CH. XXL] BLOOD -PRESSURE IN MAN 293 



us a reading of arterial pressure. If the mean pressure inside and 

 outside an artery be made equal, then the wall of the vessel is able 

 to vibrate at each pulse with the greatest freedom. The mean 

 pressure is less than the systolic, but greater than the diastolic 

 pressure; thus during the heart's systole the artery is opened out 

 to its fullest extent, while during the heart's diastole its lumen is 

 obliterated ; hence the vessel wall swings with the greatest amplitude. 

 If the pressure exerted by the sphygmometer is less than the mean 

 arterial pressure, the artery 'will not be compressed to its utmost 

 during diastole ; if, on the other hand, the pressure exerted is greater 

 than the mean, the artery will not fully expand during systole. In 

 either case, the pulsation will not be so great as when the pressure 

 exerted on the outside of the artery equals the mean pressure within. 



By recording the arterial pressure in the dog with a mercury 

 manometer and the sphygmometer simultaneously, the instrument 

 has been found to give fairly accurate results (see note at end of 

 this chapter, p. 313). 



The normal pressure in the radial artery of healthy young adults 

 is 110 to 120 mm. Hg. It appears to be as constant as the body 

 temperature. In the recumbent posture the pressure is slightly 

 lower than in the erect position. This relation is reversed in condi- 

 tions of exhaustion. During muscular exertion the pressure is raised, 

 while in the subsequent period of rest it is subnormal. Mental 

 work raises the pressure ; during rest and sleep it is lowered. The 

 taking of food produces no noteworthy effect. In disease there are 

 naturally variations in different directions, and the study of these 

 has already yielded valuable results. 



With this instrument the venous pressure can also be obtained 

 in the manner suggested by Dr George Oliver. On the back of the 

 hand or arm a vein is chosen free from anastomoses, and the 

 sphygmometer is pressed upon the peripheral end of this. The vein 

 is then emptied centrally i.e., towards the heart by the pressure of 

 the finger. Next the pressure in the sphygmometer is gradually re- 

 laxed, and the exact height noted at which the vein refills with blood. 



Since the flow of blood through the capillaries is maintained by 

 the difference in pressure between the artery and vein, we can, by 

 obtaining readings both of the arterial and of the venous pressures, 

 estimate the comparative efficiency of the capillary circulation in 

 man under varying conditions. 



The Capillary Plow. 



When the capillary circulation is examined in any transparent 

 part of a living animal by means of the microscope the blood is seen 

 to flow with a constant equable motion ; the red blood-corpuscles 



