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MANUAL OF PHYSIOLOGY. 



them ; the blood then flows readily through the capillary net- 

 work, the veins become engorged, the arterial blood pressure 

 falls, and the circulation comes to a stand-still, in spite of the 

 heart's more rapid beats. We know, also, that after the arterioles 

 are passed the pressure falls suddenly, and in the capillary net- 

 work the pressure is always very low. 



The four great factors, then, in keeping up the arterial blood 



FIG. 131. 



Mercurial Manometer for measuring and recording the blood pressure. a. Proximate 

 limb of manometer. Z>. Union of two limbs of manometer, e. The rod floating on 

 mercury and carrying the writing point, d. Stop-cock through which the sodium bi- 

 carbonate can be introduced between the blood and mercury of manometer. 



pressure, are : 1, the heart beat ; 2, perfect aortic valves ; 3, the 

 elastic resiliency of the large arteries ; 4, the resistance offered 

 by the contraction of the muscular arterioles. 



If any of these fail, the mechanism of the circulation is at once 

 impaired. For example, the heart's beat may be stopped by the 

 stimulation of the inhibitory nerve fibres of the vagus, in which 



