CHEMICAL BASIS OF THE ANIMAL BODY. 87 



then digested with pepsin and trypsin in succession, a residue is 

 obtained which closely resembles the ordinary keratins, and con- 

 stitutes about 15 20 p.c. of the whole brain after it has been 

 freed from its medullary constituents by alcohol and ether 1 . This 

 residue is neurokeratin, so named from the source from which it is 

 obtained. It is characterised by its somewhat greater resistance to 

 those decomposing agents whose action on keratin has been already 

 described. The determination of its existence in tissues which are 

 not obviously epidermal in the adult is of considerable embryological 

 and morphological interest, since it throws some light upon the de- 

 velopmental origin of the structures in which it is present or 

 absent 2 . 



Chitin. C 15 H 26 N 2 10 3 . 



Although it is not found as a constituent of any mammalian 

 tissue, this substance composes the chief part of the exoskeleton of 

 many invertebrates. It is by many regarded as the animal analogue 

 of cellulose of plants, and from this point of view it possesses con- 

 siderable morphological interest. The most convenient source from 

 which it may be prepared is the cleansed exoskeleton of crabs or 

 lobsters. This is first thoroughly extracted with dilute hydrochloric 

 acid and caustic potash, after which it is treated with boiling alcohol 

 and ether, and may be finally completely decolorised by the action of per- 

 manganate of potash 4 . It is a white amorphous substance which often 

 retains the shape of the integument from which it has been prepared. 

 It is insoluble in any reagents other than concentrated mineral acids, 

 such as sulphuric or hydrochloric. The immediate addition of water to 

 these solutions probably reprecipitates the chitin in an unaltered form 5 . 

 When heated with concentrated hydrochloric acid it is decomposed into 

 glycosamin and acetic acid, of which the former is the characteristic 

 product 6 . A similar decomposition is observed when sulphuric acid is 

 employed. 



Glycosamin (C 6 H 13 N0 5 ). Crystallises from alcohol in fine needles, is dextro- 

 rotatory, and reduces Fehling's fluid to the same extent as does dextrose, but is 

 not fermentable. By treatment with nitrous acid a carbohydrate (C 6 H 12 6 ) (?) is 

 obtained which also reduces cupric oxide, but is similarly unfermentable. This 



1 See also Chevalier, Zt. f. pliysiol. Chem. Bd. x. (1886), S. 100. 



2 Cf. Smith, H. E., Zt. f. Biol. Bd. xix. (1883), S. 469. Steinbriigge, Ibid. Bd. 

 xxi. (1885), S. 631. 



3 Ledderhose, Zt. f. physiol. Chem. Bd. ir. (1878), S. 213. But see also Sundwik, 

 Ibid. Bd. v. (1881), S. 384. 



4 Biitschli, Arch. f. Anat. u. Physiol. Jahrg. 1874, S. 362. 



5 But see Hoppe-Seyler, Hdbch. d. physiol. -path. Anal. 5 Aufl. 1883, S. 188. 

 Krukenberg, Zt. f. Biol. Bd. xxn. (1886), S. 480. 



6 Ledderhose, loc. cit. and Ibid. Bd. iv. (1880), S. 139. 



