THE CIRCULATION OF THE BLOOD. 311 



force or power of the contraction, the frequency remaining the same. 

 An increase in the volume of blood ejected at each contraction will 

 necessarily lead to an accumulation. With the accumulation there 

 goes an increased distention of the artery and a corresponding increase 

 of pressure. In a short time, therefore, the increased pressure will 

 force out of the arteries at a 

 higher rate of speed this ex- 

 cess of blood until the outflow 

 again equals the inflow. This 

 restores the equilibrium but 

 establishes the mean pressure 

 at a higher level. 



If the peripheral resistance 

 is increased by a contraction 

 of the muscular walls of the 

 arterioles, the frequency and FIG. 145. FALL OF BLOOD-PRESSURE FROM 

 force of the heart remaining ARREST OF THE HEART'S ACTION DUE 



,i ,i MI i i TO STIMULATION OF THE VAGUS BEGUN 



the same, there will also be AT a ^ STOPPED AT b.- (Landois and 



an accumulation of blood in Stirling.) 



the arteries until their in- 

 creased distention and consequent rise of pressure become sufficient 

 to increase the rapidity of outflow until it counterbalances the inflow. 

 The converse of these statements also holds true. If when the 

 general arterial pressure is in a condition of equilibrium the heart 

 ejects into the arteries in a given period of time a decreased quantity 

 of blood, either as a result of a decrease in the rate or power or both, 



FIG. 146. FALL OF BLOOD-PRESSURE FROM DIMINUTION OF THE PERIPHERAL RESIST- 

 ANCE, THE RESULT OF A DILATATION OF THE ARTERIOLES, BROUGHT ABOUT BY 

 STIMULATION OF THE CENTRAL END OF THE DEPRESSOR NERVE. Stimulation 

 begun at d, and stopped at b. (Landois and Stirling.) 



there will soon be a diminution of the arterial distention and a con- 

 sequent fall in pressure (Fig. 146). This continues until the outflow 

 no longer exceeds the inflow. Equilibrium will again be established, 

 but the pressure will be at a lower level. 



If the peripheral resistance is diminished by a dilatation of the 



