436 PHYSIOLOGICAL CHEMISTRY. 



Fary for the tissues and organs must be of different importance; 

 thus, for instance, water and the mineral bodies have another task 

 than the organic foods, and these again must vary in importance 

 among themselves. The knowledge of the action of various nutri- 

 tive bodies on the exchange of material from a qualitative as well 

 as a quantitative point of view must be of fundamental importance 

 in determining the value of different nutritive substances relative 

 to the demands of the body for food under various conditions and 

 also in deciding many other questions, for instance the proper 

 nutrition for an individual in health and in disease. 



Such knowledge can only be attained by a series of systematic 

 and thorough observations, in which the quantity of nutritive 

 material, relative to the weight of the body, taken and absorbed in 

 a given time is compared with the quantity of final products of the 

 exchange of material which leave the organism at the same time. 

 Researches of this kind have been made by several investigators, but 

 above all should be mentioned those made by BISCHOFF and Von, 

 by PETTENKOFEE and VOIT, and by VOIT and his school. 



It is absolutely necessary in researches on the exchange of 

 material to be able to collect, analyze, and quantitatively estimate 

 the excretions of the organism so that they may be compared with 

 the quantity and composition of the nutritive bodies taken up. In 

 the first place, one must know what the habitual excretions of the 

 body are and in what way these bodies leave the organism. One 

 must also have trustworthy methods for the quantitative estimation 

 of the same. 



The organism may, under physiological conditions, be exposed 

 to accidental or periodic losses of valuable material such losses 

 as only occur in certain individuals, or in the same individual only 

 at a certain period; for instance, the secretion of milk, the produc- 

 tion of pus, the ejection of semen, or menstrul blood. It is therefore 

 apparent that these losses can only be the subject of investigation 

 and estimation in special cases. 



The regular and constant excretions of the organism are of the 

 very greatest importance in the study of the exchange of material. 

 To these belong, in the first place, the true final products of the 

 exchange CARBON DIOXIDE, UREA (uric acid, hippuric acid, 

 creatinin, and other urinary constituents), and a part of the WATER, 



