THE ALBUMINOIDS. 47 



world, and form the greater portion of the internal as well as the 

 external skeleton. The most important members of the group are 

 the keratins, the principal constituents of the epidermal structures of 

 the animal body ; elastin, which is found in the connective tissue of 

 the higher animals ; collagen, which is present also in connective 

 tissue and in the organic portions of the bones ; gelatin, or glutin, 

 which is soluble collagen, viz., collagen plus water; the skeletins, 

 spongin, conchiolin, kornein, fibroin, sericin, and elastoidin, which 

 have been mentioned as occurring among the invertebrate animals ; 

 and, finally, the so-called amyloid substance, which is encountered 

 under various pathologic conditions. Of these various substances, 

 the keratins and elastin are more closely related to the albumins 

 proper than the remainder. They both give rise to the same 

 decomposition-products as the albumins, though elastin yields but 

 very little tyrosin and no glutaminic or asparaginic acid. They give 

 the various color-reactions of the albumins, but it appears that 

 eiastin only contains in its molecule that form of sulphur which is 

 easily split off on boiling with dilute alkalies. Keratin, on the 

 other hand, contains much sulphur 3 to 5 per cent. ; and it is inter- 

 esting to note that during its decomposition a fairly large proportion 

 may be obtained in the form of cystin. Both the keratins and 

 elastin can be brought into solution only by means of superheated 

 steam or by boiling with strong alkalies, but the substance is at the 

 same time decomposed. Concentrated mineral acids also dissolve 

 elastin with varying ease, and with or without the application of 

 heat, according to the origin of the material. 



Collagen and its hydrate glutin, or gelatin, on the other hand, are 

 structurally further removed from the true albumins. They appar- 

 ently contain no aromatic radicle, and hence on decomposition yield 

 neither tyrosin nor indol. Leucin, glycocoll, glutaminic acid, and 

 asparaginic acid are, however, always obtained. Pure solutions of 

 gelatin give the biuret reaction and the xanthoproteic reaction, while 

 the reactions of Millon and Adamkiewicz are negative. The sulphur 

 is apparently present only as closely combined sulphur, as hydrogen 

 sulphide does not develop on boiling with dilute alkalies. 



Solutions of collagen gelatinize on cooling and redissolve on the 

 application of heat. The behavior of the substance is in this 

 respect exactly the contrary of what we see in the albumins proper. 

 The mineral acids, potassium ferrocyanide in the presence of acetic 

 acid, and most mineral salts do not precipitate the gelatin from its 

 solutions. 



Solutions of cartilaginous glutin, which was formerly termed 

 chondrin, possess characteristics which are different from those of 

 glutin that is obtained from connective tissue or decalcified bones. 

 These differences, however, are not owing to the glutins as such, but 

 to the presence of certain soluble compounds of chondroitin-sul- 

 phuric acid, the chondroitin radicle of which, as we have seen, 

 belongs to the so-called hyalins. 



