PROXIMATE PRINCIPLES. 25 



SECOND. Substances which are the result of the wearing 

 out of the tissues, and are not used in nutrition. 1 



The first division, which is the only one that will be taken 

 up in this connection, may be subdivided, according to the 

 classification of Robin and Yerdeil, into three classes. 



1. Inorganic Substances. This class is of inorganic ori- 

 gin, definite chemical composition, and crystallizable. The 

 substances forming it 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 principles, to form the organized fluids or 

 solids. This union is " atom-to-atom," and so intimate that 

 they are taken up with the organic elements, as the latter are 

 worn out and become effete, and are discharged from the 

 body, though themselves unchanged. To supply the place 

 of the principles thus thrown off, a fresh quantity is depos- 

 ited in the process of nutrition. They give to the various 

 organs important properties ; and, though identical with sub- 

 stances in the inorganic world, in the interior of the body 

 they behave as organic substances. They require no special 

 preparation for absorption, but are soluble and taken in un- 

 changed. They are received into the body in about ,the same 

 proportion at all periods of life, but their discharge is nota- 

 bly diminished in old age ; giving rise to calcareous incrusta- 

 tions and deposits, and a considerable increase in the calca- 

 reous matter entering into the composition of the tissues. 

 As examples of this class we may cite water, chloride of so- 

 dium, the carbonates, sulphates, phosphates, and other inor- 

 ganic salts. 



2. Organic Non-Nitrogenized Substances. This class of 



1 This division is composed of excrementitious matters, which will be fully 

 considered when treating of excretion. It is included in the second class of prox- 

 imate principles by Robin and Verdeil. 



