INFLUENCE OF THE CARDIAC NERVES ON THE FROG'S HEART 255 



decrease in the rhythm, the sinus having the same rhythm as the whole heart, 

 the auricle a considerably slower rhythm, and the ventricle with a very slow 

 rhythm or even quiet. The sinus preparation will show beside the funda- 

 mental rhythm a characteristic slow contraction and relaxation, which has 

 been described as tone, figure 170. 



10. Influence of the Cardiac Nerves on the Frog's Heart. Care- 

 fully pith a frog so as not to break the blood vessels at the base of the brain, 

 and thus permit the loss of the blood of the animal. Expose the heart as 

 previously described, make a cut through the manubrium, continue it 

 through the skin and muscles at the angle of the jaw, thus exposing the 



FIG. 216. Arrangement of Apparatus for Studying the Contractions of the Strip of the 



Apex of the Ventricle. 



vagus nerve. The vagus runs diagonally downward and backward along the 

 edge of a delicate muscle toward the heart. The glosso-pharyngeal 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 aid the student in the identification of the vagus, see figure 217. It is 

 usually better to cut the hypoglossal away, and also to cut the brachial and 

 the laryngeal nerves to prevent the stimulation of structures undesired. 



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 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 



