316 
COAGULATION OF THE BLOOD. 
will occur at the seat of injury. As a good illustration of 
this, and also as bearing upon the ammonia theory, I may 
mention the following experiment. Having squeezed the 
blood out of a limited portion of one of the veins of a sheep’s 
foot, and prevented its return by appropriate means, I 
treated the empty portion with caustic ammonia, the neigh¬ 
bouring parts of the vein being protected from the irritating 
vapour by lint steeped in olive oil. After the smell of am¬ 
monia had passed off, I let the blood flow back again, and 
left it undisturbed for a while, when I found on examination 
a cylindrical clot in the part that had been treated with 
ammonia, while in the adjacent parts of the same vessel the 
blood remained fluid. I repeated this experiment several 
times, and always with the same result—where the ammonia 
had acted there was a clot. The chemical agent used here 
was one which, so long as any of it remained, would keep 
the blood fluid ; yet its ultimate effect was to induce coagula¬ 
tion, the vital properties of the vein havingbeen destroyed by it. 
If a needle or a piece of silver wire is introduced for a 
short time into one of the veins of the sheep’s foot, it is 
found on withdrawal to be covered over with a very thin 
crust of fibrin ; whereas the wall of the vessel itself is never 
found to have fibrin or coagulum adhering to it, unless it has 
been injured. Now, this seems to imply that the ordinary 
solid is the active agent with reference to coagulation; that 
it is not that the blood is maintained fluid by any action of 
the living vessels, but that it' is induced to coagulate by an 
attractive agency on the part of the foreign solid. We see, 
at any rate, that the foreign solid has an attraction for fibrin 
which the wall of the vessel has not. 
And yet I own I was at first inclined to think that the 
blood-vessels must in some way actively prevent coagulation. 
There were two considerations that led to this view : one was 
that the blood remained fluid in the small vessels after death, 
but coagulated in the large. Now, why should that be ? It 
seemed only susceptible of explanation from there being 
some connection between the size of the vessel and the 
circumstance of coagulation. It looked as if in the small 
veins the action of the wall of the vessel was able to control 
the blood and keep it fluid, but that the large mass in the 
principal trunks could notbe so kept under control. The other 
circumstance w r as the rapid coagulation of a large quantity 
of blood shed into a basin. Why should this occur unless 
there was some spontaneous tendency in the blood to coagu¬ 
late ? It seemed scarcely credible that it was the result of 
contact with the surface of the basin. 
