276 



PRACTICAL PHYSIOLOGY. 



[LVI. 



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 sym- 

 pathetic is found before it joins the vagus emerging from the cranium (fig. 

 198). 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. The nerve 

 is usually pigrnented. Put a ligature round it as far away from the skull as 

 practicable, and cut the nerve below the ligature. 



FIG. 198. .Scheme of the Frog's Sympathetic. LAS. Levator anguli scapulas; Sym. 

 Sympathetic ; GP. Glosso-pharyngeal ; V-S. Vago-sympathetic ; <?. Ganglion of 'the 

 vagus ; Ao. Aorta ; SA. Subclavian artery (Gaskell). 



(b.) Expose the heart and attach its apex to a lever supported by an elastic 

 thread as in Gaskell's method. Record several contractions, and then stimu- 

 late the sympathetic with weak interrupted shocks by means of fine electrodes. 

 The heart beats quicker. If the heart is beating quickly, reduce the 

 number of beats by cooling it with ice. 



(c.) If desired, the vagus may be isolated and stimulated, and the effects of 

 the two nerves compared (although the vagus outside the skull is really the 

 vago-sympathetic). 



Stimulation of the intracranial vagus i.e., before it is joined by the 

 sympathetic is somewhat too difficult for the average student, and is there- 

 fore omitted here. 



N.B. It is important to note that the effect of vagus stimulation on the 

 heart varies with the season of the year, and is often different in the two vagi. 

 In some animals one vagus is inactive. 



