1014 



PHYSIOLOGY 



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 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. 456). 



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



G.J. 



G.h.V 



r.Vg. 



r.Sp. Vc. 



n.c. 



G.Tli.4.. 



D.5. 



FIG. 455. 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; G.J, ganglion 

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

 sympathetic ; G.C, inferior cervical ganglion ; A.V, annulus of Vieussens ; A.sb, sub- 

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

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

 obain. 



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 

 ' escape ' of the heart from the vagus influence. This escape is generally 

 confined to the ventricles, and the. heart beats are found on opening the chest 

 to be purely ventricular, the auricles and great veins remaining in a state of 



