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? MRS. B. This change arises from various circumstances. In the first place, during its passage through the arteries, the blood undergoes a considerable alteration, some of its constituent parts being gradually separated from it for the purpose of nourishing the body, and of supplying the various secretions. The consequence of this is, that the florid arterial colour of the blood changes by degrees to a deep purple, which is its constant colour in the veins. On the other hand, the blood is recruited during its return through the veins by the fresh chyle, or imperfect blood, which has been produced by food; and it receives also lymph from the absorbent vessels, as we have before mentioned. In consequence of these several changes, the blood returns to the heart in a state very different from that in which it left it. It is loaded with a greater proportion of hydrogen and carbon, and is no longer fit for the nourishment of the body, or other purposes of circulation. EMILY. And in this state does it mix in the heart with the pure florid blood that runs into the arteries? MRS. B. No. The heart is divided into two cavities or compartitions, called the _right_ and _left ventricles_. The left ventricle is the receptacle for the pure arterial blood previous to its circulation; whilst the venous, or impure blood, which returns to the heart after having circulated, is received into the right ventricle, previous to its purification, which I shall presently explain. CAROLINE.