THE CIRCULATION 



257 



3. Mean Blood Pressure. 



I. GENERAL DISTRIBUTION. 



That the pressure is positive greater than the pressure of 

 the atmosphere throughout the greater part of the blood 

 vessels is indicated by the fact that if a vessel is opened, the 

 blood flows out of it. The force with which blood escapes is 

 a measure of the pressure in that particular vessel. If an 



B 



FIG. 124. A, The Mercurial Manometer with recording float, used in taking 

 records of the arterial blood pressure of lower animals. B, The Hill- Barnard 

 Spbygmometer, for measuring the arterial pressure in man. 



artery be cut, the blood escapes with great force ; if a vein be 

 cut, with much less force. 



Actually to measure the pressure in arteries and veins in 

 the lower animals is easy. It is only necessary to let the 

 escaping blood act against some measured force e.g. the 

 force of gravity, or a column of mercury. The instru- 

 ment most frequently employed is a U tube containing 

 mercury, one limb of which is connected with an artery by 



17 



