NUTRITION 



a special vital principle. All the trophic (nutritive) 

 processes, without exception, are subject to the law of 

 substance. 



In all the higher plants and animals the chemical' 

 process of metabolism, with the stream of energy that 

 accompanies it, is a very complex vital activity, in which 

 many different functions and organs co-operate with 

 the common aim of self-maintenance. As a rule, they 

 are distributed in four groups — namely : ( i ) Intussuscep- 

 tion of food and digestion: (2) distribution of the food 

 in the body, or circulation; (3) respiration, or exchange 

 of gases; and (4) excretion of unusable matter. In 

 most of the histona, either tissue-plants or tissue- 

 animals, a number of organs are differentiated for the 

 accompHshment of these tasks. At the lower stages of 

 life this division of labor is not found, the entire proc- 

 ess of nutrition being accomplished by a single layer 

 of cells (lower algae, gastra^ads, sponges, lower polyps). 

 In the protists, again, it is the single cell that does 

 all these things itself; in the simplest cases, the monera, 

 a homogeneous plasma-globule. As a long gradation 

 uninterruptedly unites these lowest forms of nutrition 

 with the more complicated forms, we must regard the 

 latter no less than the former as physico-chemical 



processes. 



When we take the whole of the metabolic functions in 

 organisms together, we may look upon them as the out- 

 come of two opposite chemical processes — on the one 

 hand the building-up of living matter by taking in food 

 (assimilation), and on the other the breaking-down of it 

 in consequence of its vital activity (disassimilation). As 

 in every case the plasm is the active living matter, we 

 may say: Assimilation (or plasma-production) consists 

 in the conversion within the organism into the special 

 plasm of the particular species of food that has been 

 received from without; disassimilation (or plasma- 



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