From such accumulated evidence but one conclusion can be drawn, viz. that the fifth is the nerve of general and tactile sensation to the face and its cavities, or to the parts upon which it is distributed.
With regard to the influence of the fifth nerve upon volition, it has been already stated that Bell had announced it, as one of his regular or symmetrical nerves, to be " a muscular nerve ordering the voluntary motions." This conclusion with regard to the fifth nerve he adopted in consequence of the following ex periment, and of the result, which, as he conceived, he obtained from it. " An ass being tied and thrown, the superior maxillary branch of the fifth nerve was exposed. Touching this nerve gave acute pain. It was divided, but no change took place in the motion of the nostril ; the cartilages continued to expand regularly in time with the other parts which combine in the act of respiration ; but the side of the lip was observed to hang low, and it was dragged to the other side. The same branch of the fifth was divided on the opposite side, and the animal let loose. Ile could no longer pick up his corn; the power of elevating and projecting the lip, as in gathering food, was lost. To open the lips the animal pressed the mouth against the ground, and at length licked the oats from the ground with his tongue. The loss of motion of the lips in eating was so obvious, that it was thought a useless cruelty to cut the other branches of the fifth." The inference here indicated is obvious, viz. that the motion of the lips in eating depends upon the superior maxillary branches of the fifth pair, so far at least as the distribution of those branches extends; and what lie conceived he had thus established with regard to one branch he inferred analogically of the rest. The opinion that the fifth is a muscular nerve as well as one of sensibility Bell also maintains in later writings, and supports by additional experiments : thus, in his Exposition qf the Natural System of the Nerves, published in 1824, he says, " to confirm this opinion by experiment, the nerve of the fifth pair was exposed at its root, in an ass, the moment the animal was killed ; and on irritating the nerve the muscles of the jaw acted, and the jaw was closed with a snap. On dividing the root of the nerve in a living animal, the jaw fell relaxed." That the fifth is to a certain extent a nerve of voluntary motion is univer sally admitted, but then a question arises of equal interest and delicacy ; of interest for its own nature, and of delicacy because of the personal claims and feelings involved in it. The question is,—it being admitted that the nerve is one of double function,— is such function enjoyed equally by all its branches and by both its portions ; and if otherwise, upon which do they severally depend ? From the extracts quoted it is evident that no dis tinction in function between the different branches of the nerve was contemplated by Bell at the time the first was written, in 1821, and that lie regarded them as being all alike nerves of compound function,—nerves both of voluntary motion and sensation ; and, such being the case, either that lie had not recognised a difference between the properties of the gan glionic and the non-ganglionic portions of the nerve, or that he was then not aware of the peculiar distribution of the latter ; nor is any express information afforded us upon the subject in his earlier writings, or antecedent to 1823. The conclusion to which he had arrived with regard to the nerve generally and its superior maxillary branch in particular, in the year 1821, has been stated ; in his communication to the Royal Society in 1823, he adds, " all the nerves, without a single exception, which bestow sensibility from the top of the head to the toe have ganglia on their roots ; and those which have no ganglia are not nerves of sensation, but are for the purpose of or dering the muscular frame:" from this, when applied to the fifth nerve, it might be inferred that sensation depended upon its ganglionic, and muscular action upon its non-ganglionie portion. But between the years 1821 and 1823 additions had been made by others to the knowledge of the functions of the fifth nerve which require notice. It is to be borne in mind that Bell inferred from his first ex periment, published in 1821, that the superior maxillary nerve is one both of sensation and voluntary motion to the lips (see the preceding page): to this conclusion Magendie was the first to object, for in the Journal of Physiology for October of the same year (1821), he says, " we have repeated these experiments along with Messrs. Shaw and Dupuy, and which we have obtained agrees perfectly with that which we have just related, with the ex ception always of the influence of the section of the infra-arbital upon mastication, an in fluence which I have never been able to perceive." In August 1822 Mayo published, in his Com mentaries, his " experiments to determine the influence of the portio data of the seventh, and of the facial branches of the fifth pair of nerves." Those relating to the latter point, which have been already alluded to, are as follow. 1. The infra-orbital and inferior max illary branches of the fifth were divided on either side, where they emerge from their re spective canals; the lips did not lose their tone or customary apposition to each other and to the teeth ; but their sensibility seemed destroyed : when oats were offered it, the animal pressed its lips against the vessel which contained the food, and finally raised the latter with its tongue and teeth. On pinching with a forceps the extremities nearest the lips of the divided nerves, no movement Whatever of the lips ensued : on pinching the opposite extremities of the nerves, the animal struggled violently, as at the moment of dividing the nerves. Some days afterwards, though the animal did not raise its food with its lips, the latter seemed to be moved during mastica tion by their own muscles." 2. " Some days after, the frontal nerve was divided on one side of the forehead of the same ass, when the neighbouring surface appeared to have lost sensation, but its muscles were not paralysed." 4, 5, and 6. The branch of the fifth, that joins the portio dura, was divided on either side : in the fourth experi ment, the under lip at first appeared to fall away from the teeth • at times the lips were just closed : in the fifth and sixth, the under lip did not hang down, and no difference was observed between the action of the muscles of either side; but, he observes in a later publi cation, " the cheek loses sensation upon its division." The results of these experiments, while they confirm fully the inference drawn by Bell with regard to the influence of the nerve over sensation, are altogether at variance with that of his experiment relating to the con trol of the superior maxillary nerve over mus cular motion, and are equally incompatible with the doctrine that the branches of the nerve, which were the subjects of experiment, have any direct connexion with muscular contrac tion ; for wbile, on the one hand, the division of the nerves was followed by total loss of sensibility in the lips, on the other, the latter did not fall away either from each other or from the teeth, nor did irritation of the portions of the nerves connected with the lips excite any movement whatever of those parts, but they seemed afterwards to be moved during mastication by their own muscles. Mayo in
ferred accordingly from his experiments, " that the frontal, infra-orbital, and inferior maxillary are nerves of sensation only, to which office that branch of the fifth which joins the portio dura probably contributes." A circumstance in the first experiment doubtless seems at variance with the conclusion which Mayo has drawn, and demands consideration here, be cause, unless unexplained, the fact is inconsis tent with the inference. It has been stated that both in Bell's and Mayo's experiment, the animal ceased to take up its food with its lips after the division of the facial branches of the fifth, and from that circumstance chiefly the former appears to have inferred that the motions of the lips in eating depended on these nerves ; but the inference is objected to by Mayo as " a theoretical account of the fact that the animal did not elevate and project its lip ; this fact," he says, " was noticed in my own expe riments, but appeared to me from the first equally consistent with the hypothesis, that the lip had merely lost its sensibility, as with Mr. Bell's explanation," that it had lost its muscu lar power. The fact may be obviously ex plained by either of the two suppositions, and it is very remarkable that it should occur equally in one case as in the other. In the one, the muscles of the lips having been de prived of their power of voluntary contraction, the lips themselves cannot, of course, be made use of to take hold of an object ; and in the other, the animal not being made aware of the contact of the food in consequence of the loss of sensation, volition is not exerted, nor are the muscles called into action in order to take hold of it. To the latter cause it is attributed by Mayo, after the division of the branches of the fifth, and he confirms this view of its pro duction by reference to the effect of anaesthesia in the human subject : " in that disease the sensation of the extremities is wholly lost, while their muscular power remains. Now it is remarkable that in persons thus affected the muscles of the insensible part can only be exerted efficiently when another sense is em ployed to guide tbem, and to supply the place of that which has been lost : a person afflicted with anaesthesia is described in a case quoted by Dr. Yelloly, as liable on turning her eyes aside to drop glasses, plates, &c. which she held in safety so long as she looked at them ;" but that the absence of motion in the lips on the division of the fifth is due to the loss of sensation merely, and not of voluntary power, is positively proved by the effect of the division of the portio dura on the two sides, an experi ment performed for the first time by Mayo : in it the voluntary motion of the lips is altogether lost, while sensation continues unaffected,' and hence the division of the fifth cannot deprive them of voluntary power, but only of sensation. The explanation of Mayo has been admitted and adopted by Bell hithself in his " Exposi tion," 1824, in which he has added to the detail of his experiment, as already related, the following note : " what I attributed to the effect of the loss of motion by the division of the fifth, was in fact produced by loss of sen sation ;" and he corroborates this by the case of a gentleman in whom loss of sensation in the lip had been produced by extraction of a tooth. " On putting a tumbler of water to his lips, he said, Why, you have given me a broken glass :' lie thought that he put half a glass to his lips, because the lip had been de prived of sensation in one half of its extent ; he retained the power of moving the lip, but not of feeling with the lip." The last particu lar noted is of great value, as demonstrating satisfactorily the separation of the two faculties, and, taken in connexion with anatomical con siderations, renders it necessary to refer them to separate sources. It is manifest, then, that the circumstance of the animal not taking up the food by means of the lips, after the divi sion of the fifth nerve, is not proof that it had lost the voluntary muscular power of them, but only that it did exert it, not having been, as it were, apprised of the necessity of doing so. It is also stated by Bell, that on the division of the nerve upon one side, " the side of the lip was observed to hang low, and it was dragged to the other side." This result also is objected to by Mayo, first, as contrary to his observation, for in his first experiment, after the division of the infra-orbital and inferior maxillary nerves, " the lips did not lose their tone or customary apposition to each other and to the teeth ;" and secondly, as being the effect of an extensive division of the muscular fibres, a cause quite adequate certainly to explain the fall of the lip, independent of the influence of the nerves. The difficulty, there fore, which these circumstances appear at first to present is removed, and we are left to deter mine the question by other means, and they are abundantly furnished from other sources. In the first place, the division of the nerves completely destroys the sensation of the parts to which they are distributed, without pro ducing any effect upon the tone or contractile power of those parts, nor does irritation of the divided nerves excite muscular contractions. Secondly, were these nerves the source of the voluntary powers of the parts they supply, the division of every other nerve must fail to affect that power while the former remain entire; but Mayo, in several instances, divided the portio dura alone on both sides, and the result was, that " the lips immediately fell away from the teeth, and hung flaccid," and could not be used by the animal to take hold of food, and consequently had lost all volun tary power ; while, " when the extremity, nearest the lips, of either divided nerve was pinched, the muscles of the lips and nostrils on that side were convulsed." Bell doubtless asserts that after the division of the portio dura nerve on one side, the animal " ate without the slightest impediment ;" to this Mayo objects that " the experiment is inconclusive, because the nerve was not divided on both sides ;" but in truth the experiment is quite conclusive, for though the animal can eat, and without impediment, his eating is far from perfect, and the imperfection is not the less obvious because confined to one side.