20 THE BLOOD. 



Excluding for the present, all consideration of the products of dis- 

 assimilation, there remain the various constituents of the blood that are 

 more or less directly concerned in nutrition. 



Physiological chemists recognize certain chemical constituents of the 

 organism, which may be elementary substances, but which are more fre- 

 quently compounds. Sodium chloride is spoken of as a constituent of 

 the blood, because, as sodium chloride, it gives to the blood certain proper- 

 ties. The chemical elements, chlorine and sodium, are not regarded as con- 

 stituents of the blood, because they do not exist uncombined in the blood. 

 Still, a chemical constituent may be a chemical element, as in the case of 

 oxygen, which, as oxygen, has certain important uses in the economy; 

 although even oxygen probably is loosely combined in the body with other 

 matters. 



A chemical constituent of the blood or of any of the animal tissues or 

 fluids may be denned as a substance extracted from the body, which can not 

 be subdivided without chemical decomposition and loss of certain character- 

 istic properties. This definition will apply to all classes of chemical con- 

 stituents of the body, organic as well as inorganic. The chemical elements 

 of which the constituents are composed are properly the ingredients of 

 the body. 



The constituents of the blood, and, indeed, of the entire organism, may 

 be classified as follows : 



1. Inorganic Constituents. This class is of inorganic origin, definite 

 chemical composition and crystallizable. The substances included in this 

 class are all introduced from without and are all discharged from the body in 

 the same form in which they entered. They never exist alone, but are always 

 combined with the organic constituents, and form a part of the organized 

 fluids or solids. This union is so intimate that they are taken up with the 

 organic matters, as the latter are worn out and become effete, and are dis- 

 charged from the body, although themselves unchanged. To supply the place 

 of the constituents thus thrown off, a fresh quantity is deposited in the pro- 

 cess of nutrition. They give to the various organs important properties ; and 

 although identical with substances in the inorganic world, in the interior of 

 the body they behave as organic substances. They require no special prepara- 

 tion for absorption, but are soluble and taken in unchanged. They are re- 

 ceived into the body in about the same proportion at all periods of life, but 

 their discharge is notably diminished in old age, giving rise to calcareous in- 

 crustations and deposits and a considerable increase in the calcareous matter 

 entering into the composition of the tissues. Water, sodium chloride, the 

 carbonates, sulphates, phosphates and other inorganic salts may be cited as 

 examples of this class of constituents. 



The uses of water in the blood are sufficiently evident. It acts as a 

 solvent for the inorganic salts, the organic salts and the excrementitious 

 matters. In conjunction with the nitrogenized matters, it constitutes a 

 medium in which the corpuscles are suspended without solution. 



The various salts enumerated in the table exist in solution in water and are 



