114 



The arterial manometer registers a higher and higher pres- 

 sure. At length the pressure ceases to rise. The mercury 

 remains at a mean level broken by a slight accession at each 

 stroke. The pump now merely maintains the constant high 

 arterial pressure. This pressure suffices to drive through the 

 resistance during each stroke and the succeeding interval all 

 the liquid received from the pump during the stroke. 



The venous pressure remains very low. The capillary 

 resistance (to which must be added the resistance of the 

 smallest arteries) almost entirely exhausts the pressure in the 

 arteries. Hence the sudden and profound difference observed 

 between the arterial and the venous pressure. A second arte- 

 rial manometer placed near the aorta will show that the loss 

 of pressure between the ventricle and the smallest arteries is 

 relatively slight. The pulse is absent on the venous side of 

 the resistance. 



364. Changes in the Stroke of the Pump ; Inhibition 

 OF the Ventricle. While the arterial pressure in the artifi- 

 cial scheme is at a good height (about 120 mm. Hg.) arrest 

 the ventricular stroke, (comparable to cardiac inhibition 

 through the vagus in animals). So soon as the ventricle 

 ceases to beat, the less distended arteries will empty them- 

 selves through the peripheral resistance, and the arterial 

 manometer will show a continuous fall in blood pressure. 

 Resume the ventricular beats. The mercury in the arterial 

 manometer will rise in large leaps, corresponding to the ease 

 with which the early strokes of the pump distend the lax arte- 

 ries (the inertia of the mercury somewhat exaggerates the 

 rise at each stroke). As the blood pressure rises, however, 

 the excursion of the mercury for each ventricular stroke be- 

 comes less and less, corresponding to the smaller and smaller 

 difference between the pressure in the arteries and the maxi- 

 mum pressure within the ventricle, until at length equilibrium 

 is restored between the peripheral resistance and the force 

 and frequency of the ventricular beat. 



365. The Opening and Closing of the Valves. Se- 

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

 scheme. Now greatly slow each ventricular beat and at once 

 observe closely the action of the valves. It will be seen that 

 the mitral valve closes as soon as the ventricle begins to con- 



