364 THE CIRCULATION OF THE BLOOD. [cHAP. XXVIII. 



The calculations of Hales and others on this subject, led to ideas 

 respecting the velocity of the blood, which appear to be extrava- 

 gant. Thus Hales inferred the velocity of the blood at the 

 commencement of the aorta in man, to be at the rate of 735 feet 

 in a second ! The data upon which these calculations were founded 

 are uncertain and unsatisfactory, such as the measurement of the 

 area of the aorta at its origin, and the capacity of the ventricle 

 and the quantity of blood expelled by each systole. 



Volkmann has lately devised an instrument for the direct ad- 

 measurement of the rate of the blood's movement in the arteries. 

 He calls it the hcemodromometer. It consists of a glass tube, 

 containing water, 52 inches long, bent into the form of a hair- 

 pin, which is substituted for a segment of the blood-vessel, in 

 which it is required to measure the velocity of the blood's stream. 

 The column of blood which comes from the heart pushes the column 

 of water before it, without any great mixture of the two fluids 

 taking place ; and in passing through a determined space, it takes a 

 measurable time, whence it may be calculated how far the blood 

 moves in a second. 



The following description will explain the instrument and the 

 mode of using it. At A (fig. 202) is a metal tube, an inch and a 

 half in length ; the ends of this (a, a') are conical, and fit into two 

 corresponding conical tubes k, &'), made like the pipes of an 

 injecting syringe, so that they can be readily fitted into an artery. 

 A stopcock (&') commands the channel of this tube, not only at a', 

 but also, by two cogged wheels, at a. The mechanism of this 

 arrangement may be readily understood, by reference to the adjoin- 

 ing sections of this portion of the instrument at B and C, and the 

 view of its other su-rface at D (r, r D). At h, li are two short tubes, 

 also of metal, which are fitted into the horizontal tube below the 

 stopcock, and so that their channels (as shown at C) may commu- 

 nicate with, and be exactly equal to, that of the horizontal tube. 

 The stopcock (&') commands this communication likewise. These 

 short tubes (h, h') fit exactly upon the bent glass tube (p, p), and 

 complete the communication between its channel, and that of the 

 horizontal tube at its extremities. When the stopcock is turned so 

 as to open the channel of the horizontal tube throughout, as at B, 

 all communication with the glass tube is cut off: on the other 

 hand, when the communication with the glass tube is opened, as at 

 C, the channel of the horizontal tube is stopped, and fluid entering 

 at a', would have to pass through h' ', and to traverse both limbs of 

 the glass tube (p, p) emerging at a. For the protection of the 



