leg muscles get more; after a hearty dinner, the stomach and intestines get more than any other part of the body. Why is it difficult to do the best studying and digest a meal at the same time? We see that the muscular coat of the arteries is a very useful coat, for it enables the supply of blood to be increased in any organ which is in temporary need of it.
Why the Blood Vessels must be Smooth.—The inner coat of the heart and other blood vessels is made of tissue like the epithelial tissue which forms the epidermis and the smooth lining of the mouth and other organs. The purpose of this lining is to lessen friction, and thus save the work of the heart. The friction is greatest in the capillaries because of their small size. The inner coat of smooth cells is the only coat that is prolonged to form the capillaries (see Fig. 57).
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Fig. 57.—Capillaries Magnified, showing Cells forming their walls. Notice that each cell has a nucleus and three branches.
The capillaries are small, thin, short, and very numerous. They are very small so that they may go in between the cells of the tissues. The capillaries are very thin so that the nourishment from the blood may pass readily into the tissues, and the waste material pass readily into the blood. They are very short so that the friction may be less; and they are very numerous so that all parts of the tissues may be supplied with blood, and that the blood may flow very slowly through them. Because of the number of the capillaries, their total volume is several hundred times larger than the volume of the arteries that empty into them, or of the veins that flow from them. Hence the blood flows slowly through the capillaries, as water flows slowly through a lake along the course of a river. All the changes between the blood and the lungs, and between the blood and the tissues, take place in the capillaries, and
