WAYS AND MEANS OF LIVING 



all living matter (which depends on the laws of osmosis 

 and on chemical affinity), take for themselves whatever 

 they need from the menu offered by the blood, and with 

 this matter they build up their own substance. It is 

 evident, therefore, that the blood must contain a suffi- 

 cient quantity and variety of dietary elements to satisfy 

 all possible cell appetites; that the stomach's walls and 

 their associated glands must furnish the enzymes appro- 

 priate for making the necessary elements available from 

 the raw food matter in the stomach; and, finally, that it 

 must be a part of the instincts of each animal species to 

 consume such native foodstuffs of its environment as will 

 supply every variety of nourishing elements that the cells 

 demand. 



As we have seen, the demand for food comes from the 

 loss of materials that are decomposed in the tissues during 

 cell activity. Better stated, perhaps, the chemical break- 

 down within the cell is the cause of the cell activity, or is 

 the cell activity itself. The way in which the activity is 

 expressed does not matter; whether by the contraction of a 

 muscle cell, the secretion of a gland cell, the generation of 

 nerve energy bv a nerve cell, or just the minimum activity 

 that maintains life, the result is the same always — the loss 

 of certain substances. But, as with most chemical reduc- 

 tion processes, the protoplasmic activity depends upon the 

 presence of available oxygen; for the decomposition of the 

 unstable substances of the protoplasm is the result of the 

 affinity of some of their elements for oxygen. Conse- 

 quently, when the stimulus for action comes over a nerve 

 from a nerve center, a sudden reorganization takes place 

 between these protoplasmic elements and the oxygen 

 atoms which results in the formation of water, carbon 

 dioxide, and various stable nitrogenous compounds. 



The substances discarded as a result of the cell activi- 

 ties are waste products, and must be eliminated from the 

 organism for their presence would clog the further activity 

 of the cells or would be poisonous to them. The animal, 



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