XIII.] STRUCTURE AND ACTION OF THE HEART. 14o 



a. Observe that during and for a short time 

 after the passage of the current, the heart 

 remains with all parts in diastole. (If no 

 effect is produced push the secondary nearer 

 the primary coil.) 



6. The period of rest (inhibition) may be followed 

 by a period (reaction) in which the beats are 

 quicker and more forcible ; and then the 

 previous normal condition is regained. 



Now stimulate the pneumogastric nerve, indi- 

 cating on the curve by means of a time-marker 

 (cp. Appendix) the moment at which stimulation 

 begins, and ends. Note that the heart does not 

 stop immediately after the current is sent into 

 the nerve. 



5. Place the frog on a stage and arrange a lever 

 (cp. Appendix) so that it presses lightly on the 

 ventricle, and bring the end of the lever to mark 

 on the revolving drum. Take a tracing of the 

 ventricular pulsations with the drum at a mode- 

 rate speed. 



Note the rise and fall of the lever, indicating 

 the change of form during contraction. The 

 rise increases at first rapidly, then more slowly, 

 to a maximum, and the fall is similarly at first 

 slight, then more rapid, but finally slow again. 



6. Turn the heart over and stimulate the line of j unc- 

 tion of the sinus and right auricle ; the heart 

 will stop beating just as it did when the pneumo- 

 gastric was stimulated. 



