112 THE PROTEIN SUBSTANCES. 



resistance to reagents which dissolve proteins, or to chemical reagents 

 in general, and it is due to these external properties that they are put 

 in a special group. From a purely chemical standpoint there is no 

 reason why they should be separated from the true proteids in a special 

 group. Most of the bodies belonging to the albuminoids have been 

 given on page 91. 



The Keratins. Keratin is the chief constituent of the horny struc- 

 ture of the epidermis, of hair, wool, of the nails, hoofs, horns, feathers, 

 of tortoise shell, etc., etc. Keratin is also found as neurokeratin (KUHNE) 

 in the brain and nerves. The shell membrane of the hen's egg seems 

 also to consist of keratin, and according to NEUMEISTER 1 the organic 

 matrix of the eggshells of various vertebrate animals belongs in most 

 cases to the keratin group. 



It seems that there exist a number of keratins, and these form a special 

 group of bodies. This fact, together with the difficulty in isolating the 

 keratin from the tissues in a pure condition without a partial decom- 

 position, is sufficient explanation for the variation in the elementary 

 composition given below. As examples the analyses of a few tissues 

 rich in keratin and of keratins are given : 2 



c H N s o 



Human hair ... 50.65 6.36 17.14 5.00 20.95 (v. LAAR) 



Nail 51.00 6.94 17.51 2.80 21.75 (MULDER) 



Neurokeratin . . 56.11-58.45 7.26-8.02 11.46-14.32 1.63-2.24 (KUHNE) 



Neurokeratin. . . 56.61 7.45 14.17 2.27 (ARGIRIS) 



Horn (average). 50.86 6.94 3.20 (HORBACZEWSKI) 



Tortoise shell .. 54.89 6.56 16.77 2.22 19.56 (MULDER) 



Shell membrane 49.78 6.64 16.43 4.25 22.90 (LINDVALL) 



Egg membrane. 53.92 7.33 15.08 1.44 (PREGL) 



(Scyllium) 



MoHR 3 has determined the quantity, of sulphur in various keratin 

 substances. Sulphur is in great part in loose combination, and it is 

 chiefly removed by the action of alkalies (as sulphides), or indeed in part 

 by boiling with water. Combs of lead after long usage become black, 

 and this is due to the action of the sulphur of the hair. On heating keratin 

 with water in sealed tubes to a temperature of 150 C. or higher, it dis- 

 solves with the elimination of sulphuretted hydrogen or mercaptan 

 (BAUER), and the solution contains proteose-like substances (KRUKEN- 



1 Kiihne and Ewald, Verb. d. naturhistor.-med. Vereins zu Heidelberg (N. F.), 1; 

 also Kiihne and Chittenden, Zeitschr. f. Biologic, 26; Neumeister, ibid., 31. 



2 v. Laar, Annal. d. Chem. u. Pharm., 45; Mulder, Versuch einer allgem. physiol. 

 Chem., Braunschweig, 1844-51; Kuhne, Zeitschr. f. Biologic, 26; Horbaczewski, see 

 Drechsel in Ladenburg's Handworterbuch. d Chem., 3; Lindvall, Maly's Jahres- 

 bericht, 1881; Argiris, Zeitschr. f. physiol. Chem., 54; Pregl., ibid., 56. 



3 Zeitschr. f. physiol. Chem., 20. 



