The sounds which are heard when the ear is placed over another person’s heart are often compared to the syllables lub-dup. The “lub” is partly caused by the contraction of the ventricles; the “dup,” which immediately follows, is caused by the sudden closure of the valves at the bases of the great arteries. The throb of the heart, which can be felt from the outside, is really the thrust of the apex of the heart against the chest-wall at each beat. The sudden forcing of blood into the already-full but elastic arteries causes a wave to travel along these vessels, which can readily be felt, or even seen, at places where a fairly large artery lies just beneath the skin. This arterial wave is called the pulse. Many of the veins are [Pg 240] provided with pouch-shaped valves which permit the blood to flow freely towards the heart, but which bar the passage of blood in the opposite direction. Their action will readily be understood from Figs. 166 and 167. The importance of the capillaries.—The capillaries are the least conspicuous part of the circulatory system, but they are by far the most important. The heart, arteries, and veins exist merely to renew constantly the blood which flows through these minute channels. The excessive thinness of the walls of the capillaries makes it possible for a ready exchange to take place between the living tissue and the blood, and the vessels themselves form a network of such extremely close texture that it is practically impossible to prick any living part of the body with a fine needle without puncturing some of them and “drawing blood.” The work of the blood in supplying the various organs of the body with food has already been referred to. We have next to see how this all-important fluid is of service in supplying the organs with oxygen. 47. RESPIRATION. 1. Carbon dioxide is formed when flesh burns.—Dry a piece of meat and attach it to the end of a wire. Then light it, and when it is burning vigorously lower it into a clean glass jar. When the flame goes out remove the charred flesh, and at once pour a little clear lime-water into the jar and shake up. The lime-water turns milky, showing the presence of carbon dioxide gas in the jar. Examine what is left of the meat. It is charred, showing that meat contains carbon. How was the carbon dioxide formed during the burning? 2. Carbon dioxide is formed by the living body.—Breathe through a glass tube into clear lime-water, so that the air you expel from your lungs bubbles through the liquid. Does the lime-water remain clear, or turn milky? Does the air you breathe out contain a considerable quantity of carbon dioxide gas? [Pg 241]