150 THE BODY AT WORK 



the beginning and at the end of the period of observation. 

 Or, we may estimate the amount of potential energy 

 contained in the food, and ascertain the use to which this 

 energy is put in doing external work, in maintaining the 

 temperature of the body, and in warming the breath and other 

 excreta. 



If we are making up the balance-sheet of a fully-grown man, 

 we may take for granted that he is not making fresh tissue. 

 During the period throughout which he is under observation, 

 care is taken to avoid altering the conditions of his life in such 

 a manner as to lead him to develop additional muscle. If 

 he gains in weight while under observation, he is putting on 

 fat. If he loses in weight, he is sacrificing fat. 



The whole of the nitrogen taken in leaves the body in urea, 

 unless, as we have said, growth of tissue is taking place. The 

 body has not the same temptation to store nitrogen as it 

 has to store carbon. Consequently, it is very sensitive to any 

 deficiency of nitrogen in the diet. If food does not contain as 

 much protein as is needed, the deficit is made up at the expense 

 of the tissues. It does not necessarily follow that under these 

 circumstances a man loses in weight. He may be putting on 

 fat, although losing in strength owing to waste of muscle. 

 For observations upon the income and expenditure of the body 

 to be of any value, a condition of " nitrogenous equilibrium " 

 must be established. The nitrogen taken in must equal in 

 amount the nitrogen given out. 



Very exact determinations of income and expenditure may 

 be made by placing an animal, or even a man, in a box through 

 which air is drawn. A record is made of the volume of air 

 drawn through the box. The percentages of water vapour and 

 carbonic acid which the air contains are estimated before it 

 enters and after it leaves. The solid food consumed and the 

 urea excreted are also measured. 



If it is desired to measure the amount of heat given off, an 

 animal may be placed in a calorimeter. 



Even when most passive, the subject under examination, 

 whether an animal or a man, is expending energy in keeping 

 the body warm, in movements of respiration, and in shifting 

 position. If it is desired to ascertain the relation of oxidation 

 to external work, it is easy to devise a form of resistance, such 



