218 EXPERIMENTAL PHYSIOLOGY. 



(c.) When all is ready lift the weight off the electro- 

 magnet, whereby the secondary circuit is un-short-circuited, 

 the electro-magnet lever rises up, records its movement 011 

 the cylinder, and at the same moment the induction shocks 

 are sent through the vagus. Observe that the heart is not 

 arrested immediately, but a certain time elapses the latent 

 period usually about one beat of the heart (0-15 sec.) before 

 the heart is arrested. 



(</.) Short-circuit the secondary current again, and observe 

 how the heart gradually resumes its usual rhythm, sinus 

 venosus, auricles, and ventricle. 



(<?.) Repeat (c.) several times, noting that the heart after 

 arrest goes on beating in spite of continued stimulation. 



4. Gaskell's Method. Instead of recording the movements of 

 the heart by means of a lever resting on it, a very convenient 

 method is that of Gaskell. 



(a.) An ordinary writing-lever is placed above the frog- 

 plate on which the frog rests, and supported in the horizontal 

 position by a thin thread of india-rubber, as in Fig. 106. 

 Expose the heart of a pithed frog, and leave it in situ. Tie 

 a thread to the apex of the ventricle, clamp the aorta to fix 

 the heart in position, and then attach the apex thread to 

 the lever. Every time the heart contracts it pulls down 

 the lever, and the latter is brought into position again, when 

 the heart relaxes, by the piece of elastic. 



5. Action of the Sympathetic on the Heart of the Frog. 



(a.) Pith a frog or preferably a toad, cut away the lower 

 jaw, and continue the slit from the angle of the mouth 

 downwards for a short distance. Turn the parts well 

 aside, and expose the vertebral column where it joins the 

 skull. Remove the mucous membrane covering the roof of 

 the mouth. The sympathetic is easily found before it joins 

 the vagus emerging from the cranium (Fig. 100). Carefully 

 isolate the sympathetic. It lies immediately under the 

 levator anguli scapulae, which must be carefully removed 

 with fine forceps when the nerve comes into view, usually 

 lying under an artery. Put a ligature round it as far away 

 from the skull as practicable. 



