34 CHEMICAL CONSTITUENTS I—FIBRIN. 
deprived of this water by drying, and then becomes a bard 
and brittle substance ; but, like dried flesh, it imbibes water 
again when moistened, and recovers its original softness and 
elasticity. From the recent experiments of Dr. Eichardson, 
it appears that the coagulation of blood-fibrin depends upon 
the escape of ammonia, being accelerated by such conditions 
as favour the liberation of this gas, and retarded or prevented 
by such as cause its retention in the liquid; whilst, even 
after the clot has been formed, it may be dissolved by 
ammonia, forming again when that gas is set free. Fibrin 
differs from syntonin or muscle-substance in not being dis¬ 
solved by very dilute muriatic acid, but being merely caused 
to swell up into a gelatinous mass, which contracts again 
when more acid is added. It combines with the earthy 
phosphates, of which as much as 2\ per cent, is sometimes 
found in the ash left by its combustion. 
18. There can be no doubt that fibrin is formed in the 
blood and in the other fluids in which it presents itself, at 
the expense of albumen. What is its precise destination, 
cannot as yet be clearly specified; but there are several 
circumstances which point to the conclusion that it is to be 
regarded as a transitional stage in the metamorphosis of 
albumen into the simple fibrous tissues (§ 23.) Thus, when 
the ordinary clot of blood is examined microscopically, it is 
found to consist, not, like an albuminous coagulum, of a 
homogeneous mass of granules, but of a network of im¬ 
perfectly-formed fibres, enclosing the red corpuscles in its 
interstices. A much more distinct network of the same kind, 
may be seen in the colourless coagulum formed by the liquid 
which may be skimmed off the surface of the blood drawn 
from persons suffering under any severe inflammation’; such 
blood coagulates slowly, and its red corpuscles and the fluid 
in which they float have an unusual tendency to separate 
from each other; and the fibrin previously dissolved in the 
latter sets into definite fibres, which continue for some days 
to increase in firmness. It is a liquid of the same kind, 
charged with fibrin in a peculiarly “ plastic ” condition, that is 
poured forth for the formation of new tissue when the repa¬ 
rative processes are at work for the healing of a wound or the 
reunion of divided parts; and it is by a plug of coagulated 
fibrin, which gradually comes to present a more and more 
