THE NERVOUS REGULATION OF THE HEART 



971 



ACTION OF THE VAGUS 



The action of the vagus fibres on the heart is almost identical in frog and 

 mammal. If in the dog the peripheral end of the cut vagus be stimulated 

 while the arterial blood pressure is being recorded by means of a mercurial 

 manometer, the pulse is seen to become slower, or with a stronger stimulus to 



G.J. 



r.Vg. 



SP.AC. 



FIG. 446. Diagram of cardiac inhibitory and accelerator fibres in 



the dog. (From FOSTER.) 



r.Vg, roots of the vagus ; r.Sp.Ac, roots of the spinal accessory ; GJ, ganglion 

 jugulare ; G.h.V, ganglion trunci vagi ; Vg, trunk of vagus nerve ; C.Sy, cervical 

 sympathetic ; GC, inferior cervical ganglion ; AV, annulus of Vieussens ; A.sb, sub- 

 clavian artery ; nc, cardiac nerves ; G.St, ganglion stellatum ; D2, D3, D4, D5, 

 second, third, fourth, and fifth dorsal spinal roots ; G.Th, ganglia of the thoracic 

 chain. 



cease altogether, and the blood-pressure falls towards zero. On discon- 

 tinuing the stimulus the heart begins to beat again and the pressure rises 

 after a few beats to normal (Fig. 447). 



If the stimulation of the vagus be prolonged, the blood-pressure, on dis- 

 continuance of the stimulus, may rise above normal owing to the asphyxia 

 of the vaso- motor centres produced by the prolonged cessation of the 

 circulation. Even during the application of the stimulus the heart often 

 begins to beat again with a slow rhythm. In this case we speak of an 



