122 ON THE COAGULATION OF THE BLOOD 
while in the adjacent parts of the same vessel the blood remained fluid. I re- 
peated 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 coagulation, the vital properties of the vein having 
been 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 fibrine, whereas the wall of the vessel itself is never 
found to have fibrine 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 fibrine 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 connexion 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 not be 
so kept under control. The other circumstance was 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 coagulate? It seemed 
scarcely credible that it was the result of contact with the surface of the basin. 
Both these notions, however, have since been swept away. In the first 
place, I have observed recently that it is by no means only in small vessels 
that the blood remains fluid after death. If blood be retained within the 
jugular vein of a horse or ox by the application of ligatures, either before or 
after the animal has been struck with the pole-axe, it will often continue fluid, 
but coagulable, in that vessel, which is upwards of an inch in diameter, for 
twenty-four or even forty-eight hours after it has been removed from the body. 
I say often, but not always. The jugular vein seems to be in that intermediate 
condition, between the heart and the small vessels, in which it is uncertain 
whether it will retain its vital properties for many hours, or will lose them in 
the course of one hour or so. Unfortunately for my present purpose, it happens 
that in this jugular vein, removed from an ox six hours ago, coagulation has 
