696 
COAGULATION OF THE BLOOD. 
fluid tends to no such change except when prejudicially acted 
on by surrounding objects, which in that case might be sup- 
posed 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 'priori , 
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, however, 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 con- 
tinued fluid in the aperture for a much longer time than is 
necessary to produce coagulation of a portion of that blood 
jolaced 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 have a sure, though imperfect glimpse, of 
the operation of mysterious but potent forces, peculiar to the 
tissues of living beings, and capable of reversing the natural 
order of chemical affinities ; forces which I suspect will never 
be fully comprehended by man in his present state of exist- 
ence, and the study of which should always be approached 
with humility and reverence. 
Having thus obtained evidence of the active operation of 
the living tissues upon the blood, it occurred to me that the 
walls of the vessels might probably act to greater advantage 
upon their contents when of small than when of large calibre, 
and that in that case the blood might be found fluid in the 
small vessels of the human body after death, although coagu- 
lated in the heart and large vessels. Accordingly, I have 
