There is a striking difference in the structure and appearance of the arteries and the veins. The walls of the arteries are three layers thick, with muscles in the middle layer, the outer layer being tough and elastic. They hold their shape even when not distended with blood. The veins, in contrast, are thin and have little elasticity so that when empty they are hardly noticeable, and thus may be especially bothersome for surgeons. An artery stands boldly forth. One may see or feel its beat. If it is cut, the spurting blood tells where to seize it. But a big vein with soft weak walls cannot be told from other tissue when pressure has caused it to collapse. After the pressure is released, the blood oozes up from one knows not where.

Right here is probably the best place to talk about the control of bleeding by amateurs. The first-aid manuals have told people to use tourniquets. I believe that in the overwhelming majority of cases the tourniquet is unnecessary and is usually harmful. It is not a simple thing to apply a tourniquet efficiently. The arteries are deeply situated and stiff-walled, and hence difficult to compress to the point where bleeding is stopped. But the veins are superficial, flimsy, and easy to compress. It is difficult to tighten the tourniquet evenly to the point where it will stop arterial flow. Doing so causes great pain. I have no doubt that in a large majority of cases the blood keeps going through the arteries, but the pressure stops the return through the veins. Result — increased bleeding. Fortunately it is a rare case where bleeding is not stopped by clotting in the wound, especially as the low blood pressure resulting from shock or fainting hastens the clotting. Pressure over the point of bleeding is the best procedure for anybody but an experienced surgeon to use.

The arteries in early life are elastic and their diameter may enlarge or shrink from time to time according to the action of the muscles in their walls. The individual has no command over these muscles as their action is regulated by the involuntary, or sympathetic, nervous system. These nerves can be affected in various ways, as for instance by drugs, called vasodilators and vasoconstrictors. Nicotine is a vasoconstrictor. In such a condition as Buerger’s disease, in which the diameter of the vessels to the hands and feet is lessened, smoking is always forbidden. Emotions temporarily affect the size of the arteries. Thus blushing, resulting from shame or embarrassment, is due to the dilating of the small arteries in the skin.

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