CHAPTER XIII. 



FOODS AND THEIR PHYSIOLOGIC AL VALUE. 



Having thus described the structure of the organs con- 

 cerned in digestion, the next question naturally arising is 

 the necessity for such a system. The body is a machine, 

 and apart from the mysterious property which it possesses, 

 which we call l 'life," it is subject to all the physical laws 

 which govern other machines. We may even go further, 

 and say that although we do not understand what consti- 

 tutes life, all experiments force us to the belief that this prop- 

 erty itself is never in violation of physical and chemical 

 laws. Machines must be supplied with energy to enable 

 them to do their work. There must be the pressure of 

 steam, the electro-motive force of a dynamo, the momentum 

 of running water, or what not, to set things in motion. As 

 soon as the source of energy is cut off the machine stops. 

 The idea of creating a perpetual-motion machine, one which 

 when set going will create the force with which to run, is a 

 scientific absurdity. Not only have thousands of failures to 

 construct one proved that, but one of the most firmly estab- 

 lished laws of science, possibly one of the most fundamental 

 discoveries of this century, has been the law that energy 

 can neither be created nor destroyed. This law is called 

 the " law of the conservation of energy." A great advance 

 was made when it was proved that the energy of the living 

 body is subject to this same law. Formerly it was believed 

 that the working energy of the body was a mysterious kind 

 of vital force which seemed to be in continued spontaneous 

 creation in the body. It may be said that the real science 

 of physiology was born when this notion was abandoned. 



While, however, energy is indestructible it may easily 

 be changed from one form to another. Steam pressure may 

 (308) 



