THE CAUSES OF COAGULATION OF-THE BLOOD 79 
than the fibrine of effused liquor sanguinis, contrasts, in a very striking manner, 
with the lengthened period during which blood extravasated into the cellular 
tissue may retain its fluidity. But the fact that the liquor sanguinis is exuded 
among tissues that are in a state of inflammation, and so impaired in their 
vital energies, renders the circumstance in question easily intelligible. 
With regard to the nature of the influence exercised by the living vessels 
upon the blood within them, it might be conceived to be either of a positive 
or negative character. It might be imagined, either that the blood has a natural 
tendency within the vessels to comport itself as it does when outside the body, 
and that this tendency is counteracted by an active operation of the living 
tissues, or, that the vital fluid tends to no such change except when prejudicially 
acted on by surrounding objects, which in that case might be supposed to 
exert upon it attractive forces such as tend to group the molecules of dead 
matter together in aggregation, while the living tissues were destitute of such 
action, and simply neutral in their conduct towards the blood. Of these, the 
former has always appeared to me the more likely, a prior, but I had not 
expected to have met with any facts to give distinct evidence either in one 
direction or the other upon a subject so recondite. A simple observation, how- 
ever, made on the sheep’s foot, appears to throw clear light on the matter. 
I have frequently observed that when a vein has been opened and has remained 
patent, the blood has continued fluid in the aperture for a very much longer 
time than is necessary to produce coagulation of a portion of that blood placed 
in a saucer. When the wound in the vein has been a narrow one, I have seen 
the blood remain fluid between its lips for three hours together, though perfectly 
at rest. I have even observed where a portion of fluid blood has been pressed 
from a vein into a groove in the tissues, formed by muscle below, periosteum 
on one side and tendon on the other, this portion of blood has remained fluid 
for an hour, while another drop removed into a saucer at the same time from 
the same vein, has coagulated in a quarter of an hour. Now, in all these cases 
the blood was fully exposed to the influence of the atmosphere ; and if the air 
had been an active agent, promoting the tendency to coagulate, and the tissues 
merely neutral in their operation, coagulation must have occurred rapidly. 
On the other hand, if we admit that the tissues exert an active influence on the 
blood, and that air, oil, and other inorganic matter is inert with regard to it, 
the retarded coagulation follows naturally... Here, then, it appears to me, we 
* Since this paper was read, I have obtained further evidence which will, I trust, appear quite 
conclusive regarding the entirely negative influence of the atmosphere upon the blood, with respect 
to promoting the tendency to coagulation. Just forty-eight hours after the death of the sheep which 
furnished preparations exhibited to the Society, I exposed a vein of one of the feet and injected air 
forcibly into it, by means of a condensing syringe with a fine injection pipe adapted to it. Seven hours 
