The ovum, known as the female egg, is composed of the parts just described. If it is not fertilized when ripe it passes away and dies. If fertilized in a natural way, it enlarges in size and subsequently divides into two cells; and these, passing through similar changes, finally give rise to the various groups of cells from which the body is developed.
The Animal Body a Group Collection.
—The body is, therefore, a mass of cells; not all alike, of course, but grouped together for the purpose of doing certain special kinds of work. In this way we have various groups, with each group a community performing its own function. The brain forms one community; and these cells are concerned with mind acts. The muscle cells are busy in exerting force and action. Another group looks after the secretions and digestive functions, while another group is concerned solely with the function of generation and reproduction. And so it is throughout the body.
Both individual cells and group cells are concerned with disease. One cell may be diseased or destroyed, but the surrounding ones may go on just the same. It is when the group is disturbed that the greatest trouble results.
A Word About the Cells.
—The cell always possesses its three parts—membrane, protoplasm, and nucleus. But there is no rule as to the size or shape. Cells may be round or oblong, any shape. Substances pass in and out of the cell walls; and they are in motion, many of them, especially those that line the intestines and the air passages, and the white corpuscles of the blood. More than this, some cells, Dr. Jekyl-like, change their appearance and shape, send out finger-like bodies to catch enemies or food, and even travel all around in the body, often leaving it altogether.
BODY TISSUES
The animal body contains five forms of tissues: Epithelial, in which the cells are very compact, forming either thin or thick plates; the connective tissue, by which many organs are supported or embedded; muscle tissue, either smooth or striated, and in which the cells are in fibers that contract and shorten; nerve-tissue, that has to do with nerve and ganglion cells by which mental impulses are sent; and blood and lymph tissue or fluid tissues.
The first group is intimately connected with the secretory organs, or those organs which secrete certain substances essential for the proper work of the body. Thus we have salivary glands, mucous glands, sweat glands, and the liver and pancreas. Connective tissue includes fibrous tissue, fatty tissue, cartilage and bone. The fibrous connective tissue is illustrated when the skin is easily picked up in folds. Fatty tissue occurs where large amounts of fat are deposited in the cells. Cartilage is found where a large amount of firm support is required. With muscle we are all familiar; it is the real lean meat of the body.