80 GELATIN. 



accompanied by animal-gum, the latter has by no means been proved 

 to take its origin from the former. The whole subject requires 

 further investigation. 



Gelatin or G-lutin. 1 



The ultimate fibrils of connective tissue and the organic matter 

 of which bones are largely composed consist of a substance named 

 in the first case ' collagen,' in the second ' ossein.' They are ob- 

 tained either by digesting carefully cleansed tendons with trypsin, 

 which dissolves up all the tissue-elements except the true collage- 

 nous (gelatiniferous) fibrils, 2 or by extracting bones with dilute 

 acids in the cold, by means of which the inorganic salts are dis- 

 solved and the ossein remains as a swollen elastic mass which re- 

 tains the shape of the original bone. As thus prepared they are 

 insoluble in water, saline solutions, and either cold dilute acids or 

 alkalis ; in the former, however, (acids) they swell up to a trans- 

 parent gelatinous mass. When subjected to prolonged boiling 

 with water, more especially under pressure as in a Papin's diges- 

 ter, they are gradually dissolved, and the solution now contains 

 true gelatin into which they have been converted by hydrolysis, 

 and has acquired the characteristic property of solidifying into a 

 jelly on cooling. The conversion of collagen into gelatin may be 

 still more easily effected by a shorter boiling in presence of dilute 

 acids, but in this case, unless the process be carefully regulated, 

 the first-formed gelatin is further hydrolysed into what are often 

 spoken of as gelatin-peptones. Although insoluble in dilute acids 

 collagen is readily dissolved by digestion with pepsin in presence 

 of an acid passing rapidly through the condition of gelatin into 

 that of gelatin-peptone, and although collagen is not acted upon 

 by trypsin in alkaline solution, it is readily hydrolysed by this 

 enzyme after a short preliminary treatment with dilute acid or 

 boiling water, the products as before being known as gelatin-pep- 

 tones. When gelatin is exposed for some time in the dry condi- 

 tion to a temperature of 130 it is reconverted into a substance 

 closely resembling collagen, which may be again converted into 

 gelatin by treatment with water under pressure at 120 . 3 



Gelatin obtained by the above means from connective tissue or 

 bones is, when dry, a transparent, more or less coloured and brittle 

 substance. 4 It is insoluble in cold water, but swells up into an 

 elastic flexible mass which now dissolves readily in water when 

 warmed. When the solution is again cooled it solidifies charac- 



1 Glutin must not be confounded with the vegetable proteid ' gluten.' 



2 Kuhne u. Ewald, Verhnnd. d. naturhist.-med. Ver. Heidelb. Bd. i. N.F. (1877), 

 S. 3. See also Etzinger, Zt.f. Biol. Bd. x. (1874), S. 84. Ewald, Ibid. Bd. xxvi. 

 (1889), S. 1. 



3 Hofmeister, Zt.f. physiol. Chem. Bd. ir. (1878), S. 313. Weiske, Ibid. vn. 

 (1883), S. 460. 



4 Pure gelatin is colourless, e. g. fine isinglass prepared from the bladder of the 

 sturgeon. Glue is impure gelatin made from hides, &c. 



