INFLUENCE OF THE CARDIAC NERVES ON THE FROG'S HEART 233 



the skin and muscles, at the angle of the jaw, thus exposing the vagus nerve. 

 The vagus runs diagonally downward and backward along the edge of the 

 delicate muscle toward the heart. The glossopharyngeal is just in front 

 of the vagus and the hypoglossal just behind it. The latter runs parallel 

 with the vagus near its origin, but lower down turns across the vagus and runs 

 to its distribution in the tongue muscles. These two nerves serve to aicl the 

 student in the identification of the vagus, see figure 216. It is usually better 

 to cut the hypoglossal away, and also to cut the brachial and the laryngeal 

 nerves. 



Prepare an induction coil, see Laboratory experiments on muscle. Use 

 platinum electrodes of the Harvard pattern, set the coil for a mild stimulus when 

 tested by the lips or the tongue, lift up the vagus gently and lay it on the platinum 

 tips of the electrodes, taking care that the electrodes do not come in contact 



FIG. 216. Diagram Showing the Relations of the Vago-sympathetic Nerve to the Heart, in the 

 Frog. Hy, Hypoglossal; Gl, glosso-pharyngeal; Lar, laryngeal; V, vago- sympathetic; H, heart; 

 L, lung. 



with the adjacent tissue. Arrange a signal magnet as shown in the diagram, 

 so that the signal magnet and the stimulating key of the induction coil may 

 be closed and opened at the same instant. When all 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 experiment 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 sugges- 



