105 



350. Induction in Nerves. Remove the secondary coil 

 and with the aid of a glass plate, lay the nerve of a -well 

 moistened nerve-muscle preparation upon the primary coil in 

 such a way that the free end of the nerve touches the nerve 

 near the muscle or touches the muscle itself, so as to form a 

 closed circuit. Make and break the primary circuit. Make 

 and break currents will be induced in the nerve. and the mus- 

 cle will contract. 



351. Telephone Experiment. Arrange a nerve-muscle 

 preparation with its nerve over a pair of electrodes. Connect 

 the latter with a short-circuiting key. Attach the key to 

 the telephone by means of two wires. Open the short-cir- 

 cuiting key ; shout or blow into the telephone, and note that 

 the muscle contracts vigorously. Remove the electrodes from 

 the key and connect directly with the telephone. What is 

 the result ? 



XXVIII 



352. Cardic Vagus of the Frog. Refer to the previous 

 dissection of the vagus. Pith the frog. Lay it on its back on 

 a frog board. Expose the heart, remove the sternum and pull 

 the fore legs well apart. Distend the oesophagus by intro- 

 ducing a glass rod or tube ; the nerves leaving the cranium are 

 better seen winding from behind when the oesophagus is dis- 

 tended. Remove such muscles as may be necessary. Three 

 nerves are seen coursing round the pharynx. The lowest is 

 the hypoglossal, easily recognized by tracing it forward to 

 the tongue, next is the vagus in close relationship with a 

 blood vessel, and finally the glossopharyngeal. Observe the 

 laryngeal branch of the vagus. The vagus as here exposed, 

 outside the cranium, is really the vago-sympathetic as it con- 

 tains fibers from the sympathetic system. The glosso-pharyn- 

 geal and vagus leave the cranium through the same foramen, 

 in the exoccipital bone, and through the same foramen the 

 sympathetic enters the skull. 



Stimulation of the vagus. Adjust a heart lever so as to 

 record the contractions of the heart upon a drum moving at a 

 medium rate of speed. Place well-insulated electrodes under 



