THE PHENOMENA OF LIFE. 



3. The Power of Digestion, Respiration, and Nutrition. This con- 

 sists in the power which is possessed by the amosba and similar animal 

 cells of taking in food, modifying it, building up tissue by assimilating 

 it, and rejecting what is not assimilated. These various processes are 

 effected by the protoplasm simply flowing round and inclosing within 

 itself minute organisms such as diatoms and the like, from which it 

 extracts what it requires, and then rejects or excretes the remainder, 

 which has never formed part of the body. This latter proceeding is 

 done by the cell withdrawing itself from the material to be excreted. 

 The assimilation constantly taking place in the body of the amoeba, is 

 for the purpose of replacing waste of its tissue consequent upon mani- 

 festation of energy. The respiratory process 



of absorbing oxygen goes on at the same time. 



The processes which take place in cells, 

 both animal and vegetable, are summed up 

 under the term metabolism (from jj.TaftoAr h 

 change). The changes which go on are of 

 two kinds, viz., assimilation, or building up, 

 and disassimilation, or breaking down ; they 

 may be also called composition or decom- 

 position, or, using the nomenclature of Gas- 

 kell, anabolism or constructive metabolism, 

 and Jcatabolism or destructive metabolism. 

 In the direction of anabolism two processes 

 occur, viz., the building up of materials which 

 it takes in, and secondly, the building up of 

 its own substance by those or other mate- 

 rials. As we shall see in a subsequent para- 

 graph, the process of anabolism differs to 

 some extent in vegetable and animal cells. 

 The katabolism of the cell consists in chem- 

 ical changes which occur in the cell-substance itself, or in substances 

 in contact with it. 



The destructive metabolism of a cell is increased by its activity, but 

 goes on also during quiescence. It is probably of the nature of oxida- 

 tion, and results in the evolution of carbonic anhydride and water on 

 the one hand, and in the formation of various substances on the other, 

 some of which may be stored up in the cell for future use, and are 

 called secretions, and others, like the carbonic anhydride and certain 

 bodies containing nitrogen, are eliminated as excretions. 



4. The Power of Growth. In protoplasm then, it is seen that the 

 two processes of waste and repair go on side by side, and as long as they 

 are equal the size of the animal remains stationary. If, however, the 

 building up exceed the waste, then the animal grows / if the waste ex- 



