Thus it comes to pass that the tidal air, which at each pull of the diaphragm and push of the sternum goes into the chest as pure air with twenty-one parts oxygen to seventy-nine parts nitrogen in every hundred parts, comes out, when the diaphragm goes up and the sternum falls back, as impure air with only sixteen parts oxygen, but with five parts carbonic acid to seventy-nine of nitrogen. That lost oxygen is carried through the stationary air to the blood in the capillaries, and the gained carbonic acid came through the stationary air from the blood in the capillaries. So each breath helps to purify the blood, and the pumping of air in and out of the chest changes the impure, hurtful, venous, to pure, refreshing, arterial blood; the blood breathes air in the lungs, that all the body may in turn breathe blood. HOW THE BLOOD IS CHANGED BY FOOD: DIGESTION. § VII. =45.= The blood is not only purified by air, it is also renewed and made good by food. The food we eat becomes blood. But our food, though frequently moist, is for the most part solid. We cut it into small pieces on the plate, and with our teeth we crush and tear it into still smaller morsels in our mouth. Still, however well chewed, a great deal of it, most of it in fact, is swallowed solid. In order to become blood it must first be dissolved. It is dissolved in the alimentary canal, and we call the dissolving =digestion=. Let us see how digestion is carried on. Your skin, though sometimes quite moist with perspiration, is as frequently quite dry. The inside of your mouth is always moist--very frequently quite filled with fluid; and even when you speak of it as being dry, it is still very moist. Why is this? The inside of your mouth is also very much redder than your skin. The redness and the moisture go together. In speaking of the capillaries, I said that almost all parts of the body were completely riddled with them, but =not quite all=. A certain part of the skin, for instance, has no capillaries or blood-vessels at all. You know that where your skin is thick, you can shave off pieces of skin without “fetching blood;” if your [Illustration: FIG. 15.--_Section of Skin, highly magnified._ _a_, horny epidermis; _b_, softer layer; _c_, dermis; _d_, lowermost vertical layer of epidermic cells; _e_, cells lining the sweat duct continuous with epidermic cells; _h_, corkscrew canal of sweat duct. To the right of the sweat duct the dermis is raised into a papilla, in which the small artery, _f_, breaks up into capillaries, ultimately forming the veins, _g_. ]