1204 KERATIN. 



some of the primary substances (elastoses) make their appear- 

 ance, since they are completely precipitated by saturation with 

 neutral ammonium sulphate. Elastin is also rapidly corroded 

 and dissolved by the action of papain. 



Keratin. 



Hair, nails, feathers, horn and the epidermal structures in 

 general are composed chiefly of keratin, admixed however with 

 small quantities of proteids and other substances, from which 

 it may be freed by thorough extraction with water, alcohol, 

 ether and dilute acids in succession, followed by digestion with 

 pepsin and trypsin and a renewed washing with the above 

 reagents. A convenient source which readily yields a pure 

 product, owing to the comparatively simple composition of the 

 mother substance, is found in the shell-membrane of ordinary 

 eggs. The percentage composition of keratin is in general 

 allied to that of the true proteids, but varies within somewhat 

 wide limits according to the source from which it has been pre- 

 pared and particularly with regard to the sulphur which it con- 

 tains. This latter element varies in amount from '5 to 5*0 p.c. 

 and leads to the formation of sulphides of the metal when kera- 

 tin is dissolved in alkalis. Unlike the proteids, gelatin and 

 elastin, keratin is quite unaffected by the most prolonged and 

 active digestion with either pepsin or trypsin. On the other 

 hand, when decomposed at high temperatures by either caustic 

 baryta or strong hydrochloric acid, it yields large quantities of 

 leucine (15 p.c.), tyrosine (3 4 p.c.) and other products which 

 are in general identical with those obtained by the similar 

 treatment of proteids. It is soluble in strong alkalis when 

 heated, and is further stated to be dissolved by prolonged 

 treatment with superheated water ; in the latter case a product 

 is obtained to which, since it somewhat resembles an albumose, 

 the name keratinose has been given, and which may now be 

 digested by means of pepsin. Further investigation in this 

 direction is however needed before any positive statements can 

 be made respecting any truly digestive products derivable from 

 keratin, or indeed as to the characteristic differences of the 

 keratins from different sources. 



Neurokeratin. 



When the substance of the brain or any mass of medullated 

 nerves is thoroughly extracted with water, alcohol and ether, 

 and then digested with pepsin and trypsin in succession, a 

 residue is obtained which closely resembles the ordinary kera- 

 tins, and constitutes about 15 20 p.c. of the whole brain after 

 it has been freed from its medullary constituents by alcohol 

 an-1 ether. This residue is neurokeratin, so named from the 



