ELEMENTAEY EXPERIMENTAL PHYSIOLOGY 



135 



piled up in the artery, for at each systole a greater quantity of blood 

 is driven into the artery than can escape through the capillaries. With 

 each succeeding systole, therefore, the pressure in the artery rises, and 

 the pressure in the vein falls. Venous pressure cannot sink below the 

 atmospheric pressure, for the flaccid walls of the veins collapse. The 

 venous side is capacious, and possesses little elasticity. Thus the 

 changes of pressure in the venae cavae, when the heart is arrested or 

 starts beating, are insignificant. A slight positive pressure is maintained 

 in the veins by the action of the muscles, which, at every movement of 



Fio. 134. Another model of artificial schema of circulation. 



the body, forces the blood on past the venous valves and overcomes the 

 effect of gravity. Raise the end of the board to which the pump 

 (heart) is attached. The water under the influence of gravity distends 

 the lower part of the vein, the upper part empties, and the circulation 

 is impossible. Compress the lower part of the vein with your hands 

 and the circulation is restored. This shows the effect of relaxation of 

 the muscles in fainting and the method of restoring the subject by 

 compressing the abdomen. 



The continuous flow of blood established through the capillaries is 

 due to the difference between the pressure in the arteries and veins. 

 This difference depends: on (1) the energy of the heart, (2) the elasticity 

 of the arteries, (3) the peripheral resistance. The energy of the heart 

 is spent in overcoming the resistance, and is dissipated into heat. 



Vary (1) by lessening the rate of the pump; vary (2) by opening 

 the screw-clip the difference in pressure diminishes in either case, 

 and the flow becomes intermittent. When the screw-clip is open a 



