94 
OF THE VESSELS 
ty and importance. It was necessary that the blood should 
be successively brought into'contact, or contiguity, or prox¬ 
imity, with the air. I do not know that the chemical rea¬ 
son, upon which this necessity is founded, has been yet 
sufficiently explored. It seems to be made to appear, that 
the atmosphere which we breathe is a mixture of two kinds 
of air; one pure and vital, the other, for the purposes of 
life, effete, foul, and noxious: that when we have drawn 
in our breath, the blood in the lungs imbibes from the air, 
thus brought into contiguity with it, a portion of its pure in¬ 
gredient, and, at the same time, gives out the effete or 
corrupt air which it contained, and which is carried away, 
along with the halitus, every time we respire. At least, 
by comparing the air which is breathed from the lungs 
with the air which enters the lungs, it is found to have 
lost some of its pure part, and to have brought away with 
it an addition of its impure part. Whether these experiments 
satisfy the question, as to the need which the blood stands 
in of being visited by continual accesses of air, is not for 
us to inquire into, nor material to our argument: it is suf¬ 
ficient to know, that in the constitution of most animals, 
such a necessity exists, and that the air, by some means or 
other, must be introduced into a near communication with 
the blood. The lungs of animals are constructed for this 
purpose. They consist of blood-vessels and air-vessels, ly¬ 
ing close to each other; and wherever there is a branch 
of the trachea or windpipe, there is a branch accompanying 
it of the vein and artery, and the air-vessel is always in the 
middle between the blood-vessels.* The internal surface 
of these vessels, upon which the application of the air to 
the blood depends, would, if collected and expanded, be, 
in a man, equal to a superficies of fifteen feet square. Now, 
in order to give the blood in its course the benefit of this 
organization, (and this is the part of the subject with which 
we are chiefly concerned,) the following operation takes 
place. As soon as the blood is received by the heart 
from the veins of the body, and before that is sent out 
again into its arteries, it is carried by the force of the 
contraction of the heart, and by means of a separate and 
supplementary artery, to the lungs, and made to enter 
the vessels of the lungs; from which, after it has under¬ 
gone the action, whatever it be, of that viscus, it is 
brought back by a large vein once more to the heart, in 
order, when thus concocted and prepared to be thence 
* Keill’s Anat. p. 121. 
