LXTI.] RIGID AND ELASTIC TUBES. 297 



(c.) Set the tuning-fork vibrating, allow the drum to move, compress the 

 elastic pump interruptedly— to imitate the action of the heart — and propel 

 water through the tube. The compression may be done by means of a lemon- 

 squeezer, the extent of the excursion being regulated by a screw, and, to 

 secure regularity, arrange the number of pulsations to the beating of a 

 metronome. On doing so, as one pumps in water, the tube distends and 

 raises the lever ; in the interval between the beats, as the water flows out 

 at the other end, the tube becomes smaller, and the levers fall. Feel the 

 tube ; with each contraction of the pump, a beat — the pulse-beat— can be 

 felt. 



(d.) Fix and study the tracing. The tracing due to the rise of the lever 

 next the pump begins sooner, and is higher than the one from the lever near 

 the outflow. Make two ordinates to intersect the three tracings, one where 

 the lower pulse-curve rises from the abscissa, and the other where the u])per 

 curve begins. Count the number of D.V. of the tuning-fork between these 

 lines. Measure the length of the tube between the two levers, and from 

 these data it is easy to calculate the velocity of the pulse- wave in feet per 

 second. 



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

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

 an elastic pump (Higginson's syringe), 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 tlie 

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

 whole system is filled with water. 



(''/.) IJse two mercury manometers, 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 on« 

 below the other in the same vertical Hne on a drum. 



(b.) Unclamp all the arteries, and work the pump, regulating 

 the number of beats by means of a metronome beating thirty per 

 minute, and compress the heart to the same extent each time with 

 a lemon-squeezer. Both manometers will 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 con- 

 tinue to 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 rapidly. 



