CHAP. VII. ] THE CONNECTIVE TISSUES. 257 



property of the substance, but this view is probably incorrect, the 

 absorption of the silver salt being merely due to the physical condi- 

 tions of the connecting substance rather than to any peculiar chemi- 

 cal property which it possesses 1 . 



Solubility When any of the forms of connective tissue proper 



if cttn C sub are macerate( l f r some days in baryta- or lime-water, 

 s^ancefnsoiu- tne var ius tissue elements fall asunder, in consequence 

 tions of tiie of the solution of the connecting substance. If 

 Alkalies. ^ the alkaline solution thus obtained be treated with 

 a dilute acid, a precipitate insoluble in excess of the reagent is 

 obtained, which, after careful washing with water, is found to be 

 insoluble in alcohol and ether, and when burned on platinum leaves 

 no perceptible amount of ash. This body is now looked upon 

 as identical with a^substance which is pretty widely distributed, 

 and which will be conveniently described in this place, viz. Mucin. 



Mucin. 



Distribu- This body, besides forming apparently a small 



tion - proportion of all connective tissue proper, is present in 



specially large quantities in embryonic connective tissue, and in that 

 form of the tissue occasionally met with in the adult animal and 

 which is termed Gelatinous connective tissue. 



It is found in the epidermis, where it connects together the 

 epithelial cells. It is found in considerable proportions in synovia. 



It is a frequent product of the activity of certain epithelial cells 

 and is the chief constituent in the viscid tenacious liquid called 

 Mucus, which often covers epitheliated surfaces. 



Mucus is a colourless, viscid, semi-liquid substance varying very 

 greatly in consistence. It is sometimes transparent, but often turbid 

 from the presence of epithelial cells or foreign matters. It contains, 

 besides mucin, which is its chief organic constituent, small quantities 

 of proteid substances, and salts, amongst which common salt pre- 

 ponderates. 



Mucin constitutes the chief ingredient of the tissues of certain in- 

 vertebrates, and indeed much of our knowledge of mucin is derived 

 from Eichwald's investigations of this body obtained from Helix 

 pomatia. It is not however certain that mucin from this source 

 is identical with that of the mucous membranes and tissues of man. 



Modes of (&) From connective tissue (Rollett's method 2 ). Ten- 



preparation dons are finely divided and treated with large quantities 

 of distilled water, with the object of removing albumi- 

 nous and saline matters soluble in that liquid. They are then digested 

 for many days in large quantities of lime- (or baryta-) water. The 

 solution is precipitated by acetic acid, -which throws down a pre- 



1 Consult Eobinsky, "Die Kittsubstanz auf Eeaction des Argentum nitricurn." 

 Archivf. Anat. u. Physiol., 1871, p. 184. 



2 Eollett, Sitzuntjsberichte der Wiener Akadernie, Bel. 39, p. 308 : Strieker's Hand- 

 book, Vol. i. p. 72. 



G. 17 



