280 THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXI. 



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 Jc 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 h, 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 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 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 

 7rr 2 ; r is the radius (2 mm.), and ?r = 31416. From these data we 

 get 



V 0-5 c.c. 500 cubic millimetres 



Velocity =-g-= 3 . ul6x22 = 3 . U 16 x 4 = 39 ' 8 mm ' P er sec ' 



Many modifications of Ludwig's original instrument have been 

 devised. Fig. 281 shows Tigers tedt's. 



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

 of the cut artery as before ; there is also a turn-table arrangement at 

 F, by means of which the two upper tubes C and D may be connected 

 as in the figure ; or by twisting it through two right angles, D can be 

 made to communicate with A, and C with B. In place of the two 

 bulbs of Ludwig's instrument there is a glass cylinder H which 

 contains a metal ball E. The whole instrument is washed out with 

 oil to delay clotting, and filled with defibrinated blood. As soon as 

 blood is allowed to flow from the artery, the ball E is driven over by 

 the current till it reaches the other end of the cylinder ; the instru- 

 ment is then rapidly rotated through two right angles, and once 

 more the ball is driven to the opposite end. This is repeated several 

 times, and the number of revolutions during a given period is noted. 

 The capacity of the cylinder minus that of the ball is ascertained, 



