Comparative anatonry.—It has been asserted by Professor Mayer, that in the ox and some other Mammalia he has discovered a small posterior root to the ninth nerve, having on it a ganglion ; to the investigation of this I have paid particular attention. I have repeatedly and with care sought for this posterior root and ganglion in the ox, and have never been able to satisfy myself as to the existence of a true posterior root to this nerve.
The anatomy, however, of this part in the ox is extremely interesting, and when examined may, perhaps, explain Mayer's opinion.
In the dissections which I have made of the ninth pair in the ox, the nerve was found to arise in the depression between the corpus olivare and pyramidale by several delicate roots, in a manner very similar to what is ob served in the human subject ; these roots uni formly formed two bundles which perforated the dura mater separately, l'refore doing which, however, the most inferior of these two bundles received a twig, which at first sight appeared to be given off by the spinal accessory ; but, upon further and careful dissection, this twig was found not to come from the spinal acces sory, but to arise by a number of distinct roots from the side of the medulla spinalis, anterior ' to the roots of the spinal accessory, in front of and distinct from which it passed up into the cranium, and joined the inferior of the two bundles, which formed the origins of the ninth, and uniting with this passed out through the anterior condyloid foramen.
When this nerve was cleansed, the pia mater and coagulated blood removed, which always loads these parts in the slaughtered ox, no en largement or any thing resembling a ganglion could be discovered on its course. This nerv cannot be considered a posterior root to ninth pair, for its origin from the medull anterior to that of the spinal accessory ; an am inclined to think that this nerve in holds the same relation to the ninth t spinal accessory does to the eighth. Ct be what Alayer supposes to be a posterior to the ninth ? Winslow speaks of a communication tween the spinal accessory and ninth within the cranium, the existence of w the human subject is described by Sair Meckel ;* I have never been able to tr. such communication in man.
On tracing the ninth nerve in the ox tl the anterior condyloid canal, it was foun united into one trunk, and enveloped strong neurilemma; nor could any , be detected on the nerve in this part COU Be.
111 Birds, the ninth nerve is found to nicate with the par vagum, to divide into two branches, one of which is distributed to the tongue and the other to the cesophagus. Mailer states, that in the rattlesnake he has found this nerve escaping from the cranium by a special opening behind that for the eighth pair, with which it communicates, as also with the first cervical.* In Fishes, the last cerebral nerve is described by Weber as arising by three roots, the poste rior having a ganglion, and passing out of the cranium by a special foramen in the occipital bone, and being distributed to the pectoral fin.
From these circumstances MiiIler conceived that an analogy exists between the hypoglossal, or ninth nerve, and the spinal nerves, and says, " If we now take into consideration that the first spinal nerve in the human subject has sometimes only an anterior root, and that the hypoglossal in man has only an anterior root, but that in some mammalia (according to the hypothesis of Mayer) it has a posterior root also, it will be evident that the hypoglossal nerve belongs to the class of spinal nerves, and is as it were the first spinal nerve, which, how ever, generally passes out through a foramen in the cranium ; this consideration renders the analogy between the last cerebral nerve •in fishes and the hypoglossal nerve still greater.'ot Physiology. —That the ninth pair is the nerve which influences the motions of the tongue is generally admitted, and that it de serves the name given to it of motor lingua has been proved by the experiments of Mayo, Majendie, and Miiller.
When in the living animal this nerve is ex posed and excited by pinching or galvanism, violent spasms of the entire tongue are pro duced, and its division is followed by paralysis of that organ.
On this subject Mayo performed the follow ing experiment. " I divided the ninth nerve on one side of the tongue in a dog; the ani mal did not seem much incommoded, but lapped up milk readily. I then divided the nerve on the opposite side ; the animal ap peared distressed, and did not again lap up the milk offered to it, though it smelt to it; and finally, when mustard was smeared on its nos trils, it made no use of its tongue to remove it, though evidently suffering from it."/ Further, Mayo found that when the nerve was divided on both sides in a rabbit, and the tongue dratvn out of the mouth, the animal had not the power of again retracting it.