THE CIRCULATION. 287 



be twofold : On the one hand, it saves the heart a considerable dis- 

 play of force; on the other, it furnishes the small vessels with a con- 

 tinuous and constant now of blood. 



Next in importance to the elasticity of the vessels is the power of 

 contractility, by which the caliber of a vessel is changed and the sup- 

 ply of blood to any part or organ of the body altered. This property 

 co-operates with elasticity, so that the lumen of any given vessel is 

 proportionate to the pressure exerted. Were it otherwise, at some 

 times the pressure would be too small, at other times too great, for 

 the quantity of inclosed blood. The power of contractility is very 

 prominent in the small arteries. 



THE PULSE. 



At each ventricular systole the ventricular contents are forced 

 into the arterial system, but, because of the high peripheral tension, 

 they are unable to pass along as a unit. In fact, the artery just 

 beyond the heart becomes distended because of this influx, but by 

 virtue of its elasticity it strives to regain its normal caliber, thereby 

 giving to the blood some motion. The main impetus of the blood is 

 given by the succeeding systoles, until the smaller arteries are reached 

 when the vascular elasticity asserts itself more, and so helps along the 

 blood-stream. By this means the blood is caused to circulate. If the 

 vessels were inelastic, just as much blood would be forced out of the 

 veins into the heart again as the heart at each beat injects into the 

 arteries. Though the blood in the elastic vessels of the body cannot 

 move freely as in the inelastic tubes, yet there is propagated at each 

 ventricular systole a wave which runs to the periphery of the body. 

 This wave is a transmission of a wave of pressure throughout the length 

 of the arterial tree. The pulse-pressure is the difference between the 

 systolic- and diastolic-pressures. To this wave has been given the 

 name pulse. The impulsion moves very swiftly without the liquid 

 itself participating in that swiftness. 



The pulse of the radial and the impulse of the heart do not take 

 place at the same time. The radial pulse is perceived a short interval 

 after the cardiac impulse. This interval is termed the pulse delay, 

 for between the heart-impulse and the appearance of the radial pulse 

 it is seventeen hundredths of a second. The radial pulse never 

 succeeds the second sound of the heart, but is about midway in time 

 between the first and second sounds. The pulse is not synchronous 

 throughout the vascular system, the carotid pulse is felt before the 

 radial 



