CH. XXXIX.] EXCHANGE OF MATERIAL 585 



given in the preliminary parts of the report, or may be entered into 

 by still further examining the books of the company. 



In the parts of this book that precede this chapter I have 

 endeavoured to give an account of various transactions that occur in 

 the body. I now propose to present a balance-sheet. Those who 

 wish still further to investigate the affairs of the body may do so by 

 the careful study of works on physiology; still, text-books and 

 monographs, however good, wijl teach one only a small amount ; the 

 rest is to be learnt by practical study and research; and we may 

 compare physiologists to the accountants of a commercial enterprise, 

 who examine into the details of its working. Sometimes, in business 

 undertakings, a deficit or some other error is discovered, and it may 

 be that the source of the mistake is only found after careful search. 

 Under these conditions, the accountants should be compared to 

 physicians, who discover that something is wrong in the working of 

 the animal body ; and their object should be to ascertain where, in 

 the metabolic cycle, the mistake has occurred, and subsequently 

 endeavour to rectify it. 



The construction of balance-sheets for the human and animal 

 body may be summed up in the German word Stoffwechsel, or 

 " exchange of material" A large number of investigators have applied 

 themselves to this task, and from the large mass of material published, 

 it is only possible to select a few typical examples. The subject has 

 been worked out specially by the Munich school, under the lead 

 of Pettenkofer and Voit. 



The necessary data for the construction of such tables are : 



(1) The weight of the animal before, during, and after the 

 experiment. 



(2) The quantity and composition of its food. 



(3) The amount of oxygen absorbed during respiration. 



(4) The quantity and composition of urine, faeces, sweat, and 

 expired air. 



(5) The amount of work done, and the amount of heat developed. 

 (The subject of animal heat will be considered in the next chapter.) 



Water is determined by subtracting the amount of water ingested 

 as food from the quantity lost by bowels, urine, lungs, and skin. 

 The difference is a measure of the katabolism of hydrogen. 



Nitrogen. The nitrogen is derived from proteids and albuminoids, 

 and appears chiefly in the urine as urea and uric acid. Minute 

 quantities are eliminated as similar compounds in sweat and faeces. 

 From the amount of nitrogen so found, the amount of proteids 

 which have undergone katabolism is calculated. Proteids contain, 

 roughly, 16 per cent, of nitrogen ; so 1 part of nitrogen is equivalent 



