CHAPTER III. 
METHODS OF INVESTIGATION. 
An essential prerequisite for an intelligent study of the income 
and expenditure of matter by the animal body is a knowledge of 
the general nature of the current methods of investigation and of 
the significance of the results attained by means of them. It is 
not the purpose here to enter into technical details; this is not 
a treatise upon analytical or physiological methods. The present 
chapter will be confined to outlining the general principles upon 
which those methods are based and to pointing out the logical 
value of their results. It will be confined, moreover, mainly to 
those general methods by which the balance of income and ex- 
penditure of matter is determined. 
Tissue.—The animal body has already been characterized as 
consisting, from the chemical point of view, of an aggregate of 
various substances, chiefly organic, representing a certain capital 
of matter and energy. These various substances are grouped 
together in the body to form the organized structures known as 
tissues. For the sake of brevity, then, it may be permissible to use 
the word tissue as a convenient general designation for the aggre- 
gate of all the organic matter contained in the tissues of the body, 
including both their organized elements and any materials present 
in the fluids of the body or in solution in the protoplasm of the 
cells. In this sense tissue is equivalent to the whole capital or 
store of organic matter in the body. 
Gains AND Losses.—The tissue of the body, as thus defined, is 
in a constant state of flux, the processes through which the vital 
functions are carried on constantly breaking it down and oxidizing 
it (katabolism), while the processes of nutrition are as constantly 
building it up again (anabolism). If the activity of nutrition 
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