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CHAPTER V. 



THE VITAL PEOPEETIES OF THE CELL (continued). 



Metabolism and Formative Activity. 



General Characteristics. Each living cell exhibits the pheno- 

 mena of metabolism ; it absorbs nutrient material, which it 

 elaborates, retaining certain portions of it within its body, whilst 

 it rejects others ; it resembles a small chemical laboratory, for 

 the most varying chemical processes are almost continually taking 

 place in it, by means of which substances of complex molecular 

 structure are on the one hand being formed, and on the other are 

 being broken down again. The more intense is the vitality of the 

 cell, the more considerable are these processes of destruction and 

 reconstruction, the latter keeping pace with the former. In the 

 chemistry of the cell these two principal phenomena must be 

 clearly kept apart, namely the phenomena of progressive and 

 retrogressive metabolism, or, as Claude Bernard (IV. la) ex- 

 presses it, " les phenomeues de destruction et de creation 

 organique, de decomposition et de composition." 



During its destruction the living substance, as a result of its 

 own decomposition, passes through a series of intermediate stages 

 of more simple chemical combinations, the precise nature of 

 which is at present unknown. Carbon dioxide and water are the 

 simplest final products of this decomposition. Tension (potential 

 energy) is converted into active vital force (kinetic energy). 

 Intra-molecular heat becomes free, and represents the living force, 

 which is the essential condition for the production of work in the 

 cell body. The fact that the slightest shock often suffices to call 

 forth great changes and to cause work to be done shows that vital 

 substances are exceedingly unstable in composition : as Pfluger 

 (V. 25, 26) remarks : " Are not the forces which act in a ray of 

 light truly inconceivably small ? and yet they produce most marked 

 effects upon the retina and the brain. How infinitesimal are the 

 forces which serve to excite the nerves ; how extremely minute 



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