CH. XXII.] THE STROMUHR 283 



U-shaped glass tube dilated at a and a', the ends of which, h and i, 

 are of known calibre. The bulbs can be filled by a common opening 

 at k. The instrument is so contrived that at b and V, the glass part 

 is firmly fixed into metal cylinders, attached to a circular horizontal 

 table c c, capable of horizontal movement on a similar table d d', 

 about the vertical axis marked in the figure by a dotted line. The 

 openings in c c', when the instrument is in position, as in fig. 249, 

 corresponds exactly with those in d d' ; but if c c' is turned at right 

 angles to its present position, there is no communication between h 

 and a and i and a', but h communicates directly with i ; and if turned 

 through two right angles c' communicates with d, and c with cT,and 

 there is no direct communication between h and i. The experiment 

 is performed in the following way : The artery to be investigated 

 is divided and connected with two cannulse and tubes which fit it 

 accurately with h and i\ h is the central end, and i the peri- 

 pheral ; the bulb a is filled with olive oil up to a point rather lower 

 than k, and a! and the remainder of a is filled with defibrinated 

 blood ; the tube on k is then carefully clamped ; the tubes d and d' 

 are also filled with defibrinated blood. When everything is ready, 

 the blood is allowed to flow into a through A, thus driving the oil 

 over into a and displacing the defibrinated blood through i into the 

 peripheral end of the artery ; a' is then full of oil ; when the blood 

 reaches the former level of the oil in a, 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 a is known; the diameter of the artery is then 

 measured, and as the number of times a has been filled in a given 

 time is known, the velocity of the current can be calculated. 

 We may take an example to illustrate this : 



volume per second V 

 Velocity = - - = -^-. 



sectional area b 



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 instru- 

 ment would be 50 c.c. in 100 seconds, or 0'5 c.c. in 1 second. Next, 

 suppose the diameter of the artery is 4 mm. The sectional area is 

 Trr 2 ; r is the radius (2 mm.), and TT = 3*1416. From these data we 

 get 



V 0-5 c.c. 500 cubic millimetres 

 3lt y = "a = 3-1416 x V - 3-1416 x 4 8 mm ' P er sec " 



Many modifications of Ludwig's original instrument have been 

 devised. Fig. 250 shows Tigers ted t's. 



The tubes A and B are placed in connection with the two ends 



