NITROGENOUS OEGANIC CELL-CONSTITUENTS. 109 



insoluble in alcohol, ether, ammonia, or acetic acid, but it dissolves in 

 caustic potash. Its solutions, however, do not gelatinize. When once 

 dissolved in caustic potash the alkali may be neutralized without throw- 

 ing the elastin out of solution. Tannic acid is the only acid which will 

 precipitate it. Elastin gives the xantho-proteic and Millon's reactions, 

 and its place among the albuminoids, therefore, seems warranted. When 

 boiled for a long time with sulphuric acid it undergoes decomposition, 

 with the formation of leucin and tyrosin. Elastin contains no sulphur. 



4. Keratin. — The epithelial tissues of the animal bodj- — nails, bone, 

 epidermis, and epithelium, as well as horns and feathers — are mainly 

 composed of a substance closely allied to albumen, as it gives leucin and 

 tyrosin on decomposition, to which the name of keratin has been given. 



Keratin contains sulphur in loose combination, and is in some re- 

 spects closely related to elastin. Keratin is insoluble in alcohol and 

 ether, swells up in boiling water, and is soluble in the caustic alkalies. 

 It is not liable to decomposition. When one of the epithelial structures, 

 such as horn, is subjected to the action successively of boiling water and 

 alcohol, ether, and dilute acids, this substance, keratin, remains behind. 

 But when so obtained it has by no means a constant composition, 

 and it is probable, therefore, that keratin is rather a mixture of several 

 nitrogenous bodies than a single albuminoid. 



Decomposition of the Albuminous Bodies. — As already mentioned, 

 albuminous bodies are the most unstable of all organic compounds, and 

 we have the strongest reason for believing that, even while in the interior 

 of animal and vegetable organisms, the albuminous constituents of proto- 

 plasm are continually the seat of various forms of decomposition which 

 result in the production of simpler organic and inorganic forms. As we. 

 know but very little as to the molecular constitution of the proteid 

 bodies, nothing positive can be said as to the complex chemical processes 

 which result in the production of simpler organic forms. The subject 

 has been a favorite field of research for organic chemists, but as yet 

 scarcely anything tangible has resulted from their labors. An immense 

 amount of valuable information has been attained, but the applicability 

 of the facts so reached to physiological processes is not as yet clearly 

 assured. The chief end products of the decomposition of proteids in 

 the animal cell, which is essentially one of oxidation, are water, carbon 

 dioxide, and urea. What the nature of the substances are which are in- 

 termediary between these end products and albuminoids we do not clearly 

 know, except that certain ones, such as leucin, tyrosin, certain of the 

 carbo-hydrates, such as glycogen and fats, are of constant occurrence 

 and of great importance. The subject of the decomposition of albumen 

 under various chemical and physical agents is an extremely interesting 

 one, but it falls more within the province of works on organic chemistry. 



