ALBUM EN DIP SUBSTANCES. 89 



to the quantity of alkali which they contain. Mucine is unaffected by 

 most of the metallic salts, lead-subacetate being the only one which 

 produces a distinct coagulation. In some cases, as in the bile, it is dis- 

 solved in the fluid ingredients of the secretions, from which it may be 

 separated by alcohol. In others, as in the urine, it is only mechanically 

 suspended, subsiding as a light deposit after a few hours' repose. 



Mucine is useful mainly by lubricating the opposite surfaces of 

 adjacent organs, as in the synovial cavities; by protecting mucous mem- 

 branes from the air, as in the trachea and bronchi or by facilitating the 

 mastication and deglutition of food, as in the secretions of the mouth 

 and subrnaxillary glands. 



2. Gelatine. 



This substance is very widely diffused in the animal body, forming 

 the more or less homogeneous interstitial mass of the bones, perios- 

 teum, tendons, ligaments, fasciaB, and connective tissues generally. All 

 these tissues, although at first insoluble in boiling water, become dis- 

 solved after long ebullition ; and the dissolved matter solidifies, on cool- 

 ing, into a jelly-like mass. This substance is the animal principle of 

 glue. It was formerly doubted whether gelatine represents the original 

 ingredient of the fibrous and bony tissues, or an altered product due to 

 continued ebullition. Comparative analyses, however, of the gelatige- 

 nous tissues and of the gelatine extracted from them have shown that 

 there is not only no appreciable difference in their chemical constitu- 

 tioH, but that the solid residue of the dried tissue and that of the gela- 

 tine extracted from it are the same in weight. (Hoppe-Seyler.) 



A hot solution of this substance gelatinizes on cooling when present 

 in the proportion of 3 per cent. ; below this quantity, or if the boiling 

 be repeated, it may remain liquid. Its solution rotates the plane of 

 polarization to the left 130. It is thrown down by alcohol and by 

 tannic acid. The last, which is the only acid by which this substance 

 is precipitated, is a very sensitive test of its presence ; and, according 

 to Hardy,* will detect one part of gelatine in 5,000 parts of water. A 

 similar combination takes place, in the process of tanning, between 

 tannic acid and the substance of the fibrous tissues, by which they are 

 rendered harder, more impermeable to water, and incapable of putre- 

 faction. Gelatine is not affected by potassium ferrocyanide with acetic 

 acid, nor by lead subacetate. It contains sulphur as one of its ingredients. 



3, Chondrine. 



The intercellular substance of cartilage resembles that of the bones 

 and the fibrous tissues in yielding, by prolonged boiling with water, a 

 substance which will gelatinize on cooling. In the case of the carti- 

 lages, however, this substance is termed chondrine, from the source 

 from which it is derived. Chondrine, like gelatine, contains sulphur, 



*Chimie Biologique. Paris, 1871, p. 282. 



