1198 MUCINS. 



are familiar with different forms of proteids. From this it 

 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 conduce to clearness to give a brief account of this sub- 

 stance as obtained from each of the chief sources from which it 

 has been derived. 



The mucin of bile. Normal bile is not viscid when freshly 

 secreted, but becomes so during its stay in the gall-bladder. 

 The substance to which the viscidity is due is secreted by the 

 internal epithelium of the gall-bladder and was until recently 

 regarded as being a mucin. But that it is an entirely different 

 body is shewn by the fact that it yields no reducing (carbohy- 

 drate) substance when boiled with mineral acids. Furthermore 

 the precipitate formed on the careful addition of acetic acid is 

 soluble in excess of the acid and moreover it contains phos- 

 phorus, which the true mucins do not. The so-called mucin of 

 bile is hence not a mucin, but belongs to that class of substances 

 now known as nucleo-albumins (see p. 1206). 



A certain very small amount of true mucin has however 

 been stated to exist in human bile. 



The mucin of the sub-maxillary gland. The gland is finely 

 minced, washed and extracted with water: the extract is fil- 

 tered and hydrochloric acid is added up to 1-5 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 CuSO 4 and by excess of NaCl and MgSO 4 . 



The mucin of tendons. The tendo Achillis of the ox is cut 

 into thin slices, washed with distilled water and extracted with 

 half-saturated lime-water; the mucin is thus dissolved and is 

 purified by precipitation with either acetic or hydrochloric 

 acids, re-solution in dilute alkali and reprecipitation with acids. 

 In its general reactions it resembles the mucin previously 

 described, but differs from it in being insoluble in 1 2 p.c. 



