4 MR. youatt’s vkterinary lectures. 
black, from arterial to venous, from vital to effete. In the capillary 
system of the lungs, the blood is changed from black to red; it is 
enabled again to furnish the materials for the various secretions ; 
it conveys the principle of nutrition to every organ, and it also 
communicates to each organ that impulse or stimulus Nvhich ena¬ 
bles it to discharge its function. This all-important change is 
effected through the medium of the atmospheric air, for we can 
trace it along the bronchiae to these lobules, and we have no doubt 
that it enters them. The delicate membranes forming the parietes 
of the bloodvessels, and the lining, or the wall of these lobuli, al¬ 
though impervious to the air in its compound form, suffer some of 
its principles to permeate them, and especially when urged by the 
strong power of affinity; and the oxygen of the atmosphere attracts 
and combines with a portion of the superabundant carbon of the 
blood, and the expired air is poisoned with carbonic acid gas; 
while some of the constituents of the blood attract another por¬ 
tion of the oxygen of the air, which combines with that fluid, and 
probably gives its distinguishing character and properties as arterial 
blood. 
This, until we have inquired, as we should before have done, 
and in future will do, into the nature and properties of the blood, 
is all that I say of the changes produced by the process of res¬ 
piration. 
The Arterial and Venous Circulations, —Then, gentlemen, it 
is evident that the circulation of arterial blood has its commence¬ 
ment in the capillaries of the lungs, and its termination in those 
of the frame generally. The venous blood begins to flow in the 
general capillaries, and terminates in the capillaries of the lungs. 
These systems are independent of each other; the fluid is essentially 
different; the vessels by which it is conveyed are different; but 
they communicate at these points, and, as I shall have occasion 
to shew, the capillaries (too much neglected in our dissections and 
our inquiries) are by far the most important portions of the circu¬ 
lation. They are the origin and the termination of the two cir¬ 
culations, and in them the most important phenomena of life take 
place,—the change from arterial to venous and from venous to 
arterial blood. 
The Fact of the Whole of the Blood traversing the Limbs 
accountedfor. —Does it appear singular that the lungs alone should 
counterbalance the circulation of the whole body ? Let it be re¬ 
membered, that while the lungs consist of a congeries of blood¬ 
vessels, or are made up of them, this fluid is distributed very 
sparingly to many parts of the frame. The cellular membrane, 
and the ligament and tendons, have not much of it, and the 
