Page:A Text-book of Animal Physiology.djvu/38

8 class of bodies known as proteids—that is, it consists chemically of carbon, hydrogen, a little sulphur, oxygen, and nitrogen, arranged into a very complex and unstable molecule. This very instability seems to explain at once its adaptability for the manifestation of its nature as living matter, and at the same time the readiness with which it is modified by many circumstances, so that it is possible to understand that life demands an incessant adaptation of internal to external conditions. It seems highly probable that protoplasm is not a single proteid substance, but a mixture of such; or let us rather say, furnishes these when chemically examined and therefore dead.

Very frequently, indeed generally, protoplasm contains other substances, as salts, fat, starch, chlorophyl, etc.

From the fact that the nucleus stains differently from the cell contents, we may infer a difference between them, physical and especially chemical. It (nucleus) furnishes on analysis nuclein, which contains the same elements as protoplasm (with the exception of sulphur) together with phosphorus. Nuclei have great resisting power to ordinary solvents and even the digestive juices.

Inasmuch as all vital phenomena are associated with protoplasm, it has been termed the "physical basis of life" (Huxley).

Tissues.—A collection of cells performing a similar physiological action constitutes a tissue.

Generally the cells are held together either by others with that sole function, or by cement material secreted by themselves. An organ may consist of one or several tissues. Thus the stomach consists of muscular, serous, connective, and glandular tissues besides those constituting its blood-vessels, lymphatics, and nerves. But all of the cells of each tissue have, speaking generally, the same function. The student is referred to works on general anatomy and histology for classifications and descriptions of the tissues.

The statements of this chapter will find illustration in the pages immediately following, after which we shall return to the subject of the cell afresh.

Summary.— The typical cell consists of a wall, protoplasmic contents, and a nucleus. The vegetable cell has a limiting membrane of cellulose. Cells undergo differentiation and may be united into groups forming tissues which serve one or more definite purposes.

The chemical constitution of protoplasm is highly complex