290 



MANUAL OF PHYSIOLOGY. 



them ; the blood then flows readily through the capillary network, 

 the veins become engorged, the arterial blood pressure falls, and 

 the circulation comes to a standstill, in spite of the heart's more 

 rapid beats. We know also that after the arterioles are passed 



FIG. 131. 



Mercurial Manometer for measuring and recording the blood pressure. 

 a. Proximate limb of manometer. 6. Union of two limbs of manometer. 

 e. The rod floating on mercury and carrying the writing point, d. Stop- 

 cock through which the sodium bicarbonate can be introduced between the 

 blood and mercury of manometer. 



the pressure falls suddenly, and in the capillary network the pres- 

 sure is always very low. 



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

 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 



