CARDIAC MUSCLE. 69 



the apex to a heart lever. Frequently pour Ringer's solution 

 over the preparation. The writing-point of a magnetic signal 

 in the primary circuit is exactly aligned with the writing point 

 of the heart lever. The secondary coil is connected by fine 

 wires to the base and apex of the heart. A record of the normal 

 heart is made upon a fairly rapid clock-work drum. Then 

 single break shocks are sent into the heart during different 

 phases of the cardiac cycle in an attempt to secure an extra 

 contraction (Fig. 20). 



During what phase can an extra contraction be produced? 

 What follows an extra contraction? 



When you consider that the heart obtains its rest only 

 between beats, what do you think of this arrangement as a 

 safeguard against encroachment upon the rest period? 



Next, cut the ventricle across near its junction with the 

 auricles. If the ventricle does not stop beating, cut still closer 

 until the beats disappear. Now pin the cut surface of the apex 

 to the board and apply the wire which formerly connected the 

 base of the heart. Study the effects of a series of graded single 

 shocks. How does this agree with the all-or-none principle? 

 Also attempt to get the "staircase" effect. Try the effect of 

 two successive stimuli in order to discover whether there is a 

 refractory period. 



Experiment 31. Perfusion of the Turtle Heart. The differ- 

 ence of different inorganic salts upon the activity of cardiac 

 muscle can best be shown by perfusion experiments. 



In order that the experiment may have the greatest chance 

 of success you must understand and prepare the mechanical 

 arrangements before beginning the operation. A small beaker 

 containing Ringer's solution is placed on a shelf or stand so 

 that pressure is obtained for a siphon connecting the contents 

 with a rubber tube w^hich is long enough to be manipulated 

 with ease when the time comes to insert the cannula. A small 

 glass cannula fits into the lower end of the rubber tube, the 

 flow of solution being controlled by a clamp. Make sure that 

 the system of tube and cannula is absolutely free from air. 

 Expose the heart ; then carefully free a short length of the inferior 

 vena cava without puncturing it. Place a fine ligature around 



