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rly imitated by the words "lubb, dup." While the cause of the first sound is not fully understood, most authorities believe it to be due to the contraction of the heart muscle and the sudden tension on the valve flaps. The second sound is due to the closing of the semilunar valves. These sounds are easily heard by placing an ear against the chest wall. They are of great value to the physician in determining the condition of the heart. *Arteries and Veins.*--These form two systems of tubes which reach from the heart to all parts of the body. The arteries receive blood from the heart and distribute it to the capillaries. The veins receive the blood from the capillaries and return it to the heart. The arteries and veins are similar in structure, both having the form of tubes and both having three distinct layers, or coats, in their walls. The corresponding coats in the arteries and veins are made up of similar materials, as follows: 1. _The inner coat_ consists of a delicate lining of flat cells resting upon a thin layer of connective tissue. The inner coat is continuous with the lining of the heart and provides a smooth surface over which the blood glides with little friction. 2. _The middle coat_ consists mainly of non-striated, or involuntary, muscular fibers. This coat is quite thin in the veins, but in the arteries it is rather thick and strong. 3. _The outer coat_ is made up of a variety of connective tissue and is also much thicker and stronger in the arteries than in the veins. [Fig. 19] Fig. 19--Artery dissected to show the coats. Marked differences exist between the arteries and the veins, and these vessels are readily distinguished from each other. The walls of the arteries are much thicker and heavier than those of the veins (Fig. 19). As a result these tubes stand open when empty, whereas the veins collapse. The arteries also are highly elastic, while the veins are but slightly elastic. On the other hand, many of the veins contain valves, formed by folds in the inner coat (Fig. 20), while the arteries have no valves. The blood flows more rapidly through the arteries than through the veins, the difference being due to the fact that the system of veins has a greater capacity than the system of arteries. [Fig. 20] Fig. 20--Vein split open to show the valves. *Why the Arteries are Elastic.*--Th
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