CHEMICAL BASIS OF THE ANIMAL BODY. 77 



follows that no general statement of the properties of the mucins can 

 be as yet made which would be other than misleading, and it will con- 

 duce to clearness to give a brief account of this substance as obtained 

 from each of the chief sources from which it has been derived. 



The mucin of bile 1 . Mucin is not a constituent of normal bile when 

 freshly secreted, but is found in it as the result of the secretory activity 

 of the internal epithelium of the gall-bladder. It is best prepared as 

 follows (Paijkull). Bile is mixed with five volumes of absolute alcohol 

 and centrifugalised ; the precipitated mucin which is thus obtained is 

 then dissolved in water and the above process repeated two or three 

 times. An aqueous solution of this mucin is precipitated by acetic and 

 hydrochloric acids, is soluble in excess of either acid, and yields strongly 

 marked proteid reactions. This mucin differs from that obtained from 

 other sources in not yielding any reducing substance when boiled with 

 acids, and in the solubility of its precipitate obtained by means of acetic 

 acid in an excess of this acid. It also contains phosphorus, and is by 

 some regarded as more closely allied to the nucleo-albumins (see p. 89) 

 than to the true mucins. 



The mucin of the sub-maxillary gland 2 . The gland is finely minced, 

 washed and extracted with water : the extract is filtered and hydro- 

 chloric acid is added up to *1 -15 p.c. The mucin is thus precipitated 

 at first, but at once passes into solution, from which it is precipitated 

 by the addition of a volume of water equal to three to five times that 

 of the original solution. This precipitate is then again dissolved in 

 dilute hydrochloric acid and reprecipitated by water, the process being 

 repeated several times. As thus prepared and thoroughly washed it 

 possesses a distinctly acid reaction; it may be dissolved to a neutral 

 solution, by the cautious addition of very dilute alkalis, and now exhibits 

 the following properties. It is readily precipitated by acetic acid, much 

 less readily in presence of sodium chloride ; this salt on the other hand 

 greatly facilitates the precipitation of mucin by alcohol, which again 

 does not take place in presence of a trace of free alkali. Any excess 

 of alkali, especially on warming, at once changes the substance so that 

 its characteristic ropiness is permanently lost, and boiling with dilute 

 mineral acids yields a reducing substance. It gives the usual reactions 

 for proteids and is strongly precipitated by the acetates of lead and by 

 Cu SO 4 and by excess of Na Cl and MgS0 4 . 



1 Landwehr, Zt. f. physiol. Chem. Bd. v. (1881), S. 371; vin. (1883), S. 114. 

 Paijkull, Ibid. xn. (1887), S. 196. 



2 Hammarsten, Zt. f. physiol. Chem. xn. (1888), S. 163. Contains references to 

 other literature. Obolewsky, Hoppe-Seyler's med.-chem. Unters. Hfl. 4 (1871), S. 

 590. Also in Pfluger's Arch. Bd. iv. (1871), S. 336. 



