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 are in communication with the outer world. They alone know, so to speak, what food is ingested. Even these have no direct relation with the ingested material, seeing that the latter, before being taken up by the cells of the gut, has been subjected to the action of the ferments poured into the alimentary canal, and been disintegrated into simpler and indifferent particles. All nutriment of a composite nature is dissolved in stages, until finally products of decomposition result which no longer exhibit any special characters.

Generally speaking, food supplies the material for the building up of the cell, and we must remember that we are dealing with the complicated tissues of animals and plants. Each cell has a specific fabric of its own, which is dependent on the nature of its separate units, and on the manner in which they combine together. We must not look upon this from a purely chemical point of view alone, but should pay attention to its physical aspects as well. The sum of the properties resulting from the special structure of the cell conditions its special functions. When such cells, with their specific structure and functions, are taken in by an individual organism, the latter can at first do nothing with the material supplied. The special character of the different products that make up the particular cells must first be destroyed. Unit must be separated from unit, so as to leave only a mixture of simple