172 THE RATE OF FLOW. [BOOK i. 



and not the real cause of the circulation. The real cause of the 

 flow is the ventricular stroke, and this is sufficient to drive the 

 blood from the left ventricle to the right auricle, even when every 

 muscle of the body is at rest, and breathing is for a while stopped, 

 when, therefore, all the helps we are speaking of are wanting. 



Circumstances determining the Rate of the Flow. 



104. We may now pass on to consider briefly the rate at 

 which the blood flows through the vessels, and first the rate of 

 flow in the arteries. 



When even a small artery is severed, a considerable quantity 

 of blood escapes from the proximal cut end in a very short space of 

 time. That is to say, the blood moves in the arteries from the heart 

 to the capillaries with a very considerable velocity. By various 

 methods, this velocity of the blood current has been measured at 

 different parts of the arterial system ; the results, owing to imper- 

 fections in the methods employed, cannot be regarded as satis- 

 factorily exact, but may be accepted as approximately true. They 

 shew that the velocity of the arterial stream is greatest in the 

 largest arteries near the heart, and diminishes / from the heart 

 towards the capillaries. Thus in a large artery of a large animal, 

 such as the carotid of a dog or horse, and probably in the carotid of 

 a man, the blood flows at the rate of 300 or 500 mm. a second. 

 In the very small arteries the rate is probably only a few mm. a 



second. 



\, 



Methods. The Hsernadromometer of Volkmann. An artery, e.g. a 

 carotid, is clamped in two places, and divided between the clamps. Two 

 cannulse, of a bore as nearly equal as possible to that of the artery, or of 

 a known bore, are inserted in the two ends. The two cannulae are con- 

 nected by means of two stopcocks, which work together, with the two 

 ends of a long glass tube, bent in the shape of a (J, a d rilled with 

 normal saline solution, or with a coloured, innocuous fluid. The clamps 

 on the artery being released, a turn of the stopcocks permits the blood 

 to enter the proximal end of the long U tube, along which it courses, 

 driving the fluid out into the artery through the distal end. Attached 

 to the tube is a graduated scale, by means of which the velocity with 

 which the blood flows along the tube may be read off. 



The Rheometer (Stromuhr) of Ludwig. The principle of this 

 consists in measuring the time which it takes the flow through an 

 artery to fill and refill a vessel of known capacity a certain number 

 of times. The instrument (Fig. 33), which consists of two glass bulbs, 

 one being of known capacity, is connected, like the foregoing in- 

 strument, with two cannulse fixed in the two ends of a severed 

 artery, and is so arranged that the bulb of known capacity can be 



