INHIBITION OF THE HEART 47 



Direct Inhibition of the Heart. Division of the pneumogastric 

 nerves in the neck increases the frequency and diminishes the force of 

 the contractions of the heart. To anticipate a little of the history of 

 the pneumogastric nerves, it may be stated that while they are exclu- 

 sively sensory at their origin, they receive, after having emerged from 

 the cranial cavity, a number of communicating filaments from various 

 motor nerves. That they influence certain muscles, is shown by the 

 paralysis of these muscles after division of the nerves in the neck, as, 

 for example, arrest of the movements of the glottis. 



A moderate faradic current passed through both pneumogastrics 

 arrests the action of the heart in diastole. This observation has 

 been made on living animals, both with and without exposure of the 

 heart; and this kind of action is known as inhibitory, or restraining. 

 Its nervous mechanism is direct and not reflex ; and the inhibitory 

 influence is conveyed to the heart through filaments in the pneumo- 

 gastrics, derived from the spinal accessory. 



It is said that direct stimulation of the bulb has the same effect on 

 the heart as stimulation of the pneumogastrics; but it is difficult to 

 limit the stimulation to a particular point in the bulb and to avoid com- 

 plicating conditions. A sufficiently powerful stimulus applied to one 

 pneumogastric will arrest the cardiac pulsations, and in some animals 

 the inhibitory action is confined to the nerve of the right side. It is 

 not known that any such difference between the two nerves exists in 

 the human subject, and certainly there is no marked difference in most 

 of the mammalia. 



If both pneumogastrics are faradized for two or three minutes, the 

 contractions of the heart return, even though the stimulation is con- 

 tinued, provided the current be not too powerful but of sufficient 

 strength to promptly arrest the pulsations. It is probable that this is 

 due to the fact that the conductivity of the nerve after a time becomes 

 exhausted by prolonged excitation, and its inhibitory influence is for the 

 time destroyed. 



Stimulation of the pneumogastrics in any part of their course is 

 followed by the usual inhibitory phenomena, and the same results some- 

 times follow stimulation of the thoracic cardiac branches. It has also 

 been observed that when the heart's action has been arrested and the 

 organ is quiescent in diastole, direct mechanical stimulation of the 

 heart is followed by a single contraction, showing that the excitability 

 of the fibres has not been entirely suspended. 



After section of both pneumogastrics in the neck, digitalis fails 

 to diminish the number of beats of the heart ; showing that separa- 

 tion of the heart from its connections with the cerebro-spinal nerves 



