Читать книгу The Body at Work: A Treatise on the Principles of Physiology онлайн

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In the meantime it may be well to consider the attributes which appear to belong to this most living substance. Its chemical composition can be inferred only from the compounds found on analysis to be present in a mass of organized substance which there is reason for thinking was rich in protoplasm while it was alive. The compounds found vary within certain limits. The quantity of water associated with these compounds is still more variable. Water is essential to the existence of protoplasm. Its power of combining with water in variable quantities is one of its characteristics. Tissue rich in protoplasm yields on an average about 75 per cent. of water. Part of the protoplasm within a cell holds more water associated with it, part less.

Closely associated with its power of holding water is its tendency to assume an architectural form. In large vegetable cells, such as those of the hairs within the flowers of Tradescantia, the protoplasm may be seen, under the microscope, arranged in threads containing granules which are incessantly streaming up and down them. The spaces between the threads are filled with water. Such mobile protoplasm cannot be said to have a structural form. But in the greater number of cells, and especially in animal cells, the protoplasm is disposed in a network, with usually a tendency for the strands of the network to set in lines. In attempting to define these very variable networks, the microscopist is obliged to speak with caution. He finds it very difficult to distinguish between appearances which he is justified in regarding as inherent in the cell-substance, whether alive or dead, and appearances which he may have induced by the action of reagents whilst preparing the tissue for examination. Rarely can he assert that he sees a network in a living cell. When examining a dead cell, he is bound to recognize that the preservatives and hardening reagents which he used may have caused the proteins to coagulate in a particular pattern. If he obtains the same pattern with several different methods, he infers that the appearance which he sees is that of a structure existing in the living cell; but he is never quite sure that it is not an arrangement produced by reagents after death.