PRINCIPLES WHICH PASS THROUGH THE ORGAXISM. 371 



The life of a perfect animal organism is the sum of the 

 vitalities of its constituent parts ; but a being may live with 

 the vitality of certain parts abolished or seriously modified, 

 as a man exists and preserves his identity with a limb am- 

 putated. Life may continue for a long time without 

 consciousness, or with organs paralyzed or their function 

 destroyed ; but certain functions, such as respiration or cir- 

 culation, are indispensable to the nutrition of all parts, and 

 the vitality of the different tissues is speedily lost when 

 these processes are arrested, and the being then ceases to 

 exist. 



These considerations make it evident that it is difficult, 

 if not impossible, to give a single comprehensive definition 

 of life, a study of the varied phenomena of which con- 

 stitutes the science of physiology. 



The general process of nutrition begins with the intro- 

 duction of matter from without, called food. It is carried 

 on by the appropriation of this matter by the organism. 

 It is attended with the production of excrementitious prin- 

 ciples, and the development of certain phenomena that we 

 have not yet studied, the most important of which is the 

 production of heat. We shall have little to say about food, 

 beyond what we have already considered under the head of 

 alimentation, except to classify the alimentary principles 

 with reference to their relations to the general process of 

 nutrition. 



Principles which pass through the Organism. 



All of the inorganic principles taken in with the food 

 pass out of the organism, generally in the form in which 

 they enter, in the faeces, urine, and perspiration ; but it must 

 not be inferred from this fact that they are not useful as con- 

 stituent parts of the body. Some of these principles, such 

 as water and the chlorides, have very important functions 

 of a purely physical nature. It is necessary, for example, 

 that the blood should contain a certain proportion of the 



