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 sufficient 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 appropriate 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 breakdown 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 by 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 reduction 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. Consequently, 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 activities 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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