Page:A Text-book of Animal Physiology.djvu/57

Rh  when the reverse process takes place. The results of this process are eliminated as excreta, or useless and harmful products. Since all the vital activities of cells can only be manifested when supplied with food, it follows that living organisms convert potential or possible energy into kinetic or actual energy. When lifeless, immobile matter is taken in as food and, as a result, is converted by a process of assimilation into the protoplasm of the cell using it, we have an example of potential being converted into actual energy, for one of the properties of all protoplasm is its contractility. Assimilation implies, of course, the absorption of what is to be used, with rejection of waste matters.

The movements of protoplasm of whatever kind, when due to a stimulus, are said to indicate irritability; while, if independent of any external source of excitation, they are denominated automatic.

Among agents that modify the action of all kinds of protoplasm are heat, moisture, electricity, light, and others in great variety, both chemical and mechanical. It can not be too well remembered that living things are what they are, neither by virtue of their own organization alone nor through the action of their environment alone (else would they be in no sense different from inanimate things), but because of the relation of the organization to the surroundings.

Protoplasm, then, is contractile, irritable, automatic, absorptive, secretory (and excretory), metabolic, and reproductive.

But when it is affirmed that these are the fundamental properties of all protoplasm, the idea is not to be conveyed that cells exhibiting these properties are identical biologically. No two masses of protoplasm can be quite alike, else would there be no distinction in physiological demeano—no individuality. Every cell, could we but behold its inner molecular mechanism, differs from its neighbor. When this difference reaches a certain degree in one direction, we have a manifest differentiation leading to physiological division of labor, which may now with advantage be treated in the following section.

An animal, as we have learned, may be made up of a single cell in which each part performs much the same work; or, if there be differences in function, they are ill-defined as compared