INHIBITION AND CARDIAC VAGUS 105 



When it is considered that every act of breathing and every 

 change of posture send vagus messages to the heart, it 

 seems obvious that the nerve fibres are adapted to control 

 the heart's action, and enable it to do its work. Such 

 messages are truly physiological, and cannot be measured 

 in electrical language. But though they cannot be so 

 measured, the changes induced are of definite advantage 

 to the organism, and indicate one of the most delicate 

 adaptations to gravity, or slight efforts, to be found in the 

 mammalian body. Vagal or youthful irregularity of the 

 heart is doubtless of a similar kind. In this kind of cardiac 

 arrhythmia, still frequently mistaken by some medical 

 men for serious disease, the heart slows after every inspira- 

 tion, while, if the patient holds his breath, the irregularity 

 tends to disappear. Though it is now known, owing to the 

 work of Mackenzie, that it has no pathological significance 

 whatsoever, it certainly has a physiological signification 

 as showing to what immeasurably small stimuli the normal 

 heart can and does respond. It may eventually be found 

 that, though the slowing occurs during expiration, the 

 vagal stimulation is experienced during inspiration, when 

 the lungs expand. In a pathological case, the mere act of 

 swallowing stimulates the vagus, and produces heart-block 

 a few seconds later. The more these facts are considered 

 the less likely does it seem that a tied-up animal, with a cut 

 vagus electrically stimulated, can be held to show pheno- 

 mena on a cardiogram which are remotely relevant to 

 normal vagal action. Moreover, even when pressure over 

 the vagus is applied at the neck and slowing results, there 

 is no evidence to prove that the result is directly due to 

 the pressure, since it is, on these general lines, far more 

 likely to be due to the partial interruption of the circulation, 

 and a vagal attempt at vaso-motor readjustment. 



