PROXIMATE PRINCIPLES OF THE THIRD CLASS. 95 



CHAPTER IV. 



PROXIMATE PRINCIPLES OF THE THIRD CLASS. 



THE substances belonging to this class are very important, and 

 form by far the greater part of the entire mass of the body. They 

 are derived both from animal and vegetable sources. They have 

 been known by the name of the "protein compounds" and the 

 " albuminoid substances." The name organic substances was given 

 to them by Kubin and Yerdeil, by whom their distinguishing pro- 

 perties were first accurately described. They have not only an 

 organic origin, in common with the proximate principles of the 

 second class, but their chemical constitution, their physical struc- 

 ture and characters, and the changes which they undergo, are all so 

 different from those met with in any other class, that the term " or- 

 ganic substances" proper appears particularly appropriate to them. 



Their first peculiarity is that they are not crystallizable. They 

 always, when pure, assume an amorphous condition, which is some- 

 times solid (organic substance of the bones), sometimes fluid (albu- 

 men of the blood), and sometimes semi-solid in consistency, midway 

 between the solid and fluid condition (organic substance of the 

 muscular fibre). 



Their chemical constitution differs from that of bodies of the 

 second class, first in the fact that they all contain the four chemical 

 elements, oxygen, hydrogen, carbon, and nitrogen; while the 

 starches, sugars, and oils are destitute of the last named ingredient. 

 The organic matters have therefore been sometimes known by the 

 name of the " nitrogenous substances," while the sugars, starch, and 

 oils have been called " non-nitrogenous." Some of the organic mat- 

 ters, viz., albumen, fibrin, and casein, contain sulphur also, as an in- 

 gredient ; and others, viz., the coloring matters, contain iron. The 

 remainder consist of oxygen, hydrogen, carbon, and nitrogen alone. 



The most important peculiarity, however, of the organic sub- 

 stances, relating to their chemical composition, is that it is not 

 definite. That is to say, they do not always contain precisely the 

 same proportions of oxygen, hydrogen, carbon, and nitrogen ; but 



