64 PRACTICAL PHYSIOLOGY 



gus gives off a small branch on each side, which runs 

 close along the superior vena cava to the sinus ve- 

 nosus. Place the vagus trunk upon a fine pair of 

 wire electrodes passed through a flat piece of cork 

 (which must itself be fixed securely by pins to the 

 frog-cork), and connect these electrodes, and also 

 those which are fixed into the brain, to a commu- 

 tator without cross wires so that the faradizing 

 shocks can be sent to one or other pair as may be 

 desired. Place the frog-cork upon the stand of a 

 frog cardiograph (Fig. 44), and by means of a 

 thread and fine steel hook attach the apex of the 

 ventricle to the light lever of the apparatus, and 

 record the contractions of the heart upon a very 

 slowly moving drum (one revolution in four or five 

 minutes). Be careful not to injure the heart more 

 than is absolutely necessary. It is well, in order 

 to fix the preparation securely, to pass a pin close 

 to the base of the heart through the vertebral 

 column into the frog-cork. The following experi- 

 ments may be performed upon this preparation : 



1. Take a normal tracing of the beats during 

 about a minute. 



2. Whilst this is proceeding stimulate not too 

 strongly the medulla oblongata, allowing the result 

 to be recorded. 



3. Cut the vagi nerves near the skull and repeat 

 the above experiment. No effect should be obtained. 



4. After the normal contractions are resumed 

 stimulate the vagus, and again record the result. 

 With weak stimulation of this nerve the heart 

 may beat faster and more strongly owing to exci- 



