CHEMICAL CONSTITUENTS :—FIBRIN, GELATIN. 35 
distinctly fibrous structure, tliat tbe mouths of divided blood¬ 
vessels are closed up, when tbe flow of blood from tliem 
spontaneously stops. In all sucb cases, tbe fibrous network, 
if formed out of connexion witb a living body, passes after a 
time into decay; but if it be formed in apposition witb living 
parts, blood-vessels gradually extend into it from these, its 
nutrition is maintained and improved, and it progressively 
comes to present tbe ordinary characters of tbe simple fibrous 
tissues (§ 22). 
19. Although tbe tissues most actively concerned in 
carrying on tbe vital operations, retain for tbe most part tbe 
composition of albumen, yet that very large proportion of tbe 
fabric of tbe 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 sohibility 
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 55 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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