262 THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXI. 



some of the fluid will be still distending the tube, i.e., the pressure 

 will not have fallen to zero. With the second inflow the maximum 

 pressure reached will be higher than before, but still the force acting 

 throughout the whole of the cycle may not be sufficient to empty the 

 tube to the extent found at the commencement of that inflow, and we 

 may find a further volume of fluid retained within the tube, and the 

 pressure just before the next inflow still higher than in the preceding 

 instance. This summation will continue, until at last a point is 

 reached at which the mean pressure during a complete cycle will be 

 just sufficient to drive out exactly the same volume of fluid as is sent 

 in at each pumping. When this point is reached, the pressure will 

 simply oscillate about this mean position and the outflow from the 

 tube will therefore be continuous, but not necessarily constant. If 

 we still further constrict the orifice, the result is that the mean 

 pressure will rise still further, and the outflow will be of a less 

 remittent character. The latter because the input of the same 

 volume of fluid into the distended tube will produce less marked 

 fluctuations in the driving pressure, since at the end of diastole the 

 pressure still driving the fluid will be high and not nearly zero, as in 

 the former instance. If the constriction is sufficiently increased, a 

 point will ultimately be reached at which the outflow will become 

 not only continuous but also constant. The degree of constriction 

 necessary to produce this effect will depend upon the distensibility 

 of the elastic tube. The more distensible this is, the earlier will this 

 stage be reached, and the lower will be the mean pressure. This is 

 the condition we find in the circulatory system. 



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 

 distension is transmitted as a wave along the arteries, but with 

 gradually diminishing force as the total arterial stream becomes 

 larger. This wave constitutes the pulse-wave. Between the strokes 

 of the pump, or, in other words, during the periods of diastole, the 

 arteries drive the blood on and so return to their original size. 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 pulsation having disappeared. The peripheral resistance 

 which keeps up the blood-pressure in the arteries, and like the con- 

 striction at the end of our india-rubber tube, assists in the conversion 

 of the intermittent into a continuous and constant stream, is to be 



