Again, on the one hand it has been already shewn that division of the portio dura on both sides deprives the facial muscles of all inde pendent* contractile power, whether voluntary or involuntary ; and on the other, Mayo has found that irritation of the buccal nerve does not excite contraction in those muscles : the author has taken occasion several times to repeat the experiment of Mayo upon the latter nerve after it had emerged upon the face, and he has not succeeded in obtaining contraction of the facial muscles thereby, while the strug gles of the animal, excited by the irritation of the nerve, proved it to he one of exquisite sen sibility. It appears then to the author impos sible to admit that the facial muscles either possess two contractile powers dependent on distinct nerves, or that they derive any volun tary power from the fifth.
It is extraordinary that Magendie, who was the first to detect the error into which Bell had fallen with regard to the influence of the infra orbital nerve over the motions of the muscles of the face, and has, according to his own report, divided the portio dura on animals, should, notwithstanding all that has been written upon the subject, have adopted the opinion that the muscles of the face are en dowed with the two distinct faculties of motion, one of which is derived from the fifth. Ilis view will be found at page 703-4, Anatomie des Systemes Nerveux, &c., and the opinion there expressed is implied in a note at page 191, Journal de Physiologic, t. x. In the former he says, " Now Mr. Charles Bell in England and M. Magendie in France by cutting the facial nerve have paralyzed the respiratory motions of all the side of the face correspond ing to the nerve cut. But the muscles which receive at once filaments from the facial nerve and from the fifth pair were paralyzed only in their action relative to respiration and to the expression of the physiognomy." The influence of the fifth nerve upon the tac tile sensibility of the parts with which it is con nected has been discussed : its influence upon their ordinary sensibility also requires notice. From the preceding details it appears esta blished that it is to the same nerve that this property also of the parts in general is due; but there is reason to believe that the nerve exerts a more extended control over this faculty than was at first supposed. At the commence ment of the inquiries into the functions of the nerves of the face, the opinion generally held was that the facial nerve—portio dura of the seventh pair--was devoid of sensibility. Fur ther observations, however, showed that this conclusion was erroneous, and that the insen sibility to any injury done to the nerve in question manifested by the subjects of experi ment, and from which the inference had been drawn, was only apparent, and to be referred to the constitution of the individual animal or of its species. The sensibility of the facial nerve having been established, a question arose, whether that property was independent and proper to it, or whether it was conferred by another? Those who first observed the sensi bility of the nerve adopted the former opinion ; but considerations entitled certainly to much weight led Eschricht to suspect that the facial nerve is not endowed with independent sensi bility, and that the sensibility which is manifested when it is injured is conferred on it by the fifth nerve. In order to determine the question lie performed a series of experiments in which he divided the fifth nerve within the cranium upon one side after having opened the cavity and removed so much of the corresponding hemi sphere of the brain as was necessary for the accomplishment of his purpose : the facial nerve of the same side was then exposed, and its properties tested. The faculties of the minimal are so little affected by the removal of the brain, that the result of the experiment seems free from objection, while all influence of the fifth nerve upon the sensibility of the facial or other parts must be destroyed. In his
first successful experiment irritation of the facial excited spasms of the lips, and also in dications of suffering so decided that a doubt could not be entertained : the fifth nerve bad also been fairly divided : thus far, therefore, his conjecture was disproved. Pursuing his inquiry still further, he found in his next experiment that no indication whatever of pain was manifested by the animal on irritation of the facial on the side on which the fifth nerve had been cut; but in two succeeding experi ments he ascertained that while irritation of the nerve anterior to the meatus auditorius pro duced no other effect but spasms of the nasal and labial muscles, when exerted posterior to that point it excited manifest evidence of suf fering : this latter circumstance he accounts for by the communications of the posterior part of the facial with other sentient nerves besides the fifth, and lie has come to the conclusion that the former nerve is not endowed with independent sensibility, but that it derives the property from the fifth and other sentient nerves : this ques tion, however, requires further investigation.* Relation vf the fijth pair of nerves to the special senses.—The organs of the special senses are in the higher classes and in the case of smell, sight, and hearing, each supplied with nerves from at least two sources. Besides the particular nerves, which are generally consi dered to be the source or medium of the spe cial sense, they are furnished with branches from the fifth pair; and a question must, at the outset, be asked in regard to the two nerves derived from these different sources, as to which is to be considered the proper nerve of the peculiar sense enjoyed ? In connexion with the two separate nervous supplies, it is also to be observed that each organ enjoys two kinds of sensibility, viz. the special sensibility, through which sensations of the particular sense are received, and the general sensibility, in which the several organs of the body partici pate, and which is the medium through which impressions of contact are conveyed. The exis tence of the special sense, the coincidence of the particular nerve, the impairment or loss of the special function uniformly consequent upon the injury or destruction, whether by disease or otherwise, of that nerve ; and the community both of function and distribution, displayed by the nerve from which the organs of the senses are in common supplied, have led physiologists generally to the conclusion that in each case the particular nerve is the medium of the special sense, and that the fifth nerve confers upon the organs of the spe cial senses general sensibility only. The con clusion thus commonly adopted has been at different times called in question : thus Mery and Brunet, in 1607, denied to the nerves of the first pair the function of smell, and attri bated this sense to the fifth nerve.* The ques. tion of the connexion between the fifth nerve and the special senses is one of much difficulty, and probably we are not as yet in possession of sufficient data from which to draw a positive conclusion upon it when viewed in all its bear ings. It resolves itself into three: 1. how far the nerve may be concerned in the perception of specidl sensations in those cases in which nerves, considered to be specially intended for their perception, exist: 2. how far its co operation or influence may be necessary to enable the special nerves to fulfil their func tions: 3. how far it may be capable of taking the place of those special nerves, and of be coming, under certain conditions, media of perception to sensations, for which, in other cases, peculiar nerves are conferred. We shall review the relation of the nerve to the several senses in succession, bearing in mind the three points to which our attention is to be directed. That it is a medium of perception in the case of two senses, viz. touch and taste, is already so universally acknowledged that it is unneces sary to dwell upon the point.