THE CIRCULATION OF THE BLOOD AND LYMPH 113 



FIG. 40. PITOT'S TUBES. 



3. A tube or box, in which swings a small pendulum, is inserted 

 in the course of the vessel. The pendulum is deflected by the blood, 

 and the amount of the deflection 



bears a relation to the velocity 

 of the stream (Vierordt's hcema- 

 tachometer ; Chauveau and 

 Lortet's much more perfect 

 dromograph] (Fig. 41). 



4. Pitot's Tubes. If two 

 vertical tubes, a and b, of the 

 form shown in Fig. 40, be inserted 

 into a horizontal tube in which 

 liquid is flowing in the direction 

 of the arrow, the level will be 

 higher in a than would be the 

 case in an ordinary side-tube 



without an elbow ; in b it will be lower. For the moving liquid will 

 exert a push on the column in a, and a pull on that in 6. The 

 amount of this push and pull will vary with the velocity, so that a 

 change in the latter will correspond 

 to an alteration in the difference of 

 level in the two tubes. Instruments 

 on this principle have been con- 

 structed by Marey and Cybulski, the 

 former registering the movements 

 of the two columns of blood by 

 connecting the tubes to tambours 

 provided with writing levers, the 

 latter by photography (Fig. 44). 



5. The electrical method, described 

 on p. 123, for the measurement of 

 the circulation time, can also be 

 applied to the estimation of the 

 mean velocity of the blood between 

 two cross - sections of the arterial 

 path which are separated by a 

 sufficient distance. For example, 

 salt solution can be injected into 

 the left ventricle or the beginning 

 of the aorta, and the interval which 

 it takes to reach a pair of electrodes 

 in contact with, say, the femoral 

 artery determined. Knowing the 

 distance between the point of injec- 

 tion and the electrodes, we can then 

 calculate the mean velocity. 



Of these methods, 3 and 4 are 

 alone suited for the study of the 

 velocity-pulse, that is, the change 

 of velocity occurring with every 

 beat of the heart. The curves 

 obtained by Chauveau' s dromo- 

 graph show a general agreement 

 with blood-pressure tracings taken 



FIG. 



41. CHAUVEAU'S DROMO- 



GRAPH. 



A, tube connected with blood- 

 vessel ; B, metal cylinder in com- 

 munication with A. The upper end 

 of B has a hole in the centre, which is 

 covered by a membrane, m, through 

 which a lever, C, passes ; C has a 

 small disc, p, at its end, which pro- 

 jects in'.o the lumen of A, and is d> 

 flected in the direction of the blood- 

 stream through A. The deflection is 

 registered by a recording tambour in 

 communication by the tube E with a 

 tambour D, the flexible membrane 

 of which is connected with the lever 

 or pendulum C. 



8 



