THE ARTERIAL PRESSURE 



191 



under the above conditions, are less labile, and conduct the big crest 

 of the wave better than the arm arteries. There are other possible 

 factors at work which cannot be discussed here. In cases of aortic 

 regurgitation there is present a well marked and diagnostic difference 

 of pressure between arm and leg in the horizontal position. 



When the bag of the sphygmometer is applied to an artery (such as 

 the dorsalis pedis) which lies upon bone unsupported by tissues, a far 

 lower pressure (30-35 mm. Hg) than that in the artery suffices to obliterate 

 the pulse. Under these conditions, the artery is easily deformed from 

 the round to the oval shape. This change in shape occasions an in- 

 creased resistance to the passage of the pulse-wave, and the force of 





B. CHEST 



C. ABDOMEN. 



D. CHEST. 



\'i>.. U8. INFLUENCE OF CHEST AND ABDOMINAL BREATHING ON THE PULSE. (Lewis.). 



the pulse in consequence becomes spent in the labile arterial system 

 above the point of deformation. Where the artery is surrounded by 

 tissues full of circulating blood, it cannot become deformed until the 

 blood-pressure in the vessels of the surrounding tissues is overcome. 

 The pressure of the bag first obliterates the veins; the pressure then 

 rises in the capillaries and approximates to the arterial pressure as the 

 compression is increased. Finally, the arterial pressure is overtopped, 

 and then the compression comes to act upon the main artery, and 

 this being deformed, the pulse ceases to come through. This is c?- 

 emplified in Fig. 97. In A, with the artery lying in a tube surrounded 

 by fluid, corresponding to the tissues, the pressure required to dtrnp 



