266 THE CIRCULATION IX THE BLOOD-VESSELS, [cm. xxi. 



The Use of the Elasticity of the Vessels. 



If a pump is connected with a rigid tube, such as a glass tube, 

 every time that a certain amount is forced into one end of the 

 tube an exactly equal quantity will be forced out at the other 

 end. During the intervals between the pumpings, the flow will 

 cease. If the far end of the tube is partially closed, the flow 

 will still be intermittent, only the quantity injected and the 

 quantity ejected, though still of equal volume, will be diminished. 

 If we employ an elastic tube instead of a rigid tube, and the end 

 is left freely open, the flow will still be intermittent as in the 

 case of the rigid tube ; but if the end of the elastic tube is 

 narrowed by a clamp the intermittent flow will be converted into 

 a more or less perfectly constant flow. Each stroke of the pump 

 forces a certain amount of fluid into the tube, but owing to the 

 peripheral resistance, it cannot all escape at once, and part of 

 the force of the pump is spent in distending the walls of the tube. 

 This distended elastic tube, however, tends to empty itself, and 

 forces out the fluid which distends it before the next stroke of 

 the pump takes place. One part of the fluid is therefore forced 

 out by the immediate effect of the pump, and another part by 

 the elastic recoil of the tube between the strokes. If the rate of 

 the pump and the distension of the tube which it produces is 

 sufficiently great, the fluid forced out between the strokes will 

 be equal to that entering at each stroke and thus the outflow 

 becomes continuous. 



Let us now apply this to the body. 



At each beat the left ventricle forces about three ounces of 

 blood into the already full arterial system. The arteries are 

 elastic tubes, and the amount of elastic tissue is greatest in the 

 large arteries. The first effect of the extra three ounces is to 

 distend the aorta still further; the elastic recoil of the walls 

 drives on another portion of blood which distends the next section 

 of the arterial wall, and this wave of distension is transmitted 

 along the arteries with gradually diminishing force as the total 

 arterial stream becomes larger. This wave constitutes the pulse. 

 Between the strokes of the pump, or, in other words, during the 

 periods of diastole, the arteries tend to return to their original size, 

 and so drive the blood on. The flow, therefore, does not cease 

 during the heart's inactivity, so that although the force of the heart 

 is an intermittent one, the flow through the capillaries and the veins 

 beyond is a constant one, all trace of the pulse having disap- 

 peared. The peripheral resistance which keeps up the blood- 

 pressure in the arteries, and like the clamp on our india-rubber 



