113 



end, and pierced with a hole which is covered with a rubber 

 flap fastened on both sides of the hole). (8) Place a fing-er 

 on the "aorta" near the valve and note the pressure wave 

 (pulse) as it passes along the vessel. 



With the capillary side branch open as before compress 

 the bulb rhythmically and gradually increase the frequency 

 of the stroke. It will be found that at above twenty strokes to 

 the minute the stream will be intermittent. As the interval 

 between the strokes is shortened the liquid received from the 

 pump in any one stroke cannot all escape through the resist- 

 ance during the stroke and the succeeding interval. The next 

 stroke comes before the outflow from the preceding stroke is 

 finished, and the stream becomes remittent. 



Still further increase the frequency of the stroke. A rate 

 will be reached at which one-half the quantity received from 

 the pump will pass by the resistance during the stroke of the 

 pump and the remaining half will pass in the interval between 

 that stroke and the next ; the intermittent will be converted 

 into a continuous flow. 



Observe that the duration of the intervals is greater than 

 the duration of the strokes of the pump. Thus the time dur- 

 ing which the circulation is carried on by the energy stored 

 by the pump in the elastic walls of the vessel is greater than 

 the time during which it is carried on by the direct stroke of 

 the pump. 



Note that the arterial pressure remains low even after the 

 stream becomes continuous. An increase in the frequency of 

 the beat has little influence on the blood pressure where the 

 peripheral resistance is very slight. 



Close the side branch, so that the liquid must pass through 

 a peripheral resistance. Compress the bulb at such rate that 

 the outflow shall be continuous. The frequency required to 

 make the flow continuous is now much less than when the 

 peripheral resistance was low. 



363. Relation of Peripheral Resistance to Blood 

 Pressure- Compress the bulb at a rate that will produce 

 continuous outflow. With each successive stroke the portion 

 of liquid unable to pass the resistance during the stroke and 

 the succeeding interval is added to that left behind from 

 preceding strokes. The arteries become more and more full. 



