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MANUAL OF PHYSIOLOGY. 



smaller oscillations are found to correspond with the heart-beat, 

 the larger waves have the same rhythm as the respiratory move- 

 ments, and the average elevation of the mercurial column is 

 spoken of as the mean pressure. In the large arteries of the warm- 

 blooded animals this mean pressure varies with the size of the 

 animal from 90 mm., mercury, to more than 200 mm. In cold- 

 blooded animals it is comparatively low, from 22 mm. (frog, 

 Volkmann) to 84 mm. (large fish). 



FIG. 135. 



Blood-pressure Curve, drawn by mercurial manometer. O x = zero 

 line, y y' = curve with large respiratory waves and small waves of heart 

 impulse. A scale is introduced to show height of pressure in c c of Hg. 



The general mean pressure in the arteries is increased by : (1), 

 increased action of the heart; (2), increased contraction of the 

 muscular coat of the arteries; (3), sudden increase in the quan- 

 tity of blood. When the change is gradual the vessels adapt 

 themselves to the increase. The opposite of these conditions may 

 be said to have an opposite effect. 



