The inorganic substances are water, which forms a large percentage of all the tissues and from one-fourth to one-third of the whole body weight, sodium chloride or common salt, which plays an important part in keeping substances in solution, potassium and magnesium chloride, and hydrochloric acid, found in the stomach.
The Cell.—Although the body is a very complex organism, the cell is its unit or foundation. In fact, the body begins life as a single protoplasmic cell, the ovum, which is frequently compared to the amœba, a microscopic animal consisting of a single cell of protoplasm or living substance—a substance not well understood as yet—but possessing practically all the functions of the human body. For, although it has no organs and is homogeneous in structure, the amœba can move by throwing out a process, and can surround and absorb food, which it builds up into new tissue, discarding the waste. The ovum, however, differs from the amœba in that it has a transparent limiting membrane and contains a darker spot, the nucleus. This in turn contains another smaller spot, the nucleolus, while through the protoplasm, which is semi-fluid, extends a fine network that seems to hold it in place.
The ovum is very small, about ¹/₁₂₅ inch in diameter, and after fertilization grows by segmentation, the nucleus dividing in two and the protoplasm grouping itself anew about the two nuclei. This division continues, each cell dividing and forming two, or sometimes four, new cells, all of which at first appear alike. By degrees, however, differentiation takes place and different groups of cells assume different characteristics. Thus the various tissues are gradually developed, each with a structure and a function of its own, and are distributed among the various organs, each organ consisting of several tissues. During the process of growth and even after full growth of the body is attained old cells are continually dying and being replaced by new ones.
The typical cell is circular, but through being squeezed together in the tissues or for some other reason the cells vary in shape in different parts, being at times hexagonal, spindle-shaped, or columnar. Yet, whatever their differences in shape or other characteristics, they all live the same sort of life. All protoplasm absorbs oxygen when it comes in contact with it and in the process of combining with it is in part burned or oxidized, with the consequent setting free of heat and other forms of energy and the formation of carbon dioxide. So long as the body is alive, therefore, whether it is in a state of activity or of rest, it is the seat of constant chemical change throughout all its cells, and to these chemical changes are due all the forms of energy manifested by the body. For energy is never destroyed, though it may appear in a different form, and the elements of the human body are so combined that their energy may be liberated and manifested in the different functions the body exhibits.
The fundamental tissues of the body are the [epithelial tissues], the connective tissues, including the cartilaginous and bony tissues, and the muscular and nervous tissues. Of these the epithelial tissues serve as a protection to the surface of other tissues; the connective tissues together form a framework for the support and general protection of the other tissues; while energy is expended by muscular and nervous tissue, the latter directing the former in its movements. All the tissues are inter-dependent and the organs work together. Besides cells every tissue contains a certain amount of lifeless matter, the intercellular substance, which was at some time produced by the cells.
Fig. 1.—Epithelium: 1, pavement epithelium; 2, columnar epithelium; 3, ciliated epithelium; 4, stratified epithelium.
In epithelial tissue there is little intercellular substance, the cells being close together and arranged generally as a skin or membrane covering external or internal surfaces. When there are several layers of cells, the deepest are columnar in shape and the others become more and more flattened and scale-like as they approach the surface, where they are gradually rubbed off and replaced by the growth of new cells from below. This [stratified epithelium], as it is called, is found wherever a surface is exposed to friction, as in the skin and in the mucous membrane of the mouth, pharynx, and esophagus, and in that of the vagina and the neck of the uterus. In simple epithelium, where there is only a single layer of cells, the cells may be pavement or hexagonal, columnar, glandular, or ciliated, according to their different functions. The flat [pavement cells] occur where a very smooth surface is required, as in the heart, lungs, blood-vessels, serous cavities, etc. None of these surfaces communicate directly with the external surface of the body and the name endothelium is substituted for epithelium. The columnar form of cell in the intestine facilitates the passage of leucocytes between the cells. In glandular epithelium the cells vary according to the gland in which they occur, their protoplasm being filled with the material the gland secretes. Finally, [ciliated epithelium] is composed of columnar cells with cilia or little hair-like processes upon their free surface which serve to send secreted fluids and other matters along the surfaces where they occur, as in the air passages, parts of the generative organs, the ventricles of the brain, and the central canal of the spinal cord.
Connective tissue has a great deal of intercellular substance. One form, areolar tissue, is composed of a loose network of fine white fibers with a few yellow elastic fibers interspersed and with cells lying in the spaces between the fibers. It connects and surrounds the different organs and parts, holding them together, yet allowing free motion, and is one of the most extensively distributed of the tissues. It is continuous throughout.
