310 STUDIES IN ADVANCED PHYSIOLOGY. 



may be either a motion of the whole mass, a molecular 

 motion, or even an atomic one. For this reason the body 

 which desires primarily the energy to make possible its 

 activity, must take into itself large quantities of material in 

 which the energy is bound. 



So far we have considered the human body as a machine 

 completely constructed and needing no repair, needing only 

 energy to run it. But it is evident that there are other ne- 

 cessities for introducing outside material into it. For many 

 years of its life it must increase in size, and this it can do only 

 by appropriating from the food those substances which it can 

 build into its own tissues. Even when fully matured there 

 is a continued waste which needs new material to replace 

 it. The necessity for foods, therefore, is two-fold: First, 

 to furnish the material out of which the tissues of the body 

 may be constructed; and, secondly, to furnish material out 

 of which the body may derive the energy required for its 

 activity. In order to understand how much this shall be, 

 it is desirable to examine what the losses of the body are 

 under normal conditions. 



THE LOSSES OF THE BODY. 



1. In Matter. Careful investigations upon persons of 

 average size and conditions show that in the course of a day 

 there is lost from the body in the form of matter about nine 

 pounds. In this is not included that undigested portion of 

 the food which never really becomes part of the body. 

 About five pounds of this loss is through the lungs. It 

 seems at first surprising to think that even in so short a 

 time as one day there should have been breathed out of the 

 lungs a quantity of gas reaching the amount of about five 

 pounds. It seems not quite so surprising that about three 

 pounds or more of this is eliminated from the kidneys. 

 The remainder is thrown off from the skin or poured as ex- 

 cretions into the alimentary canal. Evidently, therefore, 

 from the mere standpoint of matter it is necessary to put 

 into the body each day nine pounds of suitable substances 



