LVI.] CARDIAC VAGUS OF THE FROG. 273 



(c.) Adjust a lever marking time in seconds exactly over the 

 electro-magnet lever. 



(7.) There will be recorded two horizontal lines ; stimulate with 

 a single induction shock, the moment of stimulation will be 

 indicated by the second lever, and shortly after, the heart will 



FIG. 195. Tracing of Stanniused Heart of Frog, stimulated at S with a single Maximum 

 Induction Shock. T. Time in seconds. Gaskell's Lever. 



respond ; the interval represents the " latent period " which may 

 be about half a second according to temperature and other con- 

 ditions (fig. 195). 



(e.) Stimulate the auricle and observe the longer " delay " ; the 

 wave of contraction takes longer to travel, and is delayed at the 

 groove. 



LESSON LVI. 



CARDIAC VAGUS AND SYMPATHETIC OF THE 

 FROG AND THEIR STIMULATION. 



1. Cardiac Vagus of the Frog To Expose it. Make a pre- 

 liminary dissection before attempting to stimulate the vagus. 



Pith a frog, or destroy its brain and curarise it. Lay it on 

 its back on a frog-plate. Expose the. heart, remove the sternum 

 and pull the fore-legs well apart. Introduce a small test-tube or 

 stick of sealing-wax into the oesophagus to distend it ; the nerves 

 leaving the cranium are better seen winding round from behind 

 when the oesophagus is distended. Remove the muscles covering 

 the petrohyoid muscles, which reach from the petrous bone to the 

 posterior horn of the hyoid bone (fig. 196). Three nerves are seen 

 coursing round the pharynx parallel to these muscles. The lowest 

 is the hypoglossal (Hg), easily recognised by tracing it forward to 



