CHAPTER XXIII 



THE VELOCITY OF BLOOD-FLOW 



THE velocity of the blood at any point in a vessel may be denned 

 as the length of the column of blood flowing by that point in a second. 

 In the case of a tube supplied by a constant head of pressure, we can 

 divide the tube and measure the outflow per second ; knowing the 

 volume of this, and the cross area of the artery, we can determine 

 the length of the column. This kind of experiment cannot be done 

 on the living animal, because the 

 opening of the vessel alters the 

 resistance to flow, and the loss of 

 blood also changes the physiological 

 conditions. To determine the velo- 

 city, other means must be devised. 

 One form of instrument is called 

 the " stromuhr " (stream-clock), 

 (Fig. 106) consisting of two bulbs 

 mounted on a rotating platform 

 pierced with two holes. One bulb is 

 filled with oil, the other with blood. 

 The bulbs are connected together 

 by a tube at their upper end, and 

 the lower end of the one full of oil is 

 brought over one hole in the rotating 

 platform. The central end of the 

 artery is connected to the same 

 hole and the peripheral end to the 

 other, over which stands the bulb 

 full of blood. The blood, being 

 allowed to flow, displaces the oil out 

 of the one bulb into the other. 

 Directly this happens, the bulbs are 

 rotated, and the one full of oil is 

 again brought over the central end 



of the artery. The number of rotations per minute is counted, and 

 the volume of the bulb being known, we obtain the volume of blood 

 that passes through the instrument per minute. 



An improved form of the instrument is seen in Fig. 107. 



In using this instrument, the tube (y t ) is placed in connection 

 with the central end, and the tube (y z ) in connection with the 



203 



FIG. 106. THE STROMUHR. 



