56 CIRCULATION OF THE BLOOD ACTION OF THE HEAET. 



Direct Inhibition of the Heart. Division of the pneumogastric nerves 

 in the neck increases the frequency and diminishes the force of the contrac- 

 tions of the heart. To anticipate a little of the history of the pneumogastric 

 nerves, it may be stated that while they are exclusively sensory at their origin, 

 they receive, after having emerged from the cranial cavity, a number of fila- 

 ments 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, the arrest of the movements of the glottis. 



A moderate Faradic current passed through both pneumogastrios arrests 

 the action of the heart in diastole (Ed. Weber). This observation has been 

 made upon 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 pneumogastric which are derived from 

 the spinal accessory. 



It is said that direct stimulation of the medulla oblongata will have the 

 same effect upon the heart as stimulation of the pneumogastrics ; but it must 

 be very difficult to limit the stimulation to a particular point in the medulla 

 and to avoid conditions which would complicate such an experiment. 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 be Faradized for two or three minutes, the con- 

 tractions of the heart return, even though the stimulation be continued, pro- 

 vided 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 

 excitability of the nerve after a time becomes exhausted by the 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 sometimes 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 dias- 

 tole, direct mechanical stimulation of the heart is followed by a single con- 

 traction, 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 (Traube) ; showing that separation of the 

 heart from its connections with the cerebro-spinal nerves removes the organ 

 from the characteristic and peculiar effects of the poison. 



Feeble stimulation of one or both pneumogastrics, when it produces any 

 effect, almost always slows the action of the heart. In some animals, how- 

 ever, the pneumogastrics contain a few accelerator fibres, and feeble excita- 

 tion sometimes is followed by a slight increase in the rapidity of the cardiac 

 pulsations, but this is unusual. 



