34 PHYSIOLOGICAL CHEMISTRY. 



are well marked and easily recognizable. They first make their appear- 

 ance in the interior of organized bodies, and are riot found in the inor- 

 ganic world, excepting as the remains or products of animal or vege- 

 table life. To this group belong the several varieties of starch, sugar, 

 and oil. 



The third class comprises the ALBUMENOID or nitrogenous organic 

 matters. These substances derive their name from the albumen or 

 white of an egg, which was among the earliest to be studied, and 

 which may be considered as a representative of the whole class. They 

 differ from the substances of the two preceding groups, especially in the 

 fact that they contain nitrogen as an ingredient, in addition to the 

 three elements of the hydrocarbonaceous matters. They are exclu- 

 sively of organic origin, appearing only as ingredients of the living 

 body. Their chemical constitution is a complicated one that is, their 

 four elements are united in such a way as to form compounds of a very 

 high atomic weight, which has not, however, been determined with 

 sufficient precision to give an exact chemical formula. Their reactions 

 with other substances are not well defined, as compared with the inor- 

 ganic constituents, and their most striking physiological properties are 

 not such as can be expressed in chemical phraseology. Nevertheless, 

 they are of the first importance as ingredients of the organized frame, 

 since they form the largest portion of its mass, and contribute directly 

 to its most active phenomena. They include such substances as 

 albumen, caseine, ptyaline, pepsine, and myosine. 



The fourth class is composed of the COLORING MATTERS. These sub- 

 stances, upon which the different tints of the solids and fluids depend, 

 are present, for the most part, in small quantity, the most abundant 

 being the red coloring matter of the blood. 



Lastly, in the fifth class are embraced a group of CRYSTALLIZABLE 

 NITROGENOUS MATTERS, many, if not all, of which are derived from the 

 physiological metamorphosis of albumenoid substances. They are 

 found in some of the solid tissues, as the brain and nerves, in the secre- 

 tions of the liver, and especially in the urine, where they represent the 

 products of excretion. 



