Pets PRACTICAL ASPECTS 197 
an undisturbed coagulum formed within the body under healthy conditions 
otherwise, being free from micro-organisms, will not spread. Fibrinoplastic 
substance exists, no doubt, in the serum in the substance of the clot. But the 
experiments of the late Mr. Graham showed that diffusion of liquids is an exceed- 
ingly slow process, even between a strong saline solution and water ; and it must 
be a very slow process indeed between two liquids so nearly allied as liquor 
sanguinis and serum. 
Whether this explanation be correct or not, the fact remains that an 
undisturbed clot of healthy blood within healthy living vessels is incapable 
of self-propagation.1 On the other hand, the theory which I have sug- 
gested is in harmony with the extension of clots containing microbes, as in 
pyaemia. 
While an undisturbed clot resembles healthy and active living tissue with 
respect to coagulation, a disturbed and torn clot acts in this relation like wounded 
tissue. And as a coagulum is an easily lacerable substance, we often see coagu- 
lation induced by a blood-clot that has been disturbed. This is perhaps most 
strikingly seen in aneurysm. Let us take, for instance, a traumatic aneurysm. 
The blood escapes from the wounded artery into the surrounding tissues ; and 
these having been injured, the blood coagulates in contact with them. Every 
successive portion of blood driven in by the force of the ventricle stretches and 
lacerates the clot so formed. It is an injured clot, and induces coagulation in 
its vicinity : and the result comes to be that while, on the one hand, the force 
of the heart tends perpetually to distend the sac, inferior as it is in elasticity 
to the wall of the artery, there is on the other hand a constant tendency to deposi- 
tion of fibrine upon the interior of the sac, as if the blood were ‘ whipped’, and 
thus the wall of the sac is perpetually strengthened, exhibiting a counteracting 
agency tending to recovery. 
A beautiful converse of this state of things is presented, as it seems to me, 
by some cases of varicose aneurysm, such as used to be a not uncommon result 
of careless venesection. Here the communication with the vein was sometimes 
so free that the blood driven in by the heart distended the sac comparatively 
little, so that the aneurysm had no tendency to increase, but was merely a source 
of annoyance from the purring sensation caused by the arterial blood driven 
into the vein. And if an operation was performed on such a case, the remarkable 
fact was disclosed that instead of the aneurysmal sac being lined with layers 
? As further illustrations of this important truth may be mentioned the limitation of the coagula 
to the immediate vicinity of the wound in the veins of a stump after amputation; and also the fact 
which I have often noticed that if a varicose vein in the leg is treated by removing portions of the vessel 
at intervals of a few inches, the blood, though it coagulates in the immediate vicinity of the ligatures 
employed, remains permanently fluid in other parts of the vein. 
