348 Messrs. Hill, McQueen, and Flack. Conduction of the 



so as to prevent any considerable leak of fluid from the T-tube. The artery 

 in the T-tube was then surrounded by Einger's solution and the pressure of 

 this raised. Obliteration of the pulse wave occurred when the pressure in 

 the T-tube reached that in the artery — 190 mm. Hg. (fig. 2a). The same 

 exposed artery removed from the T-tube and placed across the dome of a 

 watch-glass required but 27 mm. Hg (fig 2b). 



This experiment demonstrates the fact that the deformation of the arterial 

 lumen is the prime cause of the obliteration of the pulse wave. When the 

 artery is circularly and equally compressed by surrounding fluid, the pulse 

 continues to come through until the full systolic pressure is overcome. In 

 the case of the artery exposed and lying on a rigid surface, when the bag is 

 pressed on it the arterial wall is pushed in above and bulged out at the sides. 

 The lumen is thus converted from a circle into an ellipse, and resistance is 

 offered to the pulse by the changed shape. The force of the pulse is spent 

 on the labile wall of the artery in front of this resistance. 



In corroboration of this experiment we have also found that if the finger 

 be gently laid along the course of the radial artery and the bag of the 

 sphygmomanometer pressed upon the finger until the pulse ceases to come 

 through under it, a less pressure is required than without such interposition 

 of the finger. This, is because the finger brings about the deformation of the 

 artery more easily than the bag. If the armlet be used and the finger be 

 inserted under the armlet to palpate the artery, one finds that the pulse 

 does not cease to come through under the finger until the full systolic 

 pressure is reached. Thus the readings were 35 and 97 mm. Hg respectively 

 in the case of a youth. 



In the aberrant radial artery the pulse was obliterated by 55-60 using the 

 bag, by 35 using finger and bag. Using armlet and finger the radial was 

 obliterated by 135 and using armlet alone by 135. 



A thin, distensible rubber bag inflated with a pressure of air can easily be 

 deformed from the spherical to elliptical or other shape without altering 

 the internal pressure. The bag may thus be made to take a shape which 

 would give great resistance to the passage of a pulse wave or flow of fluid, 

 although the total volume and pressure of the air in the bag remains 

 unaltered. 



In the experiments on animals (goat, dog, cat) it was found that the 

 pressure required to obliterate the pulse wave in the exposed artery varied 

 from 25 to 60 mm. Hg. To elucidate the cause of the higher and lower 

 readings we measured the pressure necessary for the obliteration of the 

 pulse in the same exposed artery with the animal (cat) in the head-down, 

 horizontal, and feet-down position. We found that the pressure necessary 



