VAGUS NERVE 65 



arms to the seeker. Instead of this a short glass rod 

 may be pushed down the oesophagus into the stomach 

 and the frog turned on its side. The vagus will be seen 

 on the lower edge of the slip of the petrohyoid muscle 

 mentioned above, and close to the internal jugular vein. 

 In the following dissection the vagus must not be pulled 

 on or pinched. Pick up the hypoglossal, isolate it cen- 

 trally for a short distance taking care not to injure the 

 vagus, cut it and turn it forward. Treat similarly the 

 glossopharyngeal. The vagus will now be more obvious, 

 it will be seen to consist of two branches, the smaller, 

 anterior one runs to the larynx; the larger one runs to 

 the heart, lungs and stomach. Note the much larger 

 nerve the brachial running to the arm. Trace the 

 vagus to the skull, cutting away any tissue obscuring it. 

 Pass a needle threaded with a wet silk thread under the 

 vagus near the skull (the petrohyoid muscle may be 

 taken with it), tie the thread, cut the nerve close to the 

 skull and isolate it for about a centimetre. Do not iso- 

 late near the heart. 



Cut off the arm, expose the heart if it is not already 

 exposed and pin down the frog firmly. Place a piece of 

 thin india-rubber membrane upon the tissue under the 

 vagus and stimulate the nerve for about 5 seconds with 

 weak tetanising currents, increasing the strength if neces- 

 sary till an effect is obtained (strong currents cause local 

 injury). There should be contraction in the larynx, but 

 in no other muscle. Watch the heart, and note 



The inhibition of the heart-beat. A second or two 

 after the beginning of the stimulation the heart stops 

 beating, all parts of it being flaccid (diastolic stand- 

 still) ; and it remains so during the stimulation and for a 

 short time afterwards. The inhibitory effect varies 

 L. 5 



