go THE CHEMICAL COMPOSITION OF THE BODY 



alcohol, and ether, and in the ordinary protein solvents. They are not 

 acted upon by gastric or pancreatic juices. On heating to 150 to 200 C. 

 in water the protein is hydrolyzed and dissolves. The keratins are also 

 soluble in the caustic alkalies, especially on heating. Keratins from any 

 source may be prepared in pure form by treating with artificial gastric juice, 

 artificial pancreatic juice, boiling alcohol, and boiling ether, from twenty- 

 four to forty-eight hours being devoted to each process. Several keratins, 

 so far as their chemical structure is concerned, exist. 



Collagen. Collagen can be most satisfactorily prepared from the tendo 

 achillis of the ox. It forms the principal organic constituent of this and 

 other white fibrous tissues, as shown by the analysis given in the following 

 table: 



CHEMICAL COMPOSITION OF THE WHITE FIBROUS TISSUE: Tendo Achillis 



(BERG AND GIES). 



Water 62 . 87 per cent. 



Solids 37 . 13 per cent. 



Inorganic matter 0.47 



Organic matter 36 . 66 



Fatty substance (ether-solu- 

 ble) i . 04 



Coagulable protein 0.22 



Mucoid 1.28 



Elastin 1.63 



Collagen 31 . 59 



Extractives, etc 0.90 



The collagen from various sources in common with the keratin is not 

 identical in composition. It differs from the keratin in containing less 

 sulphur. It does not give the reaction for tryptophane and contains but 

 little tyrosin. It is dissolved by pepsin and hydrochloric acid, but not by 

 pancreatic juice. 



The general characteristic of collagen is that it is hydrolyzed into gelatin 

 by boiling with water or dilute acid. Gies has shown that ammonia is 

 liberated by this procedure. Gelatin is soluble in hot water, but its solutions 

 form a jell when cooled. Inasmuch as tyrosin and tryptophane are not 

 present in the gelatin molecule, this albuminoid is not a satisfactory substi- 

 tute for the protein constituents in the normal diet. 



Elastin. Elastin is the principal solid constituent of yellow elastic 

 tissue; e.g., the ligamentum nucha. It gives the ordinary protein color re- 

 actions. It contains, however, a relatively small amount of sulphur. Elas- 

 tin is dissolved by pepsin hydrochloric acid and by pancreatic juice, and 

 unlike collagen it is not converted into gelatin on prolonged boiling with 

 water or dilute acids. 



