Before this can be explained to you, you must first allow me to complete the formation of the blood. The chyle may, indeed, be considered as forming the chief ingredient of blood; but this fluid is not perfect until it has passed through the lungs, and undergone (together with the blood that has already circulated) certain necessary changes that are effected by RESPIRATION. CAROLINE. I am very glad that you are going to explain the nature of respiration: I have often longed to understand it, for though we talk incessantly of _breathing_, I never knew precisely what purpose it answered. MRS. B. It is indeed one of the most interesting processes imaginable; but, in order to understand this function well, it will be necessary to enter into some previous explanations. Tell me, Emily, --what do you understand by respiration? EMILY. Respiration, I conceive, consists simply in alternately _inspiring_ air into the lungs, and _expiring_ it from them. MRS. B. Your answer will do very well as a general definition. But, in order to form a tolerably clear notion of the various phenomena of respiration, there are many circumstances to be taken into consideration. In the first place, there are two things to be distinguished in respiration, the _mechanical_ and the _chemical_ part of the process. The mechanism of breathing depends on the alternate expansions and contractions of the chest, in which the lungs are contained. When the chest dilates, the cavity is enlarged, and the air rushes in at the mouth, to fill up the vacuum formed by this dilatation; when it contracts, the cavity is diminished, and the air forced out again. CAROLINE. I thought that it was the lungs that contracted and expanded in breathing? MRS. B. They do likewise; but their action is only the consequence of that of the chest. The lungs, together with the heart and largest blood vessels, in a manner fill up the cavity of the chest; they could not, therefore, dilate if the chest did not previously expand; and, on the other hand, when the chest contracts, it compresses the lungs and forces the air out of them. CAROLINE. The lungs, then, are like bellows, and the chest is the power that works them. MRS. B. Precisely so. Here is a curious little figure (PLATE XV. Fig. 5.), that will assist me in explaining the mechanism of breathing. CAROLINE. What a droll figure! a little head fixed upon a glass bell, with a bladder tied over the bottom of it! MRS. B.