1090 PHYSIOLOGY 



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 discontinuing the stimulus the heart begins to 

 beat again and the pressure rises after a few beats to normal (Fig. 449). 

 If the stimulation of the vagus be prolonged, the blood pressure, 

 on discontinuance of the stimulus, may rise above normal owing to the 

 asphyxia of the vaso-motor centres produced by the prolonged cessa- 

 tion 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 diastole. Vagus escape is favoured 

 by distension of the heart cavities, and is often synchronous with the 



FIG. 449. Blood-pressure tracing from carotid of dog (taken with Hiirthle's 

 manometer), showing effect of excitation of vagus (between the arrows). 



o, abscissa line of no pressure. 



respiratory efforts, which supervene after a certain duration of inhibi- 

 tion as a result of the asphyxia of the respiratory centre. 



When the arterial system is dilated, so that the mean systemic 

 pressure, and consequently the venous pressure during cardiac inhibi- 

 tion, are low, or when the asphyxial gasps of the animal are prevented 

 by anaesthesia or by section of the spinal cord, the heart may fail to 

 recover from the inhibition produced even by a transitory stimulation 

 of the vagus. In such cases it is necessary to knead the heart in order 

 to restore its rhythmic action. 



To study the influence of the vagus on the auricles and ventricles 

 respectively, it is necessary to work with the chest opened and to 

 record separately the contractions of the different segments of the 

 heart. It is then found that the vagus may affect the heart in one of 

 several ways. Its most marked action is on that part of the heart 

 where it enters, viz. the venous end. It may affect that part of the 

 auricle corresponding to the primitive sinus venosus, where the 

 rhythm of the whole heart is determined. In this case the sole effect 



