CARDIAC MUSCLE. 67 



Study the action of the heart and arteries. Examine the 

 pericardium, then remove it. Where does the contraction seem 

 to originate? What path does it follow in passing over the 

 heart? Is there any delay in the passage between auricle and 

 ventricle? 



A-V INTERVAL. In order to observe the time relations between 

 the contractions of the auricle and ventricle arrange to take 

 simultaneous records of the two chambers one directly above 

 the other, using heart levers. Attach the levers to the tip of 

 each chamber by means of tiny hooked pins on the ends of 

 threads, but take care not to puncture the wall. By using a 

 very rapid drum the interval between the contraction of auricle 

 and ventricle can be determined (Fig. 19). 



FIG. 19. Simultaneous tracings of the contraction of the auricle (a) and 

 ventricle (v) in the turtle. Time interval 5 sec. (contractions slower than normal). 



STIMULATION OF THE VAGUS. Dissect out both vagi high up 

 in the neck. They can be located near the carotid artery. 

 Pass threads under the vagi so that they can be lifted when 

 applying the electrodes. 



Study the effect of stimulating each vagus by tetanizing 

 currents of different strengths. Are the right and left vagus 

 nerves equally efficient in stopping the heart beat? Do they 

 both have exactly the same effect? Explain. Is there escape 

 from inhibition? . Will a weak current alter the A-V interval 

 without stopping the heart beat? Secure records to demon- 



