THE HEART AND CIRCULATION. 115 



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 incompress- 

 ible. 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 distrib- 

 uted 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 dis- 

 tributed chiefly to vessels in the skin and in the abdom- 

 inal 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 gen- 

 eral 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 pres- 

 sure, 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 evapo- 

 ration. 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- 



