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

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The specialization of a gland-cell is opposite in kind to that of a liver-cell. It takes up no more food than it requires, but it has developed a great capacity of producing from the food a substance which would no doubt be needed for its own purposes were it an isolated cell, but which the gland-cell places at the service of the body as a whole. An amœba can digest proteid substances. A cell of the pancreas produces the ferment necessary for the digestion of proteins, and secretes it into the alimentary canal.

To take another instance of specialization. An amœba responds to stimulation by changing its shape. It contracts in one direction, expands in another. A muscle-fibre has developed the capacity of contraction at the expense of all other functions. During the course of its growth it changes from a round cell into one that is elongated. The elongation is in the direction in which it acts with greatest efficiency. Its cell-substance is very highly specialized in order that it may have the maximum capacity of contraction in this direction.