440 Prof. C. S. Roy and Mr. J. G. Adami. [Feb. 11, 



put of the heart and lower the blood pressure, thereby reducing the 

 activity of the circulation as a whole. The influence of the blood 

 pressure in the systemic arteries on the degree of vagus activity and 

 the readiness with which the vagus centre is called into play by 

 raising the intercranial pressure indicate that the vagus mechanism 

 is specially employed in lowering the circulation so as to limit 

 cerebral congestion. The vagus acts chiefly in the interests of the 

 heart and central nervous system. 



The power of the vagus over the heart is limited, and the ideo- 

 ventricular mechanism, which comes into play when the vagus action 

 exceeds a certain limit, must be looked upon as the means by which 

 arrest of the circulation and death is prevented, whenever from any 

 cause the nerve exerts a maximum influence. The power of the 

 vagus to lower the excitability of the ventricles makes their tem- 

 porary arrest possible, but this reduction of the excitability of the 

 ventricles cannot be kept up, no matter how strong the stimuli applied 

 to the nerve, for a period long enough to endanger the economy. 



In Section XI we show that the function of the augmentor in the 

 economy is to increase the work and tissue waste of the heart as part 

 of the mechanism by which the nervous system governs the circu- 

 lation, and that the augmentor mechanism sacrifices the heart in 

 order to increase the output of the organ and enable the ventricles 

 to pump out their contents against a heightened arterial pressure. 

 Such excessive action of the heart is limited by the vagus, which, 

 as we have seen, readily steps in so soon as the call for an increased 

 supply of blood has ceased. It may do so earlier, presumably because 

 the increased blood pressure or the fatigue of the heart calls for vagus 

 intervention. 



In Section XII we consider the mode of interaction of the vagi and 

 augmentores ; we point out that when the vagi are paralysed by sec- 

 tion or atropin the augmentores have no control over the cardiac 

 rhythm, and that therefore they can only act by inhibiting the influ- 

 ence of the vagi on the rhythmic centre of the heart. When neither 

 nerve is acting on the auricles they contract with a certain force, 

 iiich is increased by the augmentores and diminished or inhibited 

 by the vagi. The force of the ventricular contractions is increased 

 by augmentor action : this increase can be inhibited by vagus excita- 

 tion, which latter has otherwise no power to reduce the strength of 

 ventricular contractions. 



The force of the heart's contractions is influenced by other factors 

 than the vagi, augmentores, and other nerves. The pressure of the 

 blood in the coronary arteries is one of the most important of these 

 factors. If this be lowered, the contractions of both auricles and 

 ventricles diminish in strength, while a rise of pressure in the 

 systemic arteries causes an increase in the force of the heart's con- 



