NITROGENOUS ORGANIC CELL-CONSTITUENTS. 105 



in synovial secretions. Mucin may be prepared from the salivary glands 

 by making a watery extract, filtering and precipitating mucin by acetic 

 acid ; the precipitate may then be washed with water, with alcohol, and 

 with ether to remove fat. Mucin may also be obtained from tendons by 

 washing well, cutting up into small pieces, extracting them with water to 

 remove soluble albuminous bodies and salts, and then allowing them to 

 stand for several days in lime- or baryta-water. After filtration, acetic 

 acid will precipitate the mucin, which at first is granular, but afterward 

 flocculent in appearance, and which may be washed with dilute alcohol 

 or dilute acetic acid. It also may be prepared from ox-gall by precipi- 

 tating with its own volume of alcohol to remove the coloring matter and 

 proteids, dissolving the precipitate in lime-water, after washing with 

 fresh alcohol, and precipitating the mucin from its solution in lime- 

 water by acetic acid. 



When freshly precipitated, mucin is a glutinous body which may be 

 suspended but not dissolved in water. Mucin is soluble in concen- 

 trated but not in dilute mineral acids. It is also soluble in liquor 

 potassae and lime-water, and the solution is viscid and nearly neutral. 

 When in solution it is not coagulated by boiling, but is precipitated 

 in an insoluble form by acetic acid. Its solutions are precipitated by 

 mineral acids, the precipitate being soluble in a slight excess of acid. 

 It is not precipitated by metallic salts, with the exception of acetate 

 of lead. When boiled for twenty or thirty minutes with dilute sul- 

 phuric acid it acquires the power of reducing the ordinary sugar tests. 

 It again loses this power on prolonged boiling. A body similar to 

 acid albumen is formed at the same time. No precipitate is produced 

 with solutions of mucin by acetic acid and potassium ferrocj'anide 

 unless other albuminoids are also present. It gives no precipitate 

 with mercuric chloride, and does not give the biuret reaction for albu- 

 minous bodies ; with Millon's reagent it gives a red color. It therefore 

 possesses several properties which are divergent from those of or- 

 dinary albuminous bodies, and is evidently a proteid body modified 

 through the differentiation of the protoplasm of the cells of the con- 

 nective-tissue group. 



Mucin appears to be digested by pancreatic but not by gastric juice. 

 Mucin is not soluble in water or alcohol, but swells up very much in the 

 former, particularly in the presence of certain salts. When the mixture 

 is filtered part of the mucin often passes through, and causes a turbid 

 precipitate. The mixture in water possesses no viscidity ; it, however, 

 becomes clearer and more tenacious if sodium chloride is added. 



2. Collagenous Albuminoids. — Collagenous albuminoids, of which 

 gelatin is the type, are albuminous bodies found in connective tissue, 

 cartilage, and bone. They contain a little less carbon and more nitrogen 



