124 THE VELOCITY OF THE FLOW. [BOOK i. 



falls below it at each diastole, so at any spot in the artery there is 

 for each heart-beat a temporary expansion succeeded by temporary 

 contraction, the diameter of the artery in its temporary expansions 

 and contractions oscillating, in correspondence with the oscillations 

 of the manometer, beyond and within the diameter of permanent 

 expansion. These temporary expansions constitute what is called 

 the pulse, and will be discussed more fully hereafter. 



The velocity of the flow. 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 con- 

 siderable velocity. By various methods, this velocity of the blood- 

 current has been measured at different parts of the arterial system; 

 the results, owing to imperfections in the methods employed, cannot 

 be regarded as satisfactorily exact, but may be accepted as approxi- 

 mately true. The velocity of the arterial stream is greatest in 

 the largest arteries, and diminishes from the heart to the capillaries, 

 pari passu with the increase of the width of the bed, i.e. with the 

 increase of the united sectional area. 



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 stop-cocks, 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 stop-cocks 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 cannulss 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 2i, Fig. 20, 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 Z>, which can be made to rotate, through two 

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

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

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

 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-ZT with .4 instead 

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

 sake of simplicity, by which when the disc D is turned through one 



