xi THE ALBUMINOIDS : KERATIN AND ELASTIN 569 



it forms a portion of the medullary sheath of nerves in vertebrate 

 animals, and is found therefore abundantly in the brain, spinal cord, 

 and peripheral nerves ; it also occurs in the retina ; in the central 

 nervous system it amounts to from 15-20 per cent of the dry residue 

 after the myelin-substances have been removed, and according to 

 Chevalier 0*3 per cent of the fresh sciatic nerve. In the ventral 

 ganglionated cord of the lobster neuro-keratin is replaced by chitin 

 according to Kilhne and Chittenden. 



Analysis shows it to contain a remarkably high carbon percentage 

 and a somewhat lower sulphur-content than that possessed by other 

 keratins. On being dissociated it yields leucin and tyrosin. 



To the keratins belongs also, according to Drechsel, 1 the gorgonin 

 which forms the ground matrix of the axial skeleton of the coral 

 Gorgonia Cavolinii. According to Drechsel and Henze, 2 it contains no 

 glycocoll, but histidin, lysin, arginin, leucin, ammonia, sulphuretted 

 .hydrogen, and very large amounts of tyrosin. It also contains iodine, 

 and therefore belongs to the naturally occurring iodo-albumins (see 

 p. 230). It differs from spongin, which it otherwise resembles, in con- 

 taining tyrosin. A more detailed account of allied bodies is given 

 when dealing with spongin. Mendel 3 has found an iodo-keratin also 

 in other corals. The framework of the tropical horny sponges, which 

 Hundeshagen 4 has called iodo-spongin, is probably also a .keratin 

 judging by its tyrosin-content. 



3. Elastin 



Elastin arranged into fibres forms 'elastic tissue,' which may occur 

 as a thick, coarse strand as in the yellow neck-band or ligamentum 

 nuchsB of the ox, or arranged as a membrane as in the fasciae, and in 

 the walls of the aorta, or as fibres more or less freely intermingled 

 with white fibrous tissue (as in skin, the ordinary connective tissues, 

 in the smaller blood-vessels, and in tendon), or with cartilage, as e.g. 

 in the arytenoid cartilages and many ear-cartilages. It also forms, 

 according to Neumeister, 5 the organic ground substance of the egg- 

 shells in some reptiles and fishes. The elastin of the eggs of the 

 grass-snake, which has been investigated by Hilger 6 and Engel, 7 is 



1 E. Drechsel, Zeitschr. f. Biol. 33. 85 (II. to IV.) (1896). 



2 M. Henze, Zeitschr. /. physiol. Ghem. 38. 60 (1903). 



3 L. B. Mendel, Amer. Journ. of Physiol. 4. 243 (1900). 



4 F. Hundeshagen, Zeitschr. f. angeiv. Ghem. 1895, p. 473 (according to the Ghem. 

 ZentrcdU. 1895, II. p. 570). 5 R. Neumeister, Zeitschr. f. Biol. 31. 413 (1895). 



6 Hilger, Ber. d. deutsch. chem. Oes. Q. I. 166 (1873). 



7 W. Engel, Zeitschr. f. Biol. 27. 374 (1890). 



