76 MUCIN. 



Mucin. 



This is the substance which gives to many animal secretions, such as 

 saliva, bile, synovial fluid &c., their characteristic ropy consistency. It 

 may also be obtained by the use of appropriate solvents from the tissues 

 themselves, such as submaxillary gland, tendons and umbilical cord. It 

 is peculiarly copious in the secretion which may be collected on stimu- 

 lating the mantle of Helix pomatia, or in an extract of the tissues of 

 this animal. The general phenomena of the formation of mucin by 

 mucous cells, and more particularly the characteristic behaviour of the 

 mucous granules in relation to the secretory activity of the sub-maxillary 

 gland 1 , leave but little doubt that mucin is to be regarded as derived 

 from the true proteids ; in conformity with this it yields many of the 

 reactions characteristic of the proteids (Millon's and xanthoproteic), and 

 by the action with mineral acids some form of acid-albumin is usually 

 obtained. During this treatment (or with alkalis) moreover a second 

 product generally makes its appearance, which belongs to the group 

 of carbohydrates and by heating with acids may be made to yield a 

 reducing sugar. Notwithstanding the views which have frequently 

 been advanced that mucin is in reality a mixture of proteid and carbo- 

 hydrate material, it is now known with considerable certainty that it 

 is a unitary substance which, from what has been already said, might 

 be almost regarded as an animal glucoside. It further appears that the 

 substance at first secreted by the mucous cells (of Helix) may not be 

 typical mucin but a sort of mucinogen which readily gives rise to mucin 

 on treatment with dilute ('01 p.c.) caustic potash 2 . If it be assumed 

 for the moment that there is only one kind of mucin, then the following 

 general statements as to this substance may be additionally made. It 

 is precipitated from its solutions by acetic or hydrochloric acids, the 

 precipitate being soluble in excess of the latter but not of the former 

 acid. In its precipitated form it swells up strongly in water but does 

 not go into true solution; the addition of dilute alkalis ('1 '2 p.c.) or 

 of lime water leads to its ready solution, from which it can again be 

 precipitated by the addition of an acid. It may be extracted from any 

 mucigenous tissue by the use of dilute alkalis or lime-water, and in 

 solution is somewhat characteristically precipitated by basic lead 

 acetate. Our knowledge of mucin is however in an extremely trans- 

 itional condition, and recent investigations have shown that probably 

 the mucins derived from different sources are really distinct substances, 

 just as we are familiar with different forms of proteids. From this it 



1 Langley, JL of Physiol Vol. x. (1889), p. 433. 



2 Hammarsten, Pfluger's Arch. Bd. xxxvi. (1885), S. 390. 



