

 no useful effect in that direction is obtained by attempting 

 to prevent the dissipation of that heat into the atmosphere. 

 It may therefore be not unreasonable to conclude that the 

 motion for muscular work is got directly from chemical 

 affinity, and that the evolved heat is the remaining product 

 of that source of energy. This heat is necessary, not for 

 muscular work, but for the performance of the vital functions, 

 which cannot be carried on in a state of efficiency unless the 

 system is maintained at a temperature, in the case of horses, 

 of about 99.5 F. ; the surplus heat being carried off by evapora- 

 tion from the skin, lungs, etc., and by radiation and conduction. 

 In health, when the usual supply of heat is not sufficient to 

 maintain the body at its normal degree of temperature, the 

 nerves stimulate the process of combustion in the system, so 

 that more heat is produced. Hence in cold weather, animals 

 require more food than in warm weather, and at that time, 

 food can be more or less supplemented by clothing. The 

 nerves also perform the duty of preventing the temperature 

 of the body from unduly exceeding its normal standard ; for 

 when the heat production is high, as during fast work, the 

 nerves cause an increased supply of blood to go to the surface 

 of the body, with the result that the process of evaporation is 

 proportionately stimulated. The regulation of the internal 

 temperature is therefore under nervous control. 



DUTIES OF THE SKIN. 



The following are the chief duties of the skin : 



1 . To act as an organ of feeling (or touch). 



2. To assist in removing carbonic acid and other noxious 

 products from the body. 



3. To give off perspiration , which is a fluid that consists of 

 about ninety-nine parts of water and one part of various other 

 substances. This secretion is called insensible perspiration, 



