CH. XXL] 



PITOT'S TUBE 



283 



the pressure-head ; the difference in height between the two would 

 therefore give the velocity-head. As the tubes would in this way 

 be inconveniently long, it is better to use short tubes connected at 

 the top by glass or rubber-tubing. The air contained will be com- 

 pressed, and the two pressure-heads will balance one another, so that 

 the difference in height will again represent the velocity-head ; the 

 velocity will be directly proportional to the square root of this 

 velocity-head. This is the principle of one of the best instruments 

 we possess for determining velocity, namely, Cybulski's photo-haemato- 

 chometer. The meniscus of the fluid in each tube is photographed 

 on a moving sensitive plate, and in this 

 way a graphic record is obtained of the 

 changes in velocity at times corresponding 

 to different parts of the cardiac cycle. If 

 one wishes to determine the velocity in 

 absolute measures, the instrument must be 

 previously calibrated by passing through 

 it fluids flowing at known rates. It will 

 be sufficient to give the results of one 

 experiment; in the carotid artery during 

 the ventricular systole the flow was at the 

 rate of 238-248 mm. per second; during 

 the diastole it sank to 127-156 ; in the 

 femoral artery of the same animal, these 

 numbers were 356 and 177 respectively. 



To determine the pressure gradient in 

 arteries, simultaneous measurements of 

 the lateral pressures in two vessels at 

 different distances from the heart must be 

 recorded. 



It has been found that the diastolic 

 pressures in the crural and carotid are 

 practically identical, but that the maximum 

 systolic pressure is actually higher in the 

 crural than in the carotid; in the dog the difference may amount 

 to as much as 60 mm. mercury. This difference is partly to be 

 explained in that the carotid arises from the aorta at a right angle, 

 and therefore gives the true pressure-head, while the crural, to a con- 

 siderable extent, faces the stream, and therefore gives both pressure- 

 head and velocity-head. 



Unfortunately, at present no really satisfactory measurements are 

 at hand from which the pressure gradient can be determined. 



Cybulski's instrument is not the only one we possess for obtaining 

 records of the velocity-pulse. Vierordt invented a hsemo-tachometer, 

 employing the principle of the hydrometric pendulum ; his instrument 



Fro. 282. Diagram to illustrate 

 the principle of Pitot's Tube 

 and Cybulski's Photo-hsemato- 

 chometer. 



