CHEMICAL CONSTITUENTS :—FIBRIN, GELATIN. 35 
| distinctly fibrous structure, that the mouths of divided blood- 
rvessels are closed up, when the flow of blood from them 
pontaneously stops. In all such cases, the fibrous network, 
if formed out of connexion with a living body, passes after a 
time into decay ; 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 
sumpler, because these elements are united in smaller propor- 
ons ; 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 
im 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 
ringly 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- 
tively unchangeable substance. The different tissues 
ich have gelatin for their base, yield it to boiling water 
ith different degrees of facility ; this diversity apparently 
ding in some degree upon the definiteness of their 
ization. Thus the “sound” or air-bladder of the cod, 
m, and other fish, which, when dried and cut into 
is known as isinglass, is very readily acted on; the 
D2 
