You must observe that there is another bladder within the glass, the neck of which communicates with the mouth of the figure--this represents the lungs contained within the chest; the other bladder, which you see is tied loose, represents a muscular membrane, called the _diaphragm_, which separates the chest from the lower part of the body. By the chest, therefore, I mean that large cavity in the upper part of the body contained within the ribs, the neck, and the diaphragm; this membrane is muscular, and capable of contraction and dilatation. The contraction may be imitated by drawing the bladder tight over the bottom of the receiver, when the air in the bladder, which represents the lungs, will be forced out through the mouth of the figure-- EMILY. See, Caroline, how it blows the flame of the candle in breathing! MRS. B. By letting the bladder loose again, we imitate the dilatation of the diaphragm, and the cavity of the chest being enlarged, the lungs expand, and the air rushes in to fill them. EMILY. This figure, I think, gives a very clear idea of the process of breathing. MRS. B. It illustrates tolerably well the action of the lungs and diaphragm; but those are not the only powers that are concerned in enlarging or diminishing the cavity of the chest; the ribs are also possessed of a muscular motion for the same purpose; they are alternately drawn in, edgeways, to assist the contraction, and stretched out, like the hoops of a barrel, to contribute to the dilatation of the chest. EMILY. I always supposed that the elevation and depression of the ribs were the consequence, not the cause of breathing. MRS. B. It is exactly the reverse. The muscular action of the diaphragm, together with that of the ribs, are the _causes_ of the contraction and expansion of the chest; and the air rushing into, and being expelled from the lungs, are only _consequences_ of those actions. CAROLINE. I confess that I thought the act of breathing began by opening the mouth for the air to rush in, and that it was the air alone, which, by alternately rushing in and out, occasioned the dilatations and contractions of the lungs and chest. MRS. B. Try the experiment of merely opening your mouth; the air will not rush in, till by an interior muscular action you produce a vacuum--yes, just so, your diaphragm is now dilated, and the ribs expanded. But you will not be able to keep them long in that state. Your lungs and chest are already resuming their former state, and expelling the air with which they had just been filled. This mechanism goes on more or less rapidly, but, in general, a person at rest and in health will breathe between fifteen and twenty-five times in a minute. We may now proceed to the chemical effects of respiration; but, for this purpose, it is necessary that you should previously have some notion of the _circulation_ of the blood. Tell me, Caroline, what do you understand by the circulation of the blood? CAROLINE.