76 MUCIN. 



a matter of fact they could scarcely be expected to do so, since it is ex- 

 tremely difficult to make allowance for the heat which may be simply 

 absorbed or set free as the result of the varying solubilities of the orig- 

 inal substance and the products of its decomposition. The real proof 

 of the correctness of Hoppe-Seyler's view is the fact, already stated, 

 that the heat of combustion of the products of zymolysis is less than 

 that of the substance from which they are derived. 1 



NITROGENOUS NON-CRYSTALLINE BODIES ALLIED TO PROTEIDS. 



These resemble the proteids in many general points, but exhibit 

 among themselves much greater differences than do the proteids. 

 As regards their molecular structure nothing satisfactory is known. 

 Their percentage composition approaches that of the proteids, and 

 like these they yield, under hydrolytic treatment, large quantities 

 of leucin and in some cases tyrosin. They are all amorphous. 



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 secre- 

 tion which may be collected on stimulating the mantle of Helix 

 pomatia, or in an extract of the tissues of this animal. The gen- 

 eral 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, 2 leave but little doubt that mucin is to be regarded as de- 

 rived 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 ap- 

 pearance, which belongs to the group of carbohydrates and by 

 heating with acids may be made to yield a reducing sugar. Not- 

 withstanding the views which. have frequently been advanced 

 that mucin is in reality a mixture of proteid and carbohydrate 

 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 



1 For heat of combustion of physiologically important substances see Rechenberg. 

 Inaug. Diss. Leipzig, 1880, and Jn. f. prakt. Chem. (N. F.) Bd. xxii. (1880), Sn. 1, 

 223. See also Stohmann, Ibid. Bd. xxxi. (1885), and Landwirth Jahrb. Bd. xm. S. 

 513. Rubner, Zt. f. Biol. Bde. xix. (1883), S. 313 ; xxi. Sn. 250, 337. Berthelot et 

 Andre, Compt. Rend., T. ex. (1890), p. 884. 



2 Langley, Jl. of Physiol. Vol. x. (1889), p. 433. 



