4 o8 GENERAL NUTRITION 



perspiration, a cold plunge, which checks the action of the skin almost 

 immediately, is not injurious and is decidedly agreeable. This presents 

 a striking contrast to the effects of sudden cold on a system heated and 

 exhausted by long-continued exertion. In the latter instance, when the 

 perspiration is suddenly checked, serious disorders of nutrition, with 

 inflammation etc., may supervene. When the skin acts to keep down 

 the temperature of the body in simple exposure to external heat, there 

 is no modification in nutrition, and the tendency to an elevation of the 

 body-temperature comes from causes entirely external. It is a practical 

 observation that no ill effects are produced, under these circumstances, 

 by suddenly changing the external conditions ; but when the body-heat 

 is raised by a modification of the internal nutritive processes, as in pro- 

 longed muscular work, these changes should not be suddenly arrested ; 

 and a suppression of the compensative action of the skin is likely to 

 produce disturbances in nutrition, often resulting in inflammations. 



RELATIONS OF HEAT TO FORCE 



Since the development of the theory of the conservation of forces, 

 which had its origin in an essay published by J. R. Mayer, in 1842, 

 physiologists have applied the laws of correlation and conservation of 

 forces to operations involving the production of heat and the develop- 

 ment and expenditure of force in animals. This theory, if applicable 

 to what were formerly called vital operations, certainly affords, in its 

 definite quantities of heat and force as expressed in heat-units and foot- 

 pounds, a basis for calculating the value of material changes in the 

 body. Without discussing the purely physical questions involved, the 

 laws of correlation and conservation of forces, as they are applicable to 

 human physiology, may be briefly stated as follows : 



Potential energy is something either residing in or imparted to 

 matter, which is capable of being converted directly or indirectly into 

 heat. The animal body, for example, is a storehouse of potential 

 energy. Its tissues may be made to unite with oxygen and heat is 

 produced. Any body may have potential energy imparted to it. If a 

 weight is raised to a certain height, when the force which has accom- 

 plished this work is exhausted, the potential energy imparted to the 

 weight causes it to fall ; and in this fall heat is produced. The weight 

 may be supported at the height to which it has been raised for an 

 indefinite time ; but it still possesses the potential energy that has been 

 imparted to it, and when the support is removed, this potential energy 

 is converted into force, which may be converted into heat. Potential 

 energy may be converted directly into heat, as when a body is oxidized. 



