CHAPTER V 

 METABOLISM 



i. GENERAL CONCEPTION 



201. Assimilation and excretion. The cell has already been 

 defined (73) as the biological unit of life. It is the living proto- 

 plasm of the body cells which is the seat of the multifarious 

 activities of the organism. 



Every such activity requires an expenditure of energy, de- 

 rived from the breaking down of constituents of the .proto- 

 plasm itself or of cell enclosures and solutes and their transfor- 

 mation into other forms. The presence of oxygen is essential 

 to these changes and while, as will appear, they seldom are 

 primarily direct oxidations, nevertheless, they yield products 

 which are ultimately oxidized to carbon dioxid, water and 

 other comparatively simple compounds. 



Two things, then, are necessary for the continued life of the 

 cell : first, a supply of material from without to replace that 

 consumed and, second, the removal of the waste products of 

 its activities. Both conditions are fulfilled in the higher ani- 

 mals by the circulation of the blood and lymph. In the pro- 

 cesses of digestion, the heterogeneous nutritive materials con- 

 tained in the feed are gradually brought into solution by a series 

 of molecular cleavages, so that the resorptive organs transmit 

 to the blood and lymph current a qualitatively uniform material 

 consisting of substances of comparatively simple molecular 

 structure (146, 147), while oxygen is supplied to the blood cor- 

 puscles through the lung capillaries. The mechanism of cir- 

 culation is continually distributing to each tissue and cell oxygen 

 from the lungs and nutritive material from the digestive tract 

 and carrying away the waste products of cell action to the 

 various organs of excretion which remove them from the body. 



202. Definition of metabolism. It is clear from the fore- 

 going that the body is the seat of extensive chemical trans- 



144 



