INFLAMMATION. 
393 
cavities. The idea is altogether irreconcilable to the known and 
established laws of physiology; and I am confident that a very 
little consideration will explain this to every unbiassed mind. 
But let us bestow a few moments upon this point; and first I 
will inquire what would be the consequence if congestion were 
the essence of inflammation, or if the capillaries were in such a 
debilitated or spasmodic state, that they could not propel for¬ 
ward the blood which they received from the supplying trunks ? 
The merest tyro knows there is a great difference between arterial 
and venous blood: there is a difference in their character and 
properties, and, what is more to our present purpose, there is a 
very perceptible difference in their colour, the former being a 
bright scarlet, and the latter a dark red, approaching to black. 
Of course the blood passes through the arteries before it enters 
the veins, and consequently first possesses the arterial, and 
afterwards assumes the venous peculiarities. The immediate 
cause and means of this change has been the subject of various 
speculative theories, which it is needless for me to particularize ; 
but I will mention a circumstance or two connected with and 
influencing it. Arterial or red blood becomes venous or black 
with an intensity in an inverse ratio to the celerity of circula¬ 
tion; tardiness of motion is, therefore, highly conducive to the 
change, and perfect rest still more so. We have evidence of 
this, by isolating a quantity of blood in a living vessel, by means 
of ligatures applied round it; but a much more familiar and 
demonstrable illustration is afforded by placing a bandage 
tightly round our own arm: we shall then readily perceive the 
gradual yet speedy change from red to almost black, exhibited 
by that portion of the extremity most remote from the heart: 
this change is occasioned by the detention of the blood in the 
vessels of the part. Again, in many cases of pneumonia, wherein 
we have an oppressed pulse—wherein the artery is labouring in 
vain to force on the column of blood which it has received—if 
we resort to venesection, we find the fluid much darker coloured 
than under ordinary circumstances; but by and by, when the 
oppressed vessel has been relieved by the abstraction of a por¬ 
tion of its contents, and is thence enabled to act with necessary 
vigour, and accelerate the flow of its blood, the blackness which 
we at first noticed diminishes, and gives place to a brighter 
colour. I am fully aware, that in cases of pneumonia the 
blackness may be partly attributed to the imperfect oxygena¬ 
tion which the blood undergoes in the engorged lungs ; yet it is 
evidently also influenced by the retarded progress through the 
vessels. Now, if the capillaries of an inflamed part were in a 
congested condition—if their coats were distended to such a 
VOL. V. 3 II 
