SCHEME OF THE CIRCULATION. 239 



each beat a pulse-beat is felt. The resistance at the periphery 

 brings the elasticity of the tube into play between the beats, 

 and thus converts the interrupted into a uniform flow. 

 This apparatus serves also to demonstrate why there is no 

 pulse in the capillaries, and under what circumstances a 

 pulse is propagated into the capillaries and veins. 



3. Scheme of the Circulation. Use either Rutherford's scheme 

 or the major schema. In the latter, the heart is represented 

 by an ordinary elastic pump, the arteries by long elastic tubes 

 dividing into four smaller tubes with clamps on them ; two 

 of the tubes leading into tubes filled with sponge to represent 

 the capillaries. The capillaries lead into a tube with thinner 

 walls representing the veins. The inflow tube into the heart, 

 and the outflow tube at the vein are placed in a basin of water, 

 and the whole system is filled with water. 



(a.) Use two mercury manometers, and connect one with 

 the arterial, and the other with the venous tube. Adjust 

 a float on each, and cause the writing-points of the two 

 floats to write exactly in the same vertical line on a revolving 

 cylinder, the venous one a little below the arterial one. 



(6.) Unclamp all the arteries, and work the pump, 

 regulating the number of beats by means of a metronome 

 beating 30 times per minute, and compress the heart to the 

 same extent each time with a lemon-squeezer. Observe 

 that both manometers oscillate nearly to the same extent 

 with each beat. Take a tracing on a slow-moving drum. 



(c.) Gradually clamp the arteries to offer resistance, and 

 continue to work the pump, the pressure in the arterial 

 manometer will rise more and more with each beat until it 

 reaches a mean level with a slight oscillation with each 

 beat. The pressure in the venous manometer rises much 

 less, and the oscillations are very slight or absent. 



(d.) While the mean arterial pressure is high, cease 

 pumping ; this will represent the arrest of the heart's action, 

 brought about by stimulation of the peripheral end of the 

 vagus the arterial blood-pressure falls steadily. 



(e.) Begin pumping again until the mean arterial pressure 

 is restored, and then unclamp gradually the small arteries. 



