BLOOD-PRESSURE 77 



ventricle. The dicrotic wave is due to a rebound from the 

 closed aortic valve. It cannot be due to reflected waves 

 from the periphery, since there is always the same interval 

 between it and the main wave, whatever the distance from 

 the heart. The dicrotic wave corresponds to the rise at 

 4 in the aortic-pressure tracing of Fig. 10. 



The sphygmographic record is subject to considerable 

 variation even in normal individuals. Secondary waves 

 may appear, due to reflected waves from the periphery, to 

 vibration of the arterial wall, and to instrumental error. 

 When there is a high blood-pressure owing to resistance to 

 the outflow of blood from the arteries, the upstroke is more 

 prolonged and may show upon it a secondary wave : such 



• 



Fig. 18. — Radial pulse. (From Starling's Principles of Physiology.) 



a pulse is called anacrotic. When the outflow is freer the 

 upstroke tends to be sharper, and a secondary wave 

 appears in a pre-dicrotic position on the downstroke — a 

 catacrotic pulse. Secondary waves which are post-dicrotic 

 in position are of instrumental origin. 



BLOOD-PRESSURE 



Measurement 



The arterial blood-pressure is measured directly in 

 animals by the insertion of a cannula into the artery. This 

 is connected with a mercury manometer. On the open 

 surface of the mercury there is a float which holds a writing 

 pointer. The cannula and tube between the blood and the 

 mercury are filled with sodium sulphate, which prevents 

 clotting. 



