ELEMENTARY DEMONSTRATIONS 



123 



Vary (1) by lessening the rate of the pump ; vary (2) by opening the 

 screw-clip the difference in pressure diminishes in either case, and the 

 flow becomes intermittent. When the screw-clip is open a very frequent 

 beat of the pump is required to make the flow continuous, and 

 scarcely any fluid passes through the capillary tube. By means of the 

 vaso-motor nerves the arterioles are similarly dilated or constricted, 

 and the current switched on to or off an organ, according to its 

 functional activity. 



CHAPTER XXXI. 

 CIRCULATION CONTINUED. VELOCITY OF FLOW. 



Artificial Schema. Velocity of Flow. (1) Insert the Ludwig 

 stromiihr (Fig. 101) into the artery. It is convenient to fill one side 

 with water, and leave the other full 

 of air. In actual practice one tube 

 is filled with defibrinated blood and 

 the other with oil. Set the pump 

 going, and find the number of times 

 the stromiihr must be turned per 

 minute. Turn rapidly the moment 

 the water reaches the mark X. 

 Each turn means the flow of the 

 quantity of water contained in one 

 half of the stromiihr. Measure the 

 capacity of the stromiihr and the 

 diameter of the artery. The capa- 

 city of half the stromiihr multiplied 

 by the number of revolutions gives 

 the volume, and this divided by 

 the time and the sectional area of 

 the artery gives the mean velocity 

 per second. The sectional area of 

 the artery equals the radius x 3 '14. 



Note the effect on the velocity 

 of (1) opening the clip on the 

 arteriole tube, (2) of increasing the frequency of the pump. 



If the energy of the heart is constant, then in" proportion as the 

 peripheral resistance increases so the lateral pressure rises and the 

 velocity in the aorta lessens. On the other hand, as the peripheral 

 resistance decreases the pressure falls and the velocity increases. If 



Fia. 101. The stromtthr. 



