AND MEANS OF LIVING
all living rnatter (which depends on the laws of osrnosis and on chernical affinity), take for thernselves whatever they need from the menu offered by the blood, and with this rnatter they build up their own substance. It is evident, therefore, that the blood rnust contain a suffi- cient quantity and variety of dietary elernents to satisfy all possible cell appetites; that the stornach's walls and their associated glands must furnish the enzyrnes appro- priate for rnaking the necessary elernents available from the raw food rnatter in the stornach; and, finally, that it rnust 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 elernents that the cells dernand. As we have seen, the dernand for food comes from the loss of rnaterials that are decornposed in the tissues during cell activitv. Better stated, perhaps, the chernical break- down with[n 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 rnatter; 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 rninirnurn activity that rnaintains'life, the result is the same always--the loss of certain substances. But, as with most chernical reduc- tion processes, the protoplasrnic activity depends upon the presence of available oxygen; for the decornposition of the unstable substances of the protoplasrn is the result of the affinity of sorne of their elernents for oxygen. Conse- quently, when the stimulus for action comes over a nerve from a nerve center, a sudden reorganization takes place between these protoplasrnic elernents and the oxygen atorns which results in the formation of water, carbon dioxide, and various stable nitrogenous cornpounds. The substances discarded as a result of the cell activi- ties are waste products, and nlust be elirninated from the organisrn for their presence would clog the further activity of the cells or would be poisonous to thern. The anirnai,
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INSECTS
