428 NUTRITION ANIMAL HEAT AND FORCE. 



and performs certain denned offices, as far as its organization will permit. 

 This can also be destroyed, suspended or modified by surrounding conditions. 



The life of a perfected anatomical element or tissue is the property which 

 enables it to regenerate itself and perform it offices, subject, also, to modifica- 

 tions from surrounding conditions. 



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

 constituent parts ; but a being may live with the physiological properties of 

 certain parts abolished or seriously modified, as a man exists and preserves 

 his identity with a limb amputated. Life may continue for a long time with- 

 out consciousness or with organs paralyzed ; but certain functions, such as 

 respiration and circulation, are indispensable to the nutrition of all parts, the 

 properties of the different tissues are 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 phe- 

 nomena of which constitutes the science of physiology. 



The general process of nutrition begins with the introduction of matter 

 from without, called food. It is carried on by the appropriation of this mat- 

 ter by the organism. It is attended with the production of excrementitious 

 matters and the development of certain phenomena that remain to be studied, 

 the most important of which is the production of heat. 



The term metabolism, now used by many English writers, seems destined 

 to become generally adopted. It was employed by Schwann to designate a 

 kind of action by cells, resulting in a change in the character of substances 

 brought in contact with them. Modern writers use it as a translation of the 

 German word Stoffwechsel. The literal signification of the Greek word 

 HTa(3o\7} is change. As applied to nutritive changes, metabolism is equiva- 

 lent to assimilation ; and as applied to the changes which result in the pro- 

 duction of effete matters, it is equivalent to disassimilation, a term much 

 used by the French, and one which well expresses changes that are exactly 

 the opposite of assimilation. The signification of the term metabolism seems 

 likely to be extended so as to include the acts of cells in the production of 

 the constituents of the secretions, a process which it is difficult to express in 

 a single word. 



The behavior of various substances in nutrition has already been treated 

 of, to some extent, in connection with alimentation ; but certain general rela- 

 tions of nutritive substances to assimilation remain to be considered. It is 

 convenient, as before, to divide these substances into the following classes : 

 1, Inorganic; 2, organic non-nitrogenized ; 3, organic nitrogenized. The 

 excrementitious products constitute a distinct class by themselves. 



SUBSTANCES WHICH PASS THROUGH THE ORGANISM. 



All of the inorganic matters taken in with the food pass out of the organ- 

 ism, generally in the form in which they enter, in the fseces, urine and per- 

 spiration ; but it must not be inferred from this fact that they are not useful 

 as constituent parts of the body. Some of these, such as water and the chlo- 



