The spinal accessory in its course within the spinal canal frequently forms communi cations with the posterior root of the first cervical, and much more seldom with the posterior root of the second cervical nerve.* When these communicating filaments come from the second cervical, they are generally few in number. This communication between the spinal accessory and the posterior root of the first cervical is, according to Lobstein, more frequently present than absent.-1- When the posterior root of the first cervical joins itself, either in whole or in part, to the spinal accessory, a branch of equal size generally leaves the accessory, either at the point where it is joined by the posterior root of the first cervi cal, as figured and described by Asch t, or a little above this junction, as figured by Hu ber§, and described by Bellingerill This branch, after leaving the accessory, proceeds outwards, approaches the anterior root of the first cervical, and takes the place of the posterior root of that nerve.5 When the posterior root comes from the accessory, it generally presents a ganglion in the usual position. Sometimes, however, though rarely, this ganglion is found on the accessory where the posterior root of the first cervical leaves it to join itself to the anterior root. This ganglion was first pointed out by Huber ; its existence has been denied by Lobstein, Asch, Haller, and Scarps, and it has again been described by Bellingeri. I have seen this gan glion twice, and it was present on one side only. It becomes an interesting question in a physiological point of view to know, whether or not the whole of the filaments of the pos terior roots of the spinal nerves which join themselves to the accessory, again leave it to form the posterior root of the first cervical. Bellingeri answers this question in the affirm ative. " The filaments," he says, " coming from the posterior roots to the accessory are not intermixed, but only approximated, so that they can be separated by slight traction."** And in another place he says, " I believe that the filaments from the posterior roots, which join the accessory, leave it again to proceed to the posterior root of the first cervical."* From this he concludes that the accessory contains no sensiferous filaments. MUller, on the other hand, has adduced some unusual anatomical arrangements in this nerve, which may be regarded as favouring the opinion that it contains sensiferous filaments independant of those vvhich it may receive from the pos terior roots of the spinal nerves. He men tions an instance+, which he elsewhere describes at considerable length, where the postenor root of the first cervical nerve on the right side was not present, and where its place was supplied by two bundles of fila ments from the superior part of tbe spinal accessory. The upper of these bundles, at least, came from the metlulla oblongata. Upon the posterior root of the first cervical thus consttuted, a ganglion was formed while it was still within the theca vertebralis. The upper fibres of the posterior root of the second cervical of this side joined themselves to the accessory, but no nervous filaments were attached to the spinal chord in the usual position of the posterior root of the first cervical. On the left side, the posterior root of the first cervical presented its usual appearance, and was connected to the spinal accessory by some filaments of communi cation. The filaments of the accessory arising from the medulla oblongata did not, as on the right side, divide themselves into two parts, one of these becoming the substitute of the posterior root of the first cervical : but the whole ran upwards into the accessory nerved! Minter also states that Hyrtl has often seen a ganglion upon the accessory nerve opposite the entrance of the vertebral artery into the in terior of the cranium ; and that Remak showed him an instance of a ganglion upon the spinal accessory at its passage through the foramen lacerum. " I do not, however, affirm," Mid ler remarks in reasoning from these cases, " that the spinal accessory always contains originally sensiferous filaments, but leave it doubtful." " But in the case," he continues, " where the nervus accessorius forms an inti mate connection with the posterior root of the first cervical, or any other nerve, we may suppose an interchange; and this, in the same degree, will render probable the idea of Monro, that the communication of the spinal accessory with the posterior root of the first, or with any other spinal nerve, will be an equivalent to it for a posterior root." We have already seen that Stilling concludes, on anatomical grounds, that those filaments of the accessory that come from the medulla oblongata con tain centripetal filaments.* The spinal accessory passes through the foramen lacerum posterius in a canal formed by the clura mater, common to it and the vagus, but they are occasionally separated from each other as they enter this canal by a bridle of arachnoid, or of the dura mater. Soemmering has pointed out that the acces sory does not perforate the dura mater like the other nerves, but is, as it were, insensibly surrounded by this membrane.t One or two filaments generally pass be tween the accessory and the superior gang lion or gang/ion juguktre of the vagus, as they lie in the foramen lacerum posterius. Hein states that he has ntore than once distinctly observed, as also Krause has remarked, the superior five, or even six filaments of the root of the accessory approximate very closely to the ganglion jugulare of the vagus, and partly enter into its formation, so that a junction between the vagus and accessory had already taken place in this ganglion, before the fila ments of the accessory' had been fully collected to form together the trunk of this nerve.I As the spinal accessory is passing through the foramen lacerum, it is in close proximity to the posterior surface of the par _vagum, and it there divides into its two branches —its internal and external branches.
The former, or the internal, is composed of the filaments forming the upper roots of the nerve (fig. 521, 11.), and entirely, or almost entirely, of those coming from the medulla oblongata ; and it joins itself to the vagus immediately below the ganglion jugulare of that nerve. The passage of the accessory
through the foramen lacerum posterius, its division into two branches, and the distri bution of the internal branch as far as it is known, have been already described in the art. PAR VAGUAI, VOI. iii. pp. 883. and 890 , and need not be repeated here.
The external branch, composed of those fibres which arise from the spinal chord (fig. 521,12.), proceeds downwards, outwards, and backwards behind the internal jugular vein, in front of the occipital artery, and be hind the posterior belly of the digastric and stylo-hyoid muscle, and reaches the inner sur face of the sterno-cleido-mastoid muscle at the lower part of its upper third. In con tinning its course downwards and outwards, it here generally perforates the sterno-cleido mastoid ; at other times it is only closely con nected to it by cellular tissue ; but in both cases it gives branches to this muscle. In this part of its course it is strengthened and anastomoses with twigs of the third and second cervical nerves. Continuing its pro gress downwards and backwards it anastomo ses with twigs of the fourth and fifth cervical nerves, and throws itself into the inner surface of the trapezius muscle, among whose fibres it is ultimately lost.
Comparative anatomy of the spinal accessory.
— The origin and distribution of this nerve in the mammalia does not essentially differ from what is found in the human species.* Willis states that this nerve is not only pre sent in the mammalia, but also in birds and fishes t ; but the existence of it in the two latter divisions of the vertebrata has been subsequently denied by many excellent ana tomists. " If an animal," say s Mr. Shaw, " does not perform part of the act of respira tion by muscles which run from the skull to the chest, no spinal accessory is found. The truth of this observation may be shown by thd dissection of any of the larger birds, but the most extraordinary proof is to be found in the neck of the camel. The constitution of the neck of this animal is like that of birds ; there being a succession of short muscles along the side of the neck, and attached to the vertehrw, but no long muscle passing from the jaw to the sternum to assist in breathing, as in other quadrupeds." It appears, however, that in the camel this nerve is present, but it is smaller and differently distributed from what it is in the horse.§ Serres found it in three of the larger birds, Weber in some fishes, and Us choffhas given descriptions and representations of it in several birds, reptiles, and fishes. In these animals the upper part only of this nerve seems to be present, for it does not stretch downwards along the spinal chord to the same extent in them as in the mammalia. The whole of this nerve, in these animals, throws itself into the vagus, while a branch leaves the vagus after it has escaped from the cranium, and taking the place of the external branch of the accessory is distributed to the muscles of the neck in birds and in reptiles, and to the muscles which move the pectoral fin in fishes.II In the chimpanzee, the spinal accessory, after passing through the foramen lacerum, divides into two branches. The in ternal runs towards the larynx, into which it penetrates above the os-hyoid. It is placed between the superior laryngeal nerve and stylo-hyoid ligament, and passes behind the internal carotid artery to the superior hyoi dean region (la region hyoidienne superieure), where it terminates. The external branch passes downwards below the sterno-cleido mastoid muscle to reach the trapezius, in which it is chiefly distributed.* Physiology of the accessory.— The peculiar origin and course of this nerve, and particularly its intimate connection with the par vagum, have formed the basis of most of the specula tions on its functions since the time of Willis. It was maintained by Willis that this nerve, from its connection with the par vagum, re gulates those involuntary movements of the neck and arfn caused by the emotions and passions.* Lobstein likewise believed that the spinal accessory joins the vagus for the purpose of connecting itself with the in voluntary functions, and he supposed that its paralysis might also affect the move ments of the pharynx and larynx.f Others have maintained that it is a nerve of involun tary motion from the particular portion of the spinal chord in which it is implanted. It is, as is well known, one of Sir Charles Bell's respiratory nerves, arising as he supposed from a particular tract in the spinal chord to which he gave the name of respiratory tract, and is therefore, according to this view, a nerve of involuntary motion. Bellingeri believes that the lateral tract of the spinal chord, from which the accessory arises, presides over the instinc tive and sympathetic movements, and that it is consequently a nerve of involuntary motion. t Arnold §, Scarpa Bischoff 1, Valentin**, and Longet tt, have maintained that the accessory stands in the same rela tion to the vagus as the anterior roots of the spinal nerves do to the posterior roots.$f Ac cording to this last view, the vagus does not originally possess any motor filaments, but derives them from the spinal accessory. The two first of these authors came to this con clusion on anatomical grounds alone ; the three latter, from experiments upon these nerves in living animals, as well as from their anatomy. Bernard has arrived at the conclusion that it is entirely a motor nerve, and that it enables the larynx and pharynx, and muscles of the neck in which it is distributed, to partake in the production of the phenomena of phona tion, but that it does not assist in any of the true respiratory movements.§§ Dr. Todd and Mr. Bowman !III, on the other hand, believe that the internal branch of the accessory is composed of afferent nerves, and that the mode of implantation of this nerve in the cen tral organs of the nervous system serves to bring the sentient surface of the lungs and air passages into imniediate relations with the roots of all those nerves which animate the great muscles of respiration, the phrenic, the external thoracic, the cervical plexus, and the motor fibres of the spinal accessory and vagus nerves.