When the blood pressure is high, the _pulse_ is small and travels fast, because the wall of the artery is already highly stretched. Such a pulse is hard and incompressible. A large pulse occurs where the heart is strong and the pressure is low, owing to peripheral dilatation. A low-pressure pulse is soft and compressible if the heart beat is weak. A slow pulse is generally stronger than a rapid one. The _nerve supply of the blood-vessels_ comes from the spinal cord through the vasomotor nerves, which are connected with the sympathetic system and are distributed to the smooth muscle fibers of the vessels. They are of two classes, the _vasoconstrictors_, which diminish the lumen of the vessels, and the _vasodilators_, which increase the size of the vessels. By these nerves the general tone of the arteries is kept up. They are distributed chiefly to vessels in the skin and in the abdominal organs and the constrictors are probably the more important. When the constrictors are stimulated, three phenomena occur: 1. diminished flow through the vessel, due to its diminished size; 2. increased general arterial pressure, and 3. increased flow through the other arteries. When the dilators are stimulated the opposite effect is produced: 1. the flow through the vessel is increased; 2. there is decreased arterial pressure, and 3. there is decreased flow through the other arteries. The palor of fright is due to the action of the vasoconstrictor nerves of the face and blushing to the action of the vasodilators. Heat stimulates the vasodilators so that more blood goes to the skin, perspiration begins, and the body is cooled by evaporation. Cold stimulates the vasoconstrictors and the blood is kept within the body, where it cannot cool. If a part has too much blood, an impulse passes by the vasoconstrictors to lessen the supply, while if more blood is needed a message goes to the central nervous system and an impulse passes by the vasodilators to flush the organ. The more active a part is in functioning the greater the number of capillaries, except in the brain, which has only large vessels. The vessels of the intestines contain much blood and are capable of containing all the blood in the body.