I am delighted that you come to that subject, for it is one that has long excited my curiosity. But I cannot conceive how it is connected with respiration. The idea I have of the circulation is, that the blood runs from the heart through the veins all over the body, and back again to the heart.
MRS. B.
I could hardly have expected a better definition from you; it is, however, not quite correct, for you do not distinguish the arteries from the veins, which, as we have already observed, are two distinct sets of vessels, each having its own peculiar functions. The arteries convey the blood from the heart to the extremities of the body; and the veins bring it back into the heart.
This sketch will give you an idea of the manner in which some of the principal veins and arteries of the human body branch out of the heart, which may be considered as a common centre to both sets of vessels. The heart is a kind of strong elastic bag, or muscular cavity, which possesses a power of dilating and contracting itself, for the purposes of alternately receiving and expelling the blood, in order to carry on the process of circulation.
EMILY.
Why are the arteries in this drawing painted red, and the veins purple?
MRS. B.
It is to point out the difference of the colour of the blood in these two sets of vessels.
CAROLINE.
But if it is the same blood that flows from the arteries into the veins, how can its colour be changed?