326 HAND-BOOK OF PHYSIOLOGY. 



simply to the compounds of the element Carbon, irrespective of their 

 complexity; chemists having found that these compounds are so numer- 

 ous and important, and that they include all those to which the term 

 organic was in former times exclusively given. 



Characteristics of Organic Compounds. The animal organic 

 compounds are characterized as a rule by their complexity, for in the 

 first place many elements enter into their composition, thereby distin- 

 guishing them from bodies such as water (H 2 0), hydrochloric acid (HC1), 

 and ammonia (NH 3 ), which may be taken as types of inorganic com- 

 pounds. And again, because many atoms of the same element occur 

 in each molecule. This latter fact no doubt explains also the reason of 

 the instability of organic compounds. 



Another great cause of the instability arises from the fact that many 

 such compounds contain the element Nitrogen, which may be called 

 negative or undecided in its affinities, and may be easily separated 'from 

 combination with other elements. 







Animal tissues, containing as they do these organic nitrogenous com- 

 pounds, are extremely prone to undergo chemical decomposition, and 

 this is especially the case since they also contain a large quantity of water, 

 a condition most favorable for the breaking up of such substances. It 

 is from this fact that in the consideration of the chemical basis of the 

 body we meet with an extremely large number of decomposition products. 



In treating of the various substances found in the animal organism 

 it is convenient to adopt the division into 



a ' Nitroenous. 



2. Inorganic. 



Ornrfnir 



Organic > Non _ Nitrogenous . 



1. ORGANIC. 



(a) Nitrogenous bodies take the chief part in forming the solid tissues 

 of the body, and are found to a considerable extent in the circulating 

 fluids (blood, lymph, chyle), the secretions and excretions. They contain 

 often in addition to Carbon, Hydrogen, Nitrogen, and Oxygen, the ele- 

 ments Sulphur and Phosphorus; but although the composition of most 

 of them is approximately known, no general rational formula can at 

 present be given. 



Several classes of animal nitrogenous bodies may be distinguished, 

 and it is convenient to consider them under the following heads: 



(I- 



(2. 



(3. 



Albuminoids or proteids. 

 Gelatinous substances. 

 Decomposition nitrogenous bodies. 



(4.) Certain supposed nitrogenous bodies, the exact composition of 

 which has not been made out. 



