THE VELOCITY OF BLOOD FLO W. 



33 



proportion to the decrease of cardiac energy. The tension falls and the 

 velocity remains constant. 



Since under the influence of the vasomotor mechanism any one section of 

 the arterial system may become constricted, while at the same moment any 

 other section may become dilated, it is possible that the tension and velocity 

 in the aorta may remain constant ; for a balance may be struck between such 

 variations when of opposite sign. It follows from the above that (1) the 

 velocity can vary in an opposite sense in different parts of the vascular system ; 

 (2) the velocity in any particular artery bears no absolute relation to the pulse 

 frequency or the general arterial tension. 



The velocity cannot be measured by simple division of an artery, 

 and estimation of the outflow in a given' time, for the peripheral resist- 

 ance is almost entirely abolished thereby, while the output of the 

 heart is rapidly diminished by the loss of blood. Volkmann 1 was the 

 first to more or less accurately measure the velocity of blood-flow by 

 introducing a U-shaped glass tube into the course of an artery. This 

 tube was filled with a solution of a neutral salt, and provided with a 

 tap. At a certain moment, the tap was turned and the rate of flow of 

 the column of blood along the tube directly measured. Supposing the 

 sectional area of the tube to 

 • be the same as that of the 

 artery, the velocity in the 

 latter is obtained without 

 further correction. But it 

 is practically impossible to 

 select a glass tube which 

 shall have the exact calibre 

 of the artery to be measured, 

 nor is the method suitable 

 for comparative estimations 

 of velocity during the course 

 of an experiment. Among 

 the more recent methods 

 of investigation, Ludwig's 

 stromuhr, 2 by which the 

 amount of blood passing in 

 a given time may be* accur- 

 ately measured, or its modified form, as contrived by Tigerstedt, deserves 

 description. 3 



In using this instrument, the tube (// x ) is placed in connection with the 

 central end, and the tube (//„) in connection with the peripheral end of the 

 cut artery which is under investigation. The whole instrument is washed out 

 with oil to prevent clotting, and filled with defibrinated blood. So soon as the 

 blood is allowed to flow from the artery, the metal ball (b) is driven over by 

 the current till it reaches the end of the cylinder (a). The instrument is then 

 rapidly rotated on the drum (/>■), so that the tube (x 2 ) comes to lie over the 

 tube (// 2 ), and the tube (a^) over the tube (//._,). The metal ball is now once 

 more driven by the current to the opposite end of the cylinder. This pro- 

 cedure is repeated several times, and the number of revolutions during the 

 period of observation is noted. The capacity of the cylinder (a), minus the 

 volume of the ball (b), multiplied by the number of revolutions, gives the 



1 "Die Hiimodynamik," Leipzig, 1850, S. 185. 



'-' Dogiel, Bcr. d, k. Sachs. Oesellsch. d. Wissensch., Leipzig, 1867, Bd. xx. S. 200. 



"Tigerstedt, Skandin. Arch./. Physiol., Leipzig, 1891, Bd. iii. S. 152. 



Fig. 53. — Ludwig's stromuhr, as modified 

 by Tigerstedt. 



