THE CHEMICAL COMPOSITION OF THE BODY. Ill 



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

 pounds, are extremely prone to undergo decomposition. They also con- 

 tain much water, a circumstance very favorable to the breaking up of such 

 substances. It is due to this tendency to decomposition 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 conven- 

 iently considered according to the following classification : 1. Organic 

 a. Nitrogenous and b. Non-Nitrogenous. 2. Inorganic. 



Organic Substances. 



Nitrogenous organic bodies take the chief part in forming the solid tis- 

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

 lating 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 composition of most 

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

 ent be given. 



It will be convenient to give an account of the Proteid substances in 

 this Chapter, as these constitute the most important classes of nitrogen- 

 ous organic substances. According to their chemical composition or su- 

 perficial differences (e.g., solubility) they are divided into three main 

 classes, viz. : (1) Simple proteids, (2) compound proteids, and (3) albu- 

 menoids or proteoids. The other members are Decomposition products, the 

 chief of which is Urea, found for the most part in the urine; Ferments ; 

 Pigments ; and other bodies and will be more appropriately treated of 

 later on. 



Proteids (simple proteids) are also called Albuminous substances. 

 They are the chief of the nitrogenous organic compounds and exist in 

 both plants and animals, one or more of them entering as an essential 

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

 blood, they exist abundantly. Very little is known with any certainty 

 about their chemical composition. Not a single member of the class lias 

 yet been synthesized. Their formula is unknown, the chemists who have 

 attempted to construct it differing very greatly among themselves. In 

 fact the very term proteid is an extremely arbitrary one. It simply 

 means a body which, according to Hoppe-Seyler, contains in its molecule 

 the elements carbon, hydrogen, nitrogen, oxygen, and sulphur, in certain 

 arbitrary but varying amounts, thus Carbon, from 51.5 to 54.5; Hy- 

 drogen, from 6.9 to 7.3; Nitrogen, from 15.2 to 17. ; Oxygen, from 

 20.9 to 23.5; Sulphur, from .3 to 2. Some proteids contain from .3 to 

 1.5 of phosphorus; a small amount of iron is usually associated with 



