Laryngeal branches.—When the superior laryngeal nerve is laid bare in a living animal and pinched with the forceps, the animal gives indications of severe suffering, while on re peating the same experiment on the inferior la ryngeal the animal seldom gives any indication of suffering pain. When an opening is made into the trachea, and a probe introduced through it into the interior of that tube aud passed up wards, it excites litde or no uneasiness until it reaches the interior of the larynx, when violent paroxysms of coughing and signs of great un easiness immediately follow. The division of the irrferior laryngeal nerves has no effect in diminishing the severity of these paroxysms of coughing or in quieting the suvggles of' the anirnal, while they instantly cease on cutting across the internal branch of the superior la ryngeal nerves. Before Magendie published his observations upon the functions of these nerves it appears to have been genemlly be lieved that the different intrinsic muscles of the larynx received motor filaments both frotn the superior and inferior laryngeal nerves. Magen die has, on the other hand, maintained that the superior laryngeal moves those muscles which shut the superior aperture of the larynx, and the inferior laryngeal those which open it, and he supposed that this view sufficiently ex plained the closure of the superior aperture of the larynx on the division of both Inferior la ryngeal.* We found that on applying different excitants to the superior laryngeal nerve before it gave off its external branch in several animals immediately after death, that the crico-thyroid muscle was thrown into powerful contraction and the cricoid approximated to the thyroid cartilage, while all the muscles attached to the arytenoid cartilages remained quiescent. On irritating the inferior laryngeals all the muscles attached to the arytenoid cartilages were thrown into contmction, and as the force of those mus cles which close the superior aperture of the larynx preponderates over that of' those which open it, the arytenoid cartilages were drawn forwards and inwards, and the superior aperture of the larynx was closed. By applying the ex citation to the nerves for a short time and in rapid succession,. the superior aperture of the larynx could be made to close and open alter nately,—to close during the period of excitation and to open during the intervals,—and it was also remarked that the outward movement, or that of opening, was dependent upon the elas ticity of the parts. The inferences from these results were strengthened by an examination of the anatomical distribution of the laryngeal nerves, and confirmed by experiments upon living animals:I- From these and other facts related in the paper referred to, we arrived at the following conclusions. The superior la ryngeal furnishes one only of the intrinsic muscles (the crico-thyroid) of the larynx with motor filaments, while it supplies nearly all the sensiferous and incident filaments of the larynx, and also some of those distributed upon the pharynx and back parts of the tongue, so that it is chiefly composed of scnsiferous and incident filaments. he inferior laryngeal furnishes incident and sensiferous filaments to the greater part of the trachea, to the cervical portion of the cesophagus, a few to the mucous surface of the pharynx, and still fewer to the larynx ; it supplies the motor filaments of the cervical portion of the cesophagus and of all the muscles which are attached to and move the arytenoid autilages, and is chiefly composed of motor filaments.* When any excitation is applied to the mucous membrane of the larynx in the healthy state, this does not excite the contraction of the muscles which move the arytenoid carti lages by acting directly upon these through the mucous membrane, but is the result, as Dr. M. BAH- had maintained, of a reflex or excito motory action, in the performance of which the superior laryngeal is the incident, and the infe rior laryngeal the motor nerve. In each re current nerve two sets of motor filaments are included, one set transmitting the nervous in fluence which stimulates the opening muscles of the larynx to act synchronously with the other muscles of inspimtion, the other set transmit ting, the nervous influence which calls the closing muscles into synchronous action with the muscles of expiration4 Upon these views we can readily explain how, when the inferior laryngeal nerves are cut, all the movements of the muscles of the arytenoid cartilages are arrested, and the superior aper ture of the larynx, as was first pointed out by Legallois, can no longer be dilated during in spiration. In fact, the sides of the larynx are not only no longer separated by an active in fluence, but are rendered quite passive, and yield readily within the limits of their natural movements to any external force applied to them. When the recurrent nerves are cut in an adult animal, where the cavity of the larynx is large, a quantity of air may still find its svay through the diminished aperture, adequate, in many cases, to carry on the respimtory process in a sufficient manner, particularly if the mus cles of inspiration are not acting violently. If, on the other hand, the capacity of the larynx be proportionally smaller as in young animals, the air rushes through the diminished superior aper ture of the larynx in a narrower stream and with increased force, more especially when the in spiratory movements are powerful—or in other words, when the capacity of the thorax is sud denly and greatly enlarged,—and an insufficient quantity of air reaches the lungs. This quan tity is still further reduced by the circumstance that the now passive sides of the superior aper ture of the larynx are carried inwards by the current of air, and at each inspiration the ary tenoid cartilages may be so closely approx imated as to prevent the ingress of air and suf focate the animal. It is the inspiration alone of the animal which is difficult, for the expira tion is easy. The occurrence or non-occurrence of dypsncea, or suffocation, after section of the inferior laryngeals, is to be explained by the greater or less capacity of the larynx in the indi vidual animal, and the activity and extent of its respiratory movements at the time. The crowing sound which frequently attends this condition of the larynx is a mere physical effect, and depends upon the caffein. of air rushing rapidly through the diminished aper ture of the larynx, and may be imitated in the dead larynx. Severe dyspncea amounting to suffocation may arise both from the opposite conditions of irritation and compression of the inferior laryngeal nerves or the trunks of the pneumogastric above the origin of this bmnch.
We have stated above that on irritating one recurrent nerve we observed that the arytenoid cartilages were approximated so as in some cases to shut completely the superior aperture of the larynx, and we have already explained how paralysis of this nerve by compression or any other cause should produce this effect by arresting the movements of all the muscles attached to the arytenoid cartilages.* We also found that after the section of both superios laryngeal nerves in dogs and rabbits they swal lowed solids and fluids readily, and without exciting cough or difficulty of breathing.f, Mr. Hilton has arrived at the conclusion, from the anatomical distribution of the nerves alone, that the superior laryngeal is chiefly sensitive; and that the only motor filaments which it con tains are distributed in the crico-thyroid muscle, while the inferior laryngeal supplies all the, muscles attached to the arytenoid cartilageei with motor filaments,—a view in exact aci cordance with that which we have given above.fg Volkmann in his experiments found that did movements of the glottis were not affected byi dividing thesuperior laryngealnerves.§ Longa' has- published various experiments upon these nerves very similar to those which we had per formed, and obtained nearly the same results. Longet states that the respirations become in creased in frequency after dividing the recur rents. Other experimenters have lately satisfied themselves of the accuracy of these experiments.
-Effects of the laryngeal nerves on phonation. —The effect of the lesion of the recurrent nerves in enfeebling the voice was vvell known to Galen and the older physiologists.* We found in making this experiment that the voice, as Monro Secundust and others have stated, is not altogether lost, for in some cases, at least, the animal could still emit a faint howl. Longet has observed that the voice is com pletely lost in old animals, while young animals are still able to produce acute sounds different from the natural voice if the crico-thyroid muscle moved through the external branch of the superior laryngeal be not paralysed, and he attributes this clifference to the relative size of the larynx at these ages. We can have no doubts in attributing the effects of lesion of the inferior laryngeals upon the voice to the para lysis of the muscles attached to the arytenoid carti lages. § Magendie mentions that an animal after section of the superior laryngeal nerves " loses almost all its acute sounds it acquires, besides a constant gravity which it had not previously."11 This he attributed to the arrestment of the movements of the arytenoid muscles, but we 'have shown that the section of these nerves has no such effect. Bischoff could perceive no change upon the voice after he had divided these nerves in two dogs.if Longet states that the division of thos nerves above the origin of the external branch, or of the external branch alone, is follovved by a disagreeable hoarseness of the voice.** If the variations in the length of a tube alter the graveness and acuteness of the sounds which it emits, we would expect that the lesion of the superior laryngeals should, by arresting the movements of the crico-thyroid muscle, produce some change in this respect. Lonrret believes that the crico-thyroid muscles are, eduring their contmction, tensors of the vocal chords, and that the changes upon the voice induced by dividing the superior laryn geals depend upon the effect which paralysis of these muscles has upon the tension of the vocal chords:ft (Esophageal branches.—Muscular contrac tions have been observed in the cesophagus on irritating the trunk of the vagus,by Arnemann,* Cruikshank4 Mayo,$ and others. When the trunk of the vagus is irritated above the origin of the recurrent, the muscular fibres of the cesopliagus along its whole length are thrown into active contraction. In experiments upon rabbits we found that the cesophagus became impacted with food eaten after section of the vagi in the neck, when very little of it had reached the stomach and when no efforts at vomiting had occurred, while its muscular fibres could still readily be thrown into active contraction by direct excitation. From this we inferred that before the presence of the ingesta in this tube can excite its muscular fibres to contract and propel its contents onwards, the same conditions of the nervous system are ne cessary as for the production of the excito motory movements, and that certain of the fila ments of the vagi act as incident and others as motor nerves. That the food also collects in the (esophagus in the horse and sheep after division of the vagi may be inferred from the experiments of Dupuy§ and others. In subse quent experiments upon dogs we found that substances seem to pass pretty freely along the cesophagus in that animal after section of the vagi. It would appear, however, that even in the dog the food is occasionally retained in the cesophagus after dividing the yagi.11 Arnold (opus cit. p. 144) observed in his experiments upon hens and.pigeons that the cesophagus and crop were so relaxed after section of the vagi that when the animals shook their head and neck, or kept the head in a depending position, a quantity of chyme flowed from the bill.
From a review of all these facts we are in clined to agree in the opinion lately expressed by Dr. M. Hall, that in some animals the muscular contractions of the cesophagus are excito-motory, while in others they are called into action by direct excitation. We cannot at present determine whether the propulsion of the food along the cesophagus in the human species partakes more of the former or of the latter class of rnovements. .Magendie has as certained that various muscular movements go on in the lower part of the cesophagus, more especially when the stomach is full, by which this tube is contracted during inspiration and relaxed during expimtion, and that they are suspended by dividing the vagi. These we may class among the reflex muscular move ments. The cesophag-us is endowed with little sensibility, and in the natural and healthy con dition of the organ the ingesta are propelled along it to the stomach without exciting any sensation. rrota a consideration of all the above facts we believe that the (esophageal filaments are chiefly incident and motor, and a few of them only are sensiferous.