xi THE ALBUMINOIDS : FIBROIN OF SILK 571 



with alkali, gave all the colour tests excepting the lead sulphide 

 reaction. Superheated steam, according to Horbaczewski l and 

 Schwarz, 2 and peptic and tryptic digestion, according to Horbaczewski l 

 and Chittenden and Hart, 3 give rise to albumoses. 



Elastic tissue is, however, so slowly acted upon by pepsin that 

 Stirling in 1873, while working in Lud wig's laboratory, optically 

 isolated the elastic fibres in the skin by digesting the latter with 

 artificial gastric juice, which renders all other elements, excepting 

 nerves and nuclei, indistinct. 4 Subsequently Ewald and Kiihne 5 

 treated various organs with slightly alkaline glycerine extract of the 

 pancreas. The results obtained by them are given in the author's 

 Physiological Histology, p. 153. Ewald 6 has subsequently extended his 

 research. He found elastin to be very slowly soluble in both pepsin 

 and trypsin ; and. that it was more readily acted upon by digestive 

 enzymes if it had previously been boiled or had been acted upon by 

 acids or by alcohol. Barium tetroxide renders fibrils indigestible for 

 pepsin, but more readily digestible for trypsin, while chromic acid, 

 if light has access, produces exactly the opposite effect. 



4. Fibroin and Silk Glue 



Natural silk consists of delicate fibres composed of a core of 

 fibroin surrounded by an envelope of a glue-like substance. Raw silk 

 contains therefore, apart from salts, a mixture of fibroin and silk-glue 

 or sericin. Both these bodies have been investigated by Mulder, 7 

 Stadeler, 8 Cramer,9 Weyl, 10 Vignon, 11 Wetzel, 12 and E. Fischer and 

 Skita. 13 Lombardy silk yields about 70 percent of fibroin and 30 per 

 cent of glue. Even technically purified silk contains still about 5 per 

 cent of glue. 13 



Fibroin is insoluble even in superheated water, in dilute acids and 



I J. Horbaczewski, Zeitschr. f. physiol. Chem. 6. 330 (1882) ; Monatsh. f. Chem. Q. 

 639 (1885). 2 H. Schwarz, ibid. 18. 487 (1893). 



3 R. H. Chittenden and A. S. Hart, Zeitschr. f. Biol. 25. 368 (1889). 



4 W. Stirling, Ber. Verh. d. konigl. Sachs. Gesellsch. d. Wissensch. Leipzig, 26. 

 221 (1874) ; and in Journ. of Anat. and Physiol. (1875). 



5 A. Ewald and W. Kiihne, 'Die Verdauung als histologische Methode,' Verh. 

 naturhist. Vers. Heidelberg (N.F.), 1. 451. 



6 A. Ewald, Zeitschr. f. Biol. 26. 1 (1890). 



7 According to E. Fischer and Skita. 



8 G. Stadeler, Liebig's Annalen, 111. 12 (1859). 



9 E. Cramer, Journ. f. prakt. Chem. 96. 76 (1865). 



10 Th. Weyl, Ber. d. deutsch. chem. Gesellsch. 21. II. 1407 ; 21. II. 1529 (1888). 



II L. Vignon, Compt. rend. 115. 613 (1892). 



12 G. Wetzel, Zeitschr. f. physiol. Chem. 26. 535 (1899). 



13 E. Fischer and Skita, ibid. 33. 171 (1901) ; 35. 224 (1902) 



