PNEUMOGASTRIC, OR PAR VAGUM NERVE. 655 



it is to dimmish the frequency of its pulsations. Inasmuch as our object is simply to 

 show that, imitating the nervous force by galvanism, the action of the pneumogastrics 

 is inhibitory, we shall not discuss the effects of different currents, and other experi- 

 ments, which have little relation to the natural action of the nerves, and possess slight 

 interest from a purely physiological point of view. 



The direct action of the pneumogastrics upon the heart is undoubtedly through their 

 motor filaments. All the facts developed by experiments are in accordance with this view. 

 If the nerves be divided in the neck, galvanization of the central ends has no effect 

 upon the heart, the pulsations being arrested only when the peripheral ends are stimu- 

 lated. This shows that, at least as far as the fibres passing down the neck are con- 

 cerned, the action is centrifugal and direct, not reflex. Another curious fact illustrates 

 the same point very forcibly. It is well known that the woorara-poison completely par- 

 alyzes the motor nerves, leaving the muscular irritability and the sensory nerves intact. 

 It has been found that, in animals poisoned with woorara, the action of the heart being 

 maintained by artificial respiration, galvanization of both pneumogastrics has no effect 

 upon its pulsations. This fact we have repeatedly verified in public demonstrations. 

 Still another curious fact remains bearing upon the question under consideration. If pow- 

 erful galvanization, which immediately arrests the cardiac pulsations, be continued for a 

 certain time, so that the motor filaments become temporarily exhausted and lose their 

 irritability, the heart resumes its contractions, notwithstanding that the galvanization is 

 continued; the nerves being for the time incapable of transmitting the inhibitory influence. 



The source of the motor filaments in the pneumogastrics which exert a direct inhibi- 

 tory action upon the heart becomes an important point to determine. In the original 

 experiments by the brothers Weber, it was shown that, when the galvanic stimulus was 

 applied to that portion of the centres from which the nerves take their origin, the action 

 of the heart was arrested in the same way as when the nerves themselves are galvan- 

 ized ; and it has been shown by subsequent observations that, when the heart is thus 

 arrested by galvanization of the medulla oblongata, if both pneumogastrics be divided in 

 the neck, its action is resumed. This would at first lead to the supposition that the 

 inhibitory filaments are derived from the roots themselves of the pneumogastrics; but it 

 has been conclusively demonstrated that they are really derived from the spinal acces- 

 sories, the upper filaments of origin of which are situated just below the roots of the 

 pneumogastrics. 



It has been shown that powerful galvanization of one pneumogastric will arrest the 

 heart's action, and also that this inhibitory action is much more marked in the right 

 than in the left nerve. Waller, after extirpating the spinal accessory nerve upon one 

 side, found that galvanization of the pneumogastric upon that side had no effect upon 

 the heart, provided that from ten to twelve days had elapsed after extirpation of the 

 spinal accessory, a sufficient time to secure disorganization and loss of irritability of its 

 fibres. These experiments show conclusively that the motor filaments contained in the 

 pneumogastric, which act directly upon the heart, are derived exclusively from the com- 

 municating branch of the spinal accessory. 



Reflex Influence, through the Pneumogastrics, upon the Circulation. Galvanization 

 of the central ends of the pneumogastrics, after their division in the neck, does not influ- 

 ence the action of the heart, except as the pulsations are affected by the modifications in 

 respiration. When the central ends are stimulated, the pupils become dilated, the eyes 

 protrude, sometimes vomiting occurs, and always the number of respiratory acts is 

 diminished, and, with a powerful current, are arrested in inspiration ; but the pulsations 

 of the heart are not affected. 



Depressor -Nerve. An important reflex action operating upon the circulation through 

 branches of the pneumogastrics has lately been described by Cyon and Ludwig, in a 

 memoir which received the prize for Experimental Physiology from the French Academy 

 of Sciences, in 1867. The experiments upon which this memoir is based are exceedingly 



