70 INTRODUCTION. 



which they can supply this waste, or regenerate themselves by 

 the appropriation of suitable materials, involve contact with 

 the circulating blood. They take materials from this fluid and 

 change them into their own substance. This process takes 

 place only in living bodies, and is unknown in the inorganic 

 kingdom. As it is the great characteristic of life, its accom- 

 plishment being the end and object of all the functions of 

 the organism, the study of these organic principles is mani- 

 festly of the greatest importance. We shall find that their 

 properties are peculiar to themselves, and their chemical study 

 must necessarily be eminently physiological. To arrive at 

 any definite idea of their properties, the methods of study 

 which have been generally employed by chemists must be 

 discarded, as by these they are reduced to inorganic ele- 

 ments, and treated simply as combinations of inert sub- 

 stances. They must be studied as nearly as possible in the 

 condition in which they exist in the body ; which is neces- 

 sarily the condition in which they are capable of manifesting 

 their characteristic vital phenomena. 



These principles are found in all the fluids, semi-solids, 

 and solids of the body, except the excrementitious fluids. 1 

 The nutritive fluids contain several. In each tissue an or- 

 ganic principle is found which presents certain peculiarities 

 more or less distinctive. They are all formed in the or- 

 ganism, and, with the exception of the milk, a little mucus, 

 desquamated epidermis and epithelium, and an almost inap- 

 preciable quantity exhaled by the lungs and skin, are never 

 discharged unchanged from the body, in health. They 

 assume the consistence of the part in which they are found ; 

 being, therefore, fluid, semi-solid, and solid. They constitute 

 by far the greater part of the organism ; but their quantity 

 in the whole body has never been accurately estimated. 

 Their reaction is neutral. As a peculiarity of chemical com- 

 position, they all contain nitrogen ; whence they are called 



1 The excrementitious fluids contain coloring matters, which Robin and Vcrdeil 

 put in this class, but which do not seem to be endowed with vital properties. 



