the increased pressure of the lungs on the heart and great vessels. In pericarditis the opposite is true.
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 ner-
