=Chemical Constitution of the Body.=--In the body only twenty elements have been found. These include carbon, oxygen, hydrogen, nitrogen, sulphur, phosphorus, calcium, magnesium, manganese, chlorin, potassium, and fluorin. For the most part they appear in very complex and highly unstable combinations, though oxygen and nitrogen may be said to exist uncombined in the blood, alimentary canal, and lungs. Hydrogen also occurs in simple form in the alimentary canal, but as the result of fermentation, not as an element of the body. Of the organic compounds some contain nitrogen and some do not. The most important of the former are the proteins, which are found only in living bodies and consist of carbon, hydrogen, oxygen, nitrogen, and sulphur combined in very similar proportions. The important proteins in the body are the serum albumen and fibrin found in the blood, myosin in muscle, globulin in the red blood-corpuscles, and casein in the milk. Similar to the proteins but capable of passing through membranes are the peptones, the final result of protein digestion, from which the albuminoids differ in that they contain no sulphur. Ferments containing nitrogen exist in all the cells of the body, though more particularly in those of the digestive organs, and the coloring matters, as the bilirubin of the bile, are nitrogenous. The organic substances that do not contain nitrogen are the carbohydrates or starches, the hydrocarbons or fats, and the acids, of which the most important is carbon dioxide, given off by the lungs. 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.