Do the roots of the vagus contain any motor flaments?—No one can for a moment doubt that the trunk of the vagus, in its course down the neck, does contain motor filaments, but there is every reason to believe that it derives at least the greater part of these from the spinal accessory. From the resemblance of the vagus and spinal accessory as they lie in the foramen lacerum posterius to the anterior and posterior roots of a spinal nerve,—the vagus with its supe rior ganglion corresponding to the posterior, and the spinal accessory to the anterior root,— many anatomists and physiologists have of late maintained that the roots of the vagus, like the posterior roots of the spinal nerve, contain no motiferous filarnents. It is scarcely necessary to add, that the junction of the internal branch of the accessory and the vagus immediately beyond the superior ganglion of the latter, in creases still further this resemblance between these and a spinal nerve. This opinion has been maintained on anatomical considerations alone, by Arnold, Scarpa, and Bendz,* and has been further strengthened by the experi ments of Bischoffif Valentin,r. and Longet.§ It is on theNother hand maintained, that this opinion is too-exclusive, and that, though there can be no doubt of the greater part of the fila rnents of the roots of the vagus being incident and sensiferous, yet they do contain some rno tiferous filaments. We have seen that, proba bly both in man and in some of the other rnammalia, a few of the filaments of the vagus do not pass through its superior ganglion, and consequently the anatomical argument is not so conclusive as it at first appears to be. An examination of the experimental proof adduced in favour of these two opinions skews that the former is chiefly founded upon negative, and the latter upon positive evidence. Miller'. saw muscular movements of the pharynx follow excitation of the roots of the vagus within the cranium ; but from having neglected some pre cautions in the performance of the experiment, lie himself is not disposed to attach to it much weight. I have related some experiments in which I observed muscular movements in the pharynx, larynx, and cesophagus,from irritation of the vagus within the cranium, on the dog immediately after death.f Volkmann has per formed similar experiments upon calves, sheep, goats, and cats, and perceived muscular con tractions in the levator palati, azygos the superior and inferior constrictor muscles of the pharynx, the palato-pharyngeus, and crico thyroid.T The experiments of Stilling,§ Wag ner,l1 Van Kempen,lf Hein," and Bernardtf are also all in favour of the opinion that the root of the vaaus contains motor filaments.
We believe that we are justified in concluding from the evidence here adduced, that the vagus, even at its origin, and before it has received any fibres from the accessory, does contain a few motor filaments" We shall here make a few remarks upon the immediate effects of cheinical and mechanical excitation of the trunk of the vagus as it lies in the neck, and then proceed to examine in detail the functions of its auricular, pharyngeal, laryn geal, cesopliageal, cardiac, pulmonary, and gas tric branches. When the trunk of the vagus has been exposed in the neck in a living animal, and is cut, bruised, or rendered suddenly tense by forcible stretching, the animal generally gives indications of severe suffering, while ill some cases the animal remains quiescent, and, as far as we can judge, suffers little, if any.
There can be no doubt, from the distinct testimony of numerous experimenters,-1- that the trunk of the vagus does contain sensiferous filaments, but there are good grounds for be lieving that the application of chemical agen cies or the infliction of mechanical injuries upon this nerve below the origin of its superior laryngeal branch, are not attended with the same amount of pain as would attend similar lesions of one of the ordinary spinal nerves. Dr. Marshall Bail and Mr. Broughton re marked, that when the compression of this nerve is continued " for a few moments, an act of respiration and deglutition follows, with a tendency to struggle and cough."1 Romberg observed excitation of the vagus in the neck in a horse produce cough ; § and it appears that Cruveilhier had made previously the same ob servation.11 In some of the cases in which I
made this experiment on dogs, I observed powerful respiratory muscular movements, but never succeeded in inducing cough. Longet has been equally unsuccessful in producing cough by this meanslf The respiratory mus cular movements which follow excitation of the vagus in the neck are not dependent upon any direct action transmitted downwards to the lungs or muscles of respiration, but upon a reflex action, as Dr. Marshall Hall pointed out, arising from certain impressions being carried upwards to the medulla oblongata by the inci dent fibres of the vagus, followed by the tmns mission of a motor influence outwards from this portion of the central organ of the nervous system along the motiferous nerves distributed in the muscles moved. The excitation and mechanical injury of the vagus in the neck in duces various other results, some of which may be included among their immediate effects, such as those upon the movements of the intrinsic muscles of the larynx, the diminution of the frequency of the respimtions, Fcc.; but these will be more methodically introduced among the remarks which we have to make upon the functions of the individual branches of the nerve.* Auricular branch.—From the origin of this branch from the superior ganglion of the vagus, and from being partly distributed to the inte guments of the pavilion of the external ear, it is probable that it is composed of sensiferous filaments. If the portion of this bmnch which throws itself into the portio dum be sensiferous, the portio dum may contain soine sensiferous filaments as it issues from the stylo-mastoid foramen.t Pharyngeal branches.—As a great part, some times nearly the whole, of the superior pharyn geal branch of the vagus comes directly from the internal branch of the spinal accessory, we may, on anatomical grounds alone, conclude that it contains motor filaments. In irritating this bmnch in dogs, both alive and immediately after death, we observed exiensive movements of the muscles of the pharynx and soft palate without any distinct indications of pain. As, however, the animal must necessarily be sub jected to considerable suffering before the nerve can be exposed, this result cannot be taken as a conclusive test that it contains no fila ments of common sensation. We also found that division of this branch on both sides ren dered the second stage of deglutition difficult, by paralysing the muscles of the pharynx. The morsels of food were forced through the now passive bag of the pharynx to the commence ment of the (esophagus by the repeated efforts of the muscles of the tongue and those attached to the larynx and hyoid bone. From these facts, vve concluded that the pharyngeal branches of the vagus are chiefly, perhaps entirely, com posed of motiferous filaments, and- that they convey outwards the motive influence by which the muscles of the pharynx and soft palate are excited to contraction in the reflei muscular movements of deglutition.* It is possible that they may also contain a few sensiferous and incident filaments. Valentin, on irritating these branches in different animals immediately after death, saw the pharynx contract in a marked manner through its whole length.t Volkmann states, as we have already had occasion to mention, that various muscles of the soft palate and pharynx were thrown into contraction on excitation of the vagus within the cranium.t He further observes, that he could not perceive any movements in the mus cles of the pharynx or soft palate on irritating the spinal accessory within the cranium. This last result is certainly one which we would not expect, but the remarks we have to make upon it will be more appropriately introduced in the article SPINA L ACCESSORY NERVE.§ Longet observed very marked contractions in the pha rynx on galvanizing the pharyngeal branch of the vagus in the horse and the dog.II Though the experiments we have referred to, in illus tration of the functions of the pharyngeal branches of the vagus, differ in some respects, they all agree in this, that extensive and active muscular movements of the pharynx may be produced by their excitation, and that they therefore contain many motor filaments. We have adduced some facts which would seem to shew that they contain few, if any, motor fila ments.