COAGULATION OF THE BLOOD, 
685 
again various theories have been held, which may be divided 
into mechanical, chemical, and vital. The mechanical theory 
was, that mere rest of the blood was sufficient to cause 
coagulation. I say this was the theory; but I believe it will 
be found to be still taught by many, that the cause of the 
coagulation of the blood in an artery which has been tied is 
its stagnation in the vicinity of the ligature. 
As to the chemical theories, they have been various. One 
very natural view was, that exposure to the air was the essen¬ 
tial cause of coagulation. Mr. Hewson believed that this was, 
at all events, an important element in the cause of the phe¬ 
nomenon ; and many eminent physiologists and pathologists 
have held the same view, except that, instead of the air as a 
whole, the oxygen of the air has been supposed to be the im¬ 
portant element. 
Sir Charles Scudamore considered that coagulation was 
greatly promoted by the escape of carbonic acid; and more 
recently the evolution of ammonia has been regarded as the 
essential cause of the change. According to the ammonia 
theory, due to Dr. Richardson, of this city, the fluidity of the 
blood within* the body depends on a certain amount of free 
ammonia holding the fibrin in solution, and the coagulation 
of the blood when withdrawn from the vessels is the result of 
the escape of the volatile alkali. 
Then, as to vital theories. These have been held by many 
physiologists, among whom may be mentioned Sir Astley 
Cooper and Mr. Thackrah, who, from experiments which they 
performed, were led to the inference that the living vessels 
exert an active influence upon the blood, by which coagulation 
is prevented; and Mr. Thackrah went so far as to attribute 
this action of the vessels to nervous influence. The view that 
the blood is kept fluid by the operation of its natural recep¬ 
tacles has been advocated more recently by Briicke of Vienna, 
whose essay will be found in the British and Foreign Medi¬ 
cal Tterieio {qx 1857. Briicke performed his experiments on 
turtles and frogs, in which the blood remains fluid in the 
heart for days after death ; and I feel bound to say that 
some of the facts which he has brought forward seem to 
me quite sufficient to show that the ammonia theory, whatever 
amount of truth it may contain, cannot be the whole truth, 
and cannot explain the fluidity of the blood within the body. 
For cxamj)le, Briicke found that, having shed blood from the 
heart of a living turtle into a basin, and transferred, with a 
syringe, a portion of that blood into the empty heart of 
another turtle just killed, the blood thus transferred into the 
empty heart remained fluid for hours; whereas that which 
