EXPERIMENTS WITH BLOOD-FLOW. 81 



the bulb, resistance will be felt, and the stream will be con- 

 stant, or nearly so, and will continue for some time when the 

 bulb is no longer worked. This is, clearly, because the rub- 

 ber has been stretched, chiefly laterally, and is now " stretching 

 back." That is, by the elastic reaction of the rubber tube the 

 jerky action of the bulb is converted into the steady flow that 

 we see. In the first experiment we had a rigid tube and prac- 

 tically no resistance. In the second, although the tube was 

 elastic, there was no resistance, so the elasticity was not 

 brought into play. In the third, there was resistance, but the 

 tube was inelastic. In the fourth, the resistance brought into 

 play the elasticity of the rubber tube, and the elastic reaction 

 of the tube continues (so to speak) the action of the bulb be- 

 tween two successive strokes. In this experiment the pulse 

 can be felt in the tube. 



EXPERIMENT 5. Repeat the last experiment, except with 

 the change of taking the rubber tube ten or twelve feet long. 

 Double this along a table so that the nozzle lies close to the 

 bulb. Let each pupil take hold of the tube near the bend, and 

 then advance toward the bulb, holding one part of the loop 

 in each hand, noting the time at which the pulsation is felt in 

 the two parts of the loop. Let the teacher or the person who 

 works the bulb count aloud to give the time of each compres- 

 sion of the bulb, so that it can be compared with the time of 

 the pulsations. Observe (1) whether the pulse occurs at the 

 instant of the compression of the bulb; (2) whether the pulse 

 is felt at the same instant by the two hands when one hand 

 holds the tube near the bulb and the other hand near the 

 nozzle. Compare with this the intervals between the heart- 

 beats and the pulse as felt at (1) the temple, (2) the wrist, 

 (3) the ankle. 



The pulse in the artery and in the rubber tube must not be 

 confounded with the blood-flow itself. The pulse is a wave 



