ELEMENTS, INORGANIC MATERIALS, WATER 23 



her of compounds with hydrogen, oxygen, and sometimes sul- 

 phur, phosphorus and other elements. These compounds are 

 so numerous that a separate branch of chemistry is devoted to 

 them under the name of organic chemistry. Thirty per cent or 

 more of the body consists of organic material, or compounds 

 of carbon with hydrogen, oxygen, etc. The importance of these 

 compounds is so great that special chapters will be devoted to 

 the important groups. The body receives its carbon compounds 

 in the foods, the solid portion of which is very largely organic 

 material. Aside from the organic components of the tissues, 

 carbon is also found in inorganic form, chiefly as calcium car- 

 bonate in bone, and as sodium bicarbonate in the blood. This 

 latter compound is of great importance, since, although its 

 solution is neutral, it has the power of neutralizing acids, thus 

 protecting the tissues when acids are introduced into the blood 

 either from the digestive tract or as the result of the breaking 

 down of body materials. Carbon is eliminated from the body 

 as carbon dioxide, given off through the lungs, or as more com- 

 plex compounds such as urea and other materials excreted in 

 the urine or the feces. If heated, organic compounds char, 

 leaving a black residue of carbon. 



Hydrogen and oxygen aside from being constituents of water, 

 organic and inorganic compounds, also play more specialized 

 roles in the body. Oxygen, which makes up about two-thirds 

 of the entire body weight is taken up as a constituent of the 

 compounds in the food and in the process of respiration. Hy- 

 drogen, the presence of which in ionic form is the distinguishing 

 characteristic of acids, plays an important part in the digestion 

 of proteins by gastric juice. Hydrogen and oxygen are elimi- 

 nated from the body chiefly as water and C0 2 given off through 

 the lungs, and as water and in organic compounds in the urine 

 and feces. On heating organic materials they decompose, and 

 the hydrogen and oxygen are given off as water and various 

 other compounds. 



Nitrogen, sulphur and phosphorus also play various roles in 

 the body and all three are found both in organic and inorganic 



