Page:A Text-book of Animal Physiology.djvu/37

Rh of cells known as tissues. The greater part of a tree may be said to be made up of the thickened walls of the cells, and these are destitute of true vitality, unless of the lowest order; while the really active, growing part of an old and large tree constitutes but a small and limited zone, as may be learned from the plates of a work on modern botany representing sections of the wood.

Animals, too, have their rigid parts, in the adult state especially, resulting from the thickening of a part or the whole of the cell by a deposition usually of salts of lime, as in the case of the bones of animals. But in some cases, as in cartilage, the cell wall or capsule undergoes thickening and consolidation, and several may fuse together, constituting a matrix, which is also made up in part, possibly, of a secretion from the cell protoplasm. In the outer parts of the body of animals we have a great abundance of examples of thickening and hardening of cells. Very well known instances are the indurated patches of skin (epithelium) on the palms of the hands and elsewhere.

It will be scarcely necessary to remark that in cells thus altered the mechanical has largely taken the place of the vital in function. This at once harmonizes with and explains what is a matter of common observation, that old men are less active—have less of life within them, in a word, than the young. Chemically, the cellulose wall of plant-cells consists of carbon, hydrogen, and oxygen, in the same relative proportion as exists in starch, though its properties are very different from those of that substance.

Turning to cell contents, we find them everywhere made up of a clear, viscid substance, containing almost always granules of varying but very minute size, and differing in consistence, not only in different groups of cells, but often in the same cell, so that we can distinguish an outer portion (ectoplasm) and an inner more fluid and more granular region (endoplasm).

The nucleus is a body with very clearly defined outline (in some cases limited by a membrane), through which an irregular network of fibers extends that stains more deeply than any other part of the whole cell.

Owing to the fact that it is so readily changed by the action of reagents, it is impossible to ascertain the exact chemical composition of living protoplasm; in consequence, we can only infer its chemical structure, etc., from the examination of the dead substance.

In general, it may be said that protoplasm belongs to the