168 



PRACTICAL PHYSIOLOGY 



systole is divided into three: (1) the period of rising tension, when 

 all the valves are closed; (2) the period of output; (3) the period of 

 relaxation. In simultaneous records of intra ventricular pressure and 

 aortic pressure the beginning of the aortic rise (2) marks the opening 

 of the aortic valve and beginning of output. The end of output 

 occurs when the semilunar valves close at the beginning of the 

 dicrotic notch (4). The period of rising tension lasts from the 

 beginning of systole (1) to the opening of the semilunar valves (2). 

 Velocity of Blood Flow. Insert the Pitot tubes E and F into a 

 tube A through which water is flowing from a constant head of 

 pressure B (Fig. 170). Note that the water rises to different levels 



Fir,. 170. Schema to show the velocity and resistance heads. B, Pressure bottle, 

 with piezometers. B P, Pitot tubes. 



A, Tube 



in the tubes. E represents the resistance head plus the velocity 

 head. F records the resistance head minus the velocity head. 

 Measure the outflow per minute from the tube A, and notice the 

 difference between the heights of the menisci in E and F. Lessen 

 the velocity by partly screwing up the clip on the end A. Measure 

 the outflow per minute, and note that the menisci are nearer 

 together. Close the end of A. The flow ceases, and the menisci in 

 the two tubes reach the same level as that of the head of pressure 

 B. Cybulski makes use of this principle in the construction of the 

 photohaematochometer, an instrument by which alterations in velocity 

 can be recorded. Fig. 171. 



