CII. XXT.] 



THE RATE OF BLOOD 7LO\V. 



263 



the disc c c' is turned rapidly through two right angles, and the blood 

 flowing through d into a' again displaces the oil which is driven into a. 

 This is repeated several times, and the duration of the experiment noted. 

 The capacity of a and ' is known ; the diameter of the artery is also 

 known by its corresponding with the cannulae of known diameter, and as 

 the number of times a has been filled in a given time is known, the velocity 

 of the current can be calculated. 



\\V m:iy take an example to illustrate this : 



Velocity _ volume per second _ V 

 sectional area S 



If the capacity of the bulb is 5 c.c., and it required 100 seconds to fill it 

 10 times, then the amount of blood passing through the instrument would be 

 50 c.c. in 100 seconds, or o - 5 c.c. in i second. Next, suppose the diameter is 

 4 mm. The sectional area is irr* ; r is the radius (2 mm.), and ir = 3-1416, 

 From these data we get 



Velocity = X = -^ c l c - l 50Q cubic millimetres = g mm per gec 

 S 3 1416 x2' : 3'i4i6 x 4 



Chauveau's Dromograph (fig. 259) consists of a thin brass tube, a, in one 

 side of which is a small perforation closed by thin vulcanised india-rubber. 



Fig. 259. Diagram of Chauveau'a Dromograph. a, Brass tube for introduction into the 

 lumen of the artery, and containing an index-needle, which passes through the elastic 

 membrane in its side, and moves by the impulse of the blood current, c, Graduated 

 scale, for measuring the extent of the oscillations of the needle. 



Passing through the rubber is a fine lever, one end of which, slightly flat- 

 tened, extends into the lumen of the tube, while the other moves over the 

 face of a dial. The tube is inserted into the interior of an artery, and 

 ligatures applied to fix it, BO that the " relocity pulse" ('.., the change of 

 velocity with each heart-beat, may be indicated by the movement of the 

 outer extremity of the lever on the face of the dial. 



The Iftematachometer of Vierordt resembles in principle that of Chauveau. 



The Htemadromomettr of Volkmann, one of the earliest instruments 

 devised for this purpose, is simply a long glass U-tube of the same calibre as 

 the artery under investigation. It is provided with a stop-cock, so that at a 

 given moment the blood can be admitted, and the time that the blood takes 

 to reach its other end is observed. 



In the Capillaries. The microscopic observations of E. II. 

 Weber and Valentin agree very closely as to the rate of the 

 blood-current in the capillaries of the frog; and the mean of their 



