SAPIDITY OF THE FLOW IN THE AETERIES. 77 



deviations as occurred when the instrument was connected with the blood- 

 vessel. The rapidity of the current in this tube may be easily calculated by 

 receiving the fluid 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 ascertained. This instrument is made on the same principle as 

 the one constructed by Vierordt, but in sensitiveness and accuracy it is 

 much superior. 



Rapidity of the Current in the Carotid. It has been found that three 

 currents, with different degrees of rapidity, may be distinguished in the ca- 

 rotid : 



1. At each ventricular systole, as the average of the experiments of Chau- 

 veau, the blood moves in the carotids at the rate of about 2O4 inches (510 

 mm.) 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 semilunar 

 valves, the blood moving at the rate of about 8'6 inches (215 mm.) per sec- 

 ond. This is the dicrotic impulse. 



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

 ishes 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 

 mm.) 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 violent 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 disten- 

 tion until the minimum is reached. According to the observations of Chau- 

 veau, corresponding to the first dilatation 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. 



Rapidity in Different Parts of the Arterial System. From the fact that 

 the arterial system progressively increases in capacity, there should be found 

 a corresponding diminution in the rapidity of the flow of blood. There are, 

 however, many conditions, aside from simple increase in the capacity of 

 the vessels, which modify the blood-current and render inexact any calcula- 

 tions made upon purely physical principles. There are the tension of the 

 blood, the conditions of contraction or relaxation of the smallest arteries, etc. 

 It is necessary, therefore, to have recourse to actual experiments to arrive at 

 any definite results on this point. Volkmann found a great difference in the 

 rapidity of the current in the carotid and metatarsal arteries, the averages 

 being about 10 inches (254 mm.) per second in the carotid, and about 2'2 

 inches (5G mm.) in the metatarsal. The same difference, although not quite 

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