220 THE KATE OF FLOW. [BOOK i. 



Circumstances determining the Rate of the Flow. 



122. 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 Hsemadromometer 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 cannulse 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 U, and filled 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. Even supposing 

 the cannulse to be of the same bore as the artery, it is evident that the 

 conditions of the flow through the tube are such as will only admit of 

 the result thus gained being considered as an approximative estimation 

 of the real velocity in the artery itself. 



The Rheometer (Stromuhr) of Ludwig. This consists of two glass 

 bulbs, A and B, Fig. 32, communicating above with each other and with 

 the common tube C by which they can be filled. Their lower ends are 

 fixed in the metal disc />, which can be made to rotate, through two 

 right angles, round the lower disc E. In the upper disc are two holes, 

 a and 6, continuous with A and B respectively, and in the lower disc are 

 two similar holes, a' and &', similarly continuous with the tubes G and H. 

 Hence, in the position of the discs shewn in the figure, the tube G is 

 continuous through the two discs with the bulb A and the tube H with 

 the bulb B. On turning the disc D through two right angles the tube G 

 becomes continuous with B instead of A, and the tube H with A instead 

 of B. There is a further arrangement, omitted from the figure for the 



