122 



THE PROTEIN SUBSTANCES. 



According to TEBB reticulin is only a somewhat changed, impure collagen 

 but this is disputed by SIEGFRIED. l 



It may be prepared as follows, according to SIEGFRIED: Digest intes- 

 tinal mucosa with trypsin and alkali. Wash the residue, extract with 

 ether, and digest again with trypsin and then treat with alcohol and ether. 

 On careful boiling with water the collagen present either as contamina- 

 tion or as a combination with recticulin is removed. The thoroughly 

 boiled residue consists of reticulin. 



Ichthylepidin is an organic compound, so-called by C. MORNER,* which occurs 

 with collagen in fish-scales and forms about one-fifth of their organic substance. 

 This compound, with 15.9 per cent nitrogen and 1.1 per cent sulphur, stands on 

 account of its properties rather close to elastin. It is insoluble in cold and hot 

 water, as well as in dilute acids and alkalies at the ordinary temperature. On 

 boiling with these it dissolves. Pepsin-hydrochloric acid, as well as an alkaline 

 trypsin solution, also dissolves it. It responds beautifully to MILLON'S reagent, 

 the xanthoproteic reaction, and the biuret test. At least a part of the sulphur 

 is split off by the action of alkali. Ichthylepidin stands very close to elastin 

 in regard to its solubilities; but it differs essentially in composition as it is markedly 

 poorer in glycocoll, but much richer in proline and glutamic acid (ABDERHALDEN 

 and VOITINOVTCI 3 ). 



As skeletins, KRUKENBERG 4 has designated a number of nitrogenized 

 substances which form the skeletal tissue of various classes of inverte- 

 brates. These substances are chitin, spongin, conchiolin, byssus, cornein, 

 and crude silk (fibroin and sericin). Of these, chitin does not belong to 

 the protein substances, and silk is hardly to be classed as a skeletin. 

 Only those so-called skeletins will be discussed that actually belong to 

 the protein group, and chitin will be discussed in another chapter. 



The elementary composition of certain of the bodies belonging to 

 this group is as follows : 5 



1 Tebb, Journ. of Physiol., 27; Siegfried, ibid., 28. 



2 Zeitschr. f . physiol. Chem., 24 and 37. See also Green and Tower, ibid., 3u. 



3 Zeitschr. f . physiol. Chem., 52, p. 368. 



4 Grundziige einer vergl. Physiol. d. thier. Geriistsubst. Heidelberg, 1885. 



5 Krukenberg, Ber. d. d. chem. Gesellsch., 17 and 18, and Zeitschr. f. Biologic, 22; 

 Croockewitt, Annal. d. Chem. u. Pharm., 48; Posselt, ibid., 45; Cramer, Journ. f. 

 prakt. Chem., 96; Vignon, Compt. rend., 115; Wetzel, Zeitschr. f. physiol. Chem., 29 

 and Centralbl. f. Physiol., 13, 113; Bondi, Zeitschr. f. physiol. Chem., 34. 



