22 MANUAL OF HISTOLOGY. 



boiling in the same yielding then a principle which becomes gelatinous 

 on cooling, known as glue. It is supposed that in these processes the 

 composition of the materials under consideration is not essentially altered. 

 Our knowledge of the chemistry of the formation of glue is not, how- 

 ever, at all satisfactory at present. 



From allied protein substances these materials differ in their solubility 

 in boiling water and subsequent gelatinisation. With the sugar and 

 sulphuric acid test, likewise, they do not become red, but yellowish- 

 brown. With nitric acid they assume a yellow colour, like albuminous 

 materials. 



All efforts to convert albuminous matters into glutin-yielding sub- 

 stances artificially, as well as the latter into one another, have up to the 

 present proved futile. 



Collagen and Glutin. 



Collagen, or the substance converted into ordinary glue or glutin by 

 boiling, has received but little attention, while glutin has been made the 

 object of extensive investigation as regards its reactions. A solution of 

 glue is not affected by mineral or acetic acids, or by alkalies ; tannic acid 

 alone gives a copious precipitate. Among the earthy and metallic salts, 

 the chlorides of mercury and platinum, and basic sulphate of iron, pre- 

 cipitate glutin, but not acetate of lead. A polarised ray of light is bent 

 to the left by a watery solution of the matter in question. With man- 

 ganese and sulphuric acid it yields the decomposition products of albumen; 

 with acids and alkalies, ammonia, leucin, glycin, and other compounds. 



From glutin is formed the extensive group of connective-tissue struc- 

 tures, the organic substratum of bones and ossified cartilage. Conse- 

 quently collagen is found widely throughout the body, entering into the 

 composition of tissues of low physiological dignity. From the fact that, 

 with one exception, that of leucaemic blood (Scherer), no glutin has as 

 yet been found in the fluids of the body, we infer that collagen must 

 spring from the protein substances. Connective tissue likewise, at an 

 early embryonic period, yields no glutin, but appears to consist of a 

 protein compound (Scliwann). As to the mode in which the necessary 

 changes here take place, the present state of zoochemistry does not admit 

 of an answer being given. 



Chondrigen and Chondrin. 



Chondrin, or cartilage-glue obtained from the cornea, from per- 

 manent cartilage, from bone cartilage before the commencement of 

 ossification, and likewise from a pathological growth, enchondroma is 

 allied to glutin. Most acids, however, produce precipitates in a solution 

 of chondrin, which are again dissolved by an excess of the reagent. The 

 precipitate, however, caused by acetic acid does not redissolve. Watery 

 solutions of chondrin possess greater power of left-sided polarisation than 

 those of ghitin. Heavy precipitates are also seen here on the addition of 

 alum, sulphates of the protoxide and sesquioxide of iron, sulphate of 

 copper, neutral and basic acetate of lead, nitrate of silver, and nitrate of 

 mercury. Boiled with hydrochloric acid, or digested in gastric juice, 

 chondrin yields, besides numerous other products, a sugar (cartilage 

 sugar), as far as we know, non-cry stallizable, but capable of fermentation. 

 If this last statement be correct, chondrin may be regarded as a nitro- 



