CHEMICAL COMPOSITION OF THE HUMAN BODY 39 



all the characteristic color reactions, because some do not 

 have the aromatic groups, hence all of them do not yield 

 tyrosin when boiled with alkalies. They do not dissolve 

 but swell up in water. They are not coagulated by heat. 



Physiologically they differ from the proteids in that they 

 are either indigestible, or, if they can be digested and 

 absorbed, they cannot replace the used-up body proteids as 

 the other proteids can. 



Also in regard to their functions in building up the body, 

 there is a great difference between the albuminoids and the 

 simple and combined proteids. The latter form the bases 

 of the living substance, and as such are the chief constituents 

 of the cells. The albuminoids, on the contrary, are present 

 only as intracellular substances ; they are indeed cell- 

 products and perhaps take a part in cellular metabolism, but 

 their chief physiological importance lies in their furnishing 

 the material for covering and framework. They form the 

 organic ground-substance of bones, cartilage, tendons, fascise, 

 connective tissue and of the covering of the body epidermis, 

 hair and nails. They are the most important organic con- 

 stituents which furnish form and stability to the body. 



The albuminoids are specific animal products. They are 

 formed by the cells themselves, being the intracellular sub- 

 stance. In the epidermal cells all the protoplasm is changed 

 to a certain kind of albuminoid substance. They are formed 

 from the proteids of the cells by chemical changes the 

 nature of which is still unknown. 



Albuminoids unite with acids and alkalies ; in some of 

 them the amido-acid character is even more apparent than 

 in the other proteids. By digestion, albuminoids, if at all 

 digestible, yield proteoses-like products. 



Among the albuminoids are : 



i. Keratin, the chief constituent of the horny epithelial cells, 

 hair, nails, and of the membrane of nerves (here called neuro- 

 keratin). It is rich in sulphur (25^), most of which is loosely 

 combined so that it is easily split off by alkali. It gives all the 

 proteid reactions and in decomposing yields tyrosin. It is not 



