It is plain that we cannot understand the physiology of our bodies without a knowledge of their anatomy. An engineer could not understand the working of his engine unless well acquainted with all its parts, and the manner in which they were fitted together. So, if we are to understand the principles of elementary physiology, we must master the main anatomical facts concerning the organs of the body before considering their special functions. As a branch of study in our schools, physiology aims to make clear certain laws which are necessary to health, so that by a proper knowledge of them, and their practical application, we may hope to spend happier and more useful, because healthier, lives. In brief, the study of hygiene, or the science of health, in the school curriculum, is usually associated with that of physiology.[1] 6. Chemical Elements in the Body. All of the various complex substances found in nature can be reduced by chemical analysis to about 70 elements, which cannot be further divided. By various combinations of these 70 elements all the substances known to exist in the world of nature are built up. When the inanimate body, like any other substance, is submitted to chemical analysis, it is found that the bone, muscle, teeth, blood, etc., may be reduced to a few chemical elements. In fact, the human body is built up with 13 of the 70 elements, namely: oxygen, hydrogen, nitrogen, chlorine, fluorine, carbon, phosphorus, sulphur, calcium, potassium, sodium, magnesium, and iron. Besides these, a few of the other elements, as silicon, have been found; but they exist in extremely minute quantities. The following table gives the proportion in which these various elements are present: Oxygen 62.430 per cent Carbon 21.150 ” ” Hydrogen 9.865 ” ” Nitrogen 3.100 ” ” Calcium 1.900 ” ” Phosphorus 0.946 ” ” Potassium 0.230 ” ” Sulphur 0.162 ” ” Chlorine 0.081 ” ” Sodium 0.081 ” ” Magnesium 0.027 ” ” Iron 0.014 ” ” Fluorine 0.014 ” ” ——— 100.000 As will be seen from this table, oxygen, hydrogen, and nitrogen, which are gases in their uncombined form, make up ¾ of the weight of the whole human body. Carbon, which exists in an impure state in charcoal, forms more than ⅕ of the weight of the body. Thus carbon and the three gases named, make up about 96 per cent of the total weight of the body.