972 PHYSIOLOGY 



1 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 respiratory efforts, which supervene after a 

 certain duration of inhibition 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 inhibition, are low, or 

 when the asphyxial gasps of the animal are prevented by anaesthesia or by a 



FIG. 447. Blood-pressure tracing from carotid of dog (taken with Hurthle's 

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

 o, abscissa line of no pressure. 



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 actionT 



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 of the vagus on the auricles and ventricles will consist in 

 an alteration of rhythm. They may cease to beat altogether or they may 

 give beats of normal strength but at a slower rhythm than before. Often 

 indeed under these conditions the beats of the ventricles may be increased 

 in size, since the strength and extent of their contractions are determined, 

 not by the strength of the stimulus arriving from the auricles, but by 

 the length of their fibres, and this will increase with any prolongation 

 of the diastolic period, and consequent increased diastolic filling of the 

 ventricle. 



If the vagus acts on the auricles without affecting the sinus part of the 

 auricles (sino-auricular node), the rhythm will be unaltered, but the response 

 of the auricles to the impulses received by them will be diminished, and the 

 amplitude of the excursions of the lever attached to them will therefore be 



