80 CIRCULATION OF THE BLOOD. 



withdrawing the instrument, it is applied to a tube of the size of the artery, in which a 

 current of water is made to pass with a rapidity which will produce the same devia- 

 tions as occurred when the instrument was connected with the blood-vessel. The ra- 

 pidity 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 we ascertain the rapidity of the current of blood. This instrument is on the same 

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

 superior. In the hands of Chauveau, the results, particularly those with regard to varia- 

 tions in the rapidity of the current, are very interesting. 



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

 different degrees of rapidity, may be distinguished in the carotid : 



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

 veau, the blood moving in the carotids at the rate of 20 T % inches per second. After this, 

 the rapidity quickly diminishes, the needle returning quite or nearly to zero, which would 

 indicate complete arrest. 



2. Immediately succeeding the ventricular systole, we have a second impulse given to 

 the blood, which is synchronous with the closure of the semilunar valves, the blood mov- 

 ing at the rate of 8 T 6 inches per second. Chauveau calls this 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 5^ inches per second. 



The above experiments give us, for the first time, correct notions of the rapidity and 

 variations in the flow of blood in the larger vessels ; and it is seen that they correspond 

 in a remarkable degree 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 experience 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. Chauveau shows by experiments with his instrument 

 that, 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, we have a 

 gradual decline in the rapidity up to the time of the next pulsation. 



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

 system increases in capacity as we recede from the heart, we should expect to find a cor- 

 responding diminution in the rapidity of the flow of blood. There are, however, many 

 circumstances, aside from simple increase in the capacity of the vessels, which modify 

 the blood-current and render inexact any calculations made upon purely physical principles. 

 Such 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. The experiments of Volkmann showed a 

 great difference in the rapidity of the current in the carotid and metatarsal arteries, the 

 averages being 10 inches per second in the carotid and 2*2 inches in the metatarsal. The 

 same difference, although not quite so marked, was found by Chauveau between the carot- 

 id and the facial. The last-named observer also noted an important modification in the 

 character of the current in the smaller vessels. As we recede from the heart, the sys- 

 tolic impulse becomes rapidly diminished, being reduced in one experiment about two- 

 thirds ; the dicrotic impulse becomes feeble or may even be abolished ; but the constant 

 flow is very much increased in rapidity. This fact coincides with the ideas already 

 advanced with regard to the gradual conversion, by virtue of the elasticity of the vessels, 

 of the impulse of the heart into, first, a remittent, and, in the very smallest arteries, a 

 nearly constant current. 



The rapidity of the flow in any artery must be subject to constant modifications due 



