62 CIRCULATION OF THE BLOOD 



When the circulation is arrested, the needle should be vertical and 

 mark zero on the scale. When the flow is established, a deviation of 

 the needle occurs, which varies in extent with the rapidity of the current. 

 Having removed all pressure from the vessel so as to allow the current 

 to resume its normal character, the deviations of the needle are carefully 

 noted, as they occur with the systole and with the diastole of the heart. 

 After withdrawing the instrument, it is applied to a tube of the same, 

 size of the artery, in which a current of water is made to pass with a 

 rapidity that will produce the same deviations as occurred when the 

 instrument was connected with the bloodvessel. The rapidity of the 

 current in this tube may be easily calculated by receiving the water in a 

 graduated vessel and noting the time .occupied in discharging a given 

 quantity. By this means the rapidity of the current of blood is as- 

 certained. 



It has been found that three currents, with different degrees of rapid- 

 ity, may be distinguished in the carotid : 



1. With each ventricular systole, as the average of the experiments 

 of Chauveau, the blood moves in the carotids at the rate of about 20.4 

 inches (510 millimeters) per second. After this, the rapidity quickly 

 diminishes and the needle returns quite or nearly to zero, which would 

 indicate complete arrest. 



2. Immediately succeeding the ventricular systole, a second impulse 

 is given to the blood, which is synchronous with the closure of the semi- 

 lunar valves, the blood moving at the rate of about 8.6 inches (215 milli- 

 meters) per second. This is the dicrotic impulse. 



3. After the dicrotic impulse, the rapidity of the current gradually 

 diminishes until just before the systole of the heart, when the needle is 

 nearly at zero. The average rate, after the dicrotic impulse, is about 

 5.9 inches (147.5 millimeters) per second. 



The experiments of Chauveau correspond with the experiments of 

 Marey on the form of the pulse. Marey showed that there is a marked 

 oscillation of the blood in the vessels, due to a reaction of their elastic 

 walls following the first distention by the heart ; that at the time of 

 closure of the semilunar valves, the arteries present a second, or dicrotic 

 distention, much less than the first ; and following this, there is a 

 gradual decline in the distention until the minimum is reached. Accord- 

 ing to the observations of Chauveau, corresponding with the first dilata- 

 tion of the vessels, the blood moves with great rapidity ; following this, 

 the current suddenly becomes nearly arrested ; this is followed by a 

 second acceleration in the current, less than the first ; and following this, 

 there is a gradual decline in the rapidity, to the time of the next 

 pulsation. 



