THE CIRCULATION OF THE BLOOD 525 



blood-pressure rises, however, the excursion of 

 the mercury for each ventricular stroke hecomes 

 less and less, corresponding to the smaller and 

 smaller difference between the pressure in the 

 arteries and the maximum pressure within the 

 ventricle, until at length equilibrium is restored 

 between the peripheral resistance and the force 

 and frequency of the ventricular beat. 



The Effect of Inhibition of the Heart on the 

 Blood-Pressure in the Frog. Arrange an induc- 

 torium for strong tetanizing currents. Insert 

 the electromagnetic signal in the primary circuit 

 and bring its writing point beneath that of the 

 manometer. Eaise the heart gently. Note the 

 white " crescent " between the sinus venosus and 

 the right auricle. Put the points of the elec- 

 trodes on the crescent, and close the circuit 

 for a moment. After one or two beats the 

 heart will stop. 



Observe the great fall in blood-pressure. 

 Cease the stimulation. 



The mercury returns in leaps to its former 

 level. 



THE HEART AS A PUMP 



The Opening and Closing of the Valves. Secure 

 a high arterial pressure (120 mm. Hg) in the 

 artificial scheme. Now greatly slow each ven- 



