286 



THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXII. 



vessel wall as the pressure-head, and of the kinetic energy measured 

 in terms of a pressure as the velocity -head. We could then say that 

 the velocity between any two points is determined by the difference 

 between the two pressure-heads plus the velocity -head at the first 

 point. One method of recording the velocity-head is by the use of 

 a tube (Pitot's tube) shaped as in the accompanying figure (fig. 251). 

 The blood is made to enter at A, and leave through B ; in the same 

 straight line as A is a tube C, and a second tube D is placed at right 

 angles to the tube B. If the tubes C and D are placed vertically 

 and were sufficiently long, the blood would flow up C until it 



reached a height which would balance the 

 pressure-head plus the velocity-head ; in D 

 it would only reach a height sufficient to 

 balance 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 compressed, 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-hsemata- 

 chometer. The meniscus of the fluid in 

 each tube is photographed on a moving 

 sensitive plate, and in this way a graphic 



FIG. 251. Diagram to illustrate , . r , , .' n . ,, , * i 



the principle of Pitot's Tube record is obtained of the changes in velo- 



cSom C ete b r i ; lski ' s Phot " b8emata ' city 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 



