CHEMICAL CONSTITUENTS : FIBRIN, GELATIN. 35 



distinctly fibrous structure, that the mouths of divided blood- 

 vessels are closed up, when the flow of blood from them 

 spontaneously stops. In all such cases, the fibrous network, 

 if formed out of connexion with a living body, passes after a 

 time into decay j but if it be formed in apposition with living 

 parts, blood-vessels gradually extend into it from these, its 

 nutrition is maintained and improved, and it progressively 

 comes to present the ordinary characters of the simple fibrous 

 tissues ( 22). 



19. Although the tissues most actively concerned in 

 carrying on the vital operations, retain for the most part the 

 composition 'of albumen, yet that very large proportion of the 

 fabric of the higher animals whose offices are essentially 

 mechanical, has a very different chemical constitution. If we 

 boil down either their bones, their skin, or their internal 

 membranes, we shall get a considerable quantity of the sub- 

 stance scientifically termed Gelatin, familiarly glue. Though 

 consisting of the same elements as albumen, its composition is 

 simpler, because these elements are united in smaller propor- 

 tions ; the atom or combining equivalent of gelatin being 

 made up of 13 Carbon, 10 Hydrogen, 5 Oxygen, 2 Nitrogen. 

 The distinctive character of gelatin consists in its solubility 

 in warm water, its coagulation on cooling into a uniform jelly 

 which can be liquefied again by warmth, and its formation of 

 a peculiar insoluble compound with tannin. Gelatin is very 

 sparingly soluble in cold water, though made to swell up and 

 soften by prolonged contact with it. A solution of only one 

 part of gelatin in 100 of hot water is sufficiently strong for 

 the whole to form a consistent jelly on cooling. The re- 

 action of gelatin with tannin is so decided, that the presence 

 of only one part in 5000 of water is at once detected by 

 infusion of galls ; and it is in this action that the process of 

 tanning consists, the gelatinous fibre of the skin, which 

 would speedily pass into decay, being converted into a com- 

 paratively unchangeable substance. The different tissues 

 which have gelatin for their base, yield it to boiling water 

 with different degrees of facility ; this diversity apparently 

 depending in some degree upon the definiteness of their 

 organization. Thus the " sound " or air-bladder of the cod, 

 sturgeon, and other fish, which, when dried and cut into 

 strips, is known as isinglass, is very readily acted on ; the 



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