PROXIMATE PRINCIPLES IN GENERAL. 39 



are not well defined, as compared with those of inorganic substances, 

 and their most striking properties are not such as can be accounted 

 for by ordinary chemical reactions or expressed in the usual chemical 

 phraseology. Nevertheless, they are of the first importance as ingredi- 

 ents of the organized frame, since they form a large proportion of its 

 mass, and contribute, by their peculiar properties, to its most essential 

 and characteristic active phenomena. They include such substances as 

 albumen, fibrine, caseine, and myosine. 



The fourth class is formed by 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 comprised a group of CRYSTALLIZABLE 

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

 physiological metamorphosis of albuminous principles. They are found 

 in some of the solid tissues, as the brain and nerves, in the secretions 

 of the liver, and especially in the urine, where they represent the pro- 

 ducts of excretion. 



