256 



THE CIRCULATION OF THE BLOOD 



is ready stimulate the vagus for five to ten seconds, recording the time with 

 the signal magnet and allowing the record to continue until the heart has 

 returned to its normal rate and amplitude. Most students fail in this ex- 

 periment by not allowing sufficient time in the record for a normal before 

 stimulation, and by not allowing sufficient time after stimulation for a return 

 to the normal. It will be better to take one good tracing, showing the facts 

 of the experiment, than several partial tracings, none of which are complete. 

 With these suggestions in mind, repeat the above experiment, using stimu- 

 lating currents of increasing intensity until complete cardiac inhibition is 



FIG. 217. Diagram Showing the Relations of the Vago-sym pathetic Nerve to the 

 Heart, in the Frog. Hy, Hypoglossal; Gl, glosso-pharyngeal; ~Lar, laryngeal; V, vago- 

 sympathetic; H, heart; L, lung. 



produced. Perform experiments showing the influence of the time of the 

 stimulus on the inhibition; i.e., stimuli of i second, 2 seconds, 10 seconds, 

 and 30 seconds. 



In the frog the vagus, or inhibitory, and sympathetic, or accelerator, fibers, 

 are found in one trunk, the vago-sympathetic, but the stimuli will usually 

 produce inhibitions and not acceleration. Occasionally with very weak 

 preparations direct acceleration may be produced. To get the pure inhibi- 

 tory or pure accelerator effects one must dissect back to the origin of the vagus 

 before it is joined by the sympathetic fibers; or to the sympathetic trunk 

 between the third spinal nerve and the point where it joins the vagus trunk. 

 In the study of the conditions in the above experiments one should note 

 the rate per minute and the amplitude in the normal, the period just before 

 stimulation, the rate and amplitude during the period of stimulation, and the 

 same at different times after the stimulation until constant results are ob- 



