176 



1'IIYSIOLOGY FOR DENTAL STUDENTS. 



young men varies from 110 to 130 mm. of mercury when it is 

 determined in the sitting posture. When ;i person is lying down 

 the pressure is a little less, and after hard exercise a little higher. 

 The blood pressure under ordinary conditions is relatively con- 

 stant, and is dependent on a delicate adjustment of the relation- 

 ship existing between the force of the heart, the amount of blood 



Fig. 22. Apparatus for measuring the arterial blood pressure in num. 

 The pressure in the cuff is raised by means of the syringe until the pulse 

 can no longer be felt at the wrist. This pressure is read off on the mercury 

 manometer (systolic pressure). 



pumped at each beat, the resistance which the walls of the blood 

 vessels offer to the flow of the blood, the size of the vascular sys- 

 tem, and the amount of blood in the body. Since the amount of 

 blood in the body is relatively constant, we may say that the 

 factors which change are the heart and the blood vessels. How 



