APPENDIX. 733 



elements may enter into their composition, thereby distinguishing them 

 from bodies as simple as water (H 2 0), hydrochloric acid (HC1), and am- 

 monia (N H 5 ), which may be taken as types of inorganic compounds. 

 Many atoms of the same element also may occur in each molecule. This 

 latter fact no doubt explains also the reason of the instability of these 

 compounds. Another great cause of the instability is the frequent 

 presence of 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. They 

 also contain a large quantity of water, a condition most favorable for the 

 breaking up of such substances. It is due to this tendency to decompo- 

 sition that we meet with so large a number of decomposition products 

 among the chemical substances forming the basis of the animal body. 



The various substances found in the animal organism may be conve- 

 niently considered according to the following classification: 



1 n ( a. Nitrogenous. 



1. Ogamc, j b< Non _f itrogenous . 



2. Inorganic. 



1. Organic. 



(a. ) Nitrogenous bodies take the chief part in forming the solid tis- 

 sues of the body, and are found also to a considerable extent in the cir- 

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

 They often contain in addition to Carbon, Hydrogen, Nitrogen, and 

 Oxygen, the elements Sulphur and Phosphorus ; but although the com- 

 position of most of them is approximately known, no general rational 

 formula can at present be given. 



Several classes of organic nitrogenous bodies may be distinguished, 

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



(1.) Pro teids or albuminoids. 



(2.) Gelatinous substances. 



(3. ) Decomposition nitrogenous bodies. 



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

 not been made out. 



(1.) Proteids or Albuminoids are the most important of the nitro- 

 genous animal compounds, one or more of them entering as essential 

 parts into the formation of all living tissue. In the lymph, chyle, and 

 blood, they also exist abundantly. Their atomic formula is uncertain. 

 Their composition, according to Hoppe-Seyler, may be taken to be: 



Carbon, from 51.5 to 54.5; Hydrogen, from 6.9 to 7.3; Nitrogen, 

 from 15.2 to 17.; Oxygen, from 20.9 to 23.5; Sulphur, from .3 to 2. 



