Introduction 7 



master a few of the main anatomical facts concerning 

 the organs of the body before considering their special 

 functions. 



NOTE. A more accurate title for the subject-matter of this 

 branch of study as now treated in text-books for elementary schools 

 would be " Anatomy, Physiology, and Hygiene." By common usage, 

 however, the word " Physiology " is quite generally used as a more 

 condensed and convenient name. 



CHEMICAL COMPOSITION OF THE BODY 



9. Chemical Elements in the Body. All the various com- 

 plex substances found in nature can be reduced by chemi- 

 cal 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 a human body is submitted to chemi- 

 cal analysis, it is found that the bones, muscles, teeth, 

 blood, etc., may be reduced to a few chemical elements. 



In fact, the human body is almost entirely composed of 

 1 3 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 and iodine, have been found ; but 

 they exist in extremely minute quantities. 



Oxygen, hydrogen, and nitrogen, which are gases in 

 their uncombined form, make up three-fourths of the weight 

 of the whole human body. Carbon, which exists in an 

 impure state in charcoal, forms more than one-fifth 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. 



V We must keep in mind that, with slight exceptions, none 

 of these elements exist in their elementary form in the 



