APP.] CHEMICAL BASIS OF THE ANIMAL BODY. 727 



acids. The sulphur in keratin is evidently very loosely united to the 

 substance, and in all its reactions there appears to be a want of 

 similarity between keratin and either proteids, mucin or gelatin. The 

 most common of its products of decomposition are leucin (10 p.c.), and 

 tyrosin (3 '6 p.c.), and some aspartic acid; no glycin is formed. What 

 is generally known as keratin is probably a compound body, which has 

 not yet been resolved into its components. 



Ewald and Kiihne l have described a new body to which, since it occurs as a 

 constituent of nervous tissue (both of nerves and of the central nervous system), 

 and is yet closely identical with ordinary horny tissue, they give the name of neuro- 

 keratin. It is prepared in quantity from the brain by extracting this tissue with 

 alcohol and aether, and subjecting the residue to the action of pepsin and trypsin. 

 The final residue is neuro-keratin, and amounts to 15 20 p.c. of the original tissue. 



Nuclein. C^ H^ N 9 P 3 O^ 



Discovered by Miescher 2 in the nuclei of pus corpuscles and in the 

 yellow corpuscles of yolk of egg. Other observers have subsequently 

 obtained it from yeast, from semen, from the nuclei of the red blood-cor- 

 puscles of birds and amphibia, from hepatic cells, and it is probably 

 present in all nuclei. 



"When newly prepared it is a colourless amorphous body, soluble to a 

 slight extent in water, readily soluble in many alkaline solutions; but 

 its solubilities alter on keeping. If added gradually in sufficient 

 quantity to a solution of caustic alkali it first neutralises the solution 

 and then renders it acid. It seems to possess an indistinct xantho- 

 proteic reaction, but gives no reaction with Millon's fluid. It yields 

 precipitates with several salts, e.g. zinc chloride, argentic nitrate, and 

 cupric sulphate. 



Preparation 3 . Since nuclein is very resistent to the action of 

 pepsin, it may be obtained from the granular residue consisting chiefly 

 of nuclei, which occurs after digesting pus with pepsin. The most 

 remarkable feature of this body is its large percentage of phosphorus, 

 9-59 per cent. This phosphorus is readily separated by boiling with 

 strong hydrochloric acid or caustic alkalis; the same occurs when 

 solutions of nuclein are acidulated and allowed to stand. 



Chitin. CuHagN.Ojo 4 . 



Although not found as a constituent of any mammalian tissue, this 

 substance composes the chief part of the exoskeleton of many inverte- 

 brates. It may probably be regarded as the animal analogue of the 



1 Verhand. naturhist. med. Ver Heidelberg. Bd. i. (1876), Heft 5. 



2 Med. Chem. Untersuch. Hoppe-Seyler, Heft 4, 1872, S. 441 und 502. 



3 See Kossel, Zeitsch. f. physiol. Chem. Bd. in. (1879), S. 284 iv. (1880), S. 290. 

 vn. (1883), S. 7. "Untersuch. iiber d. Nuclein u. ihre Spaltungsprod," Strassb., 

 1881. 



4 Ledderhose, Zeitsch. f. physiol. Chem. Bd. n. (1878), S. 213. 



