The importance of the fifth nerve in the three other senses of smell, sight, and hearing, has been advocated by several physiologists, and more particularly by Magendie, who ap pears disposed to view the fifth nerve as the source or medium of all the three. His appli cation of this doctrine, however, has reference more particularly to the sense of smelling, upon which he has performed a series of expe riments, of which the following is a summary: he destroyed entirely the olfactory nerves within the cranium, and lie found the animal still sensible to strong odours, such as ammonia, acetic acid, essential oil of lavender. The sen sibility of the interior of the nasal cavity had lost nothing of its energy; the introduction of a stylet had the same effect as upon a dog which had not been touched. This experiment he performed several times, and always with the same results. Ile next divided the fifth nerves within the cranium, of course before they had given branches to the nostrils, and found all trace of the action of strong odours to disappear. Ile hence concluded that smell, in so far as pungent smells are con cerned, is exercised by the branches of the fifthpair, and that the first is not concerned in the function. To this conclusion he himself starts the objection that the agents used are not odours, properly speaking, but chemical, pun gent, irritating vapours, and that by the section of the fifth we destroy not the sense of smell, but only the sensibility of the membrane of the nose to these irritating vapours, and he admits the force of the objection with respect to some of the vapours alluded to ; hut be denies that it will apply to the oil of lavender or that of Dippel, the effect of which in the experiments is the same. In order to remove the difficulty he destroyed the olfactory nerves of a dog of particularly fine nose, and then enclosing por tions of food of various kinds in paper, he presented them to the animal, and it always undid the paper and possessed itself of the food; but, he adds, " I do not regard this ex periment as satisfactory, because in other cir cumstances it appeared to me to want smell to discover food which I put near him without his knowledge" (el son insu). Ilowever, the latter circumstance is overlooked by Magendie, and his conclusion is, " une fois le nerf trifa cial coupe, toute trace de sensibilitd disparait, aucun corps odorant a distance on en contact, les corrosifs me mes n'affectent plus en aucune facon la pituitaire."* Doubtless this conclu sion is qualified by another immediately suc ceeding, " that does not prove that the seat of smell is in the branches of the fifth pair ; but it proves at least that the olfactory nerve has an indispensable need of the branches of the fifth pair to be able to enter into action ; that it is devoid of general sensibility, and that it can have only a special sensibility relative to odorous bodies."-J- The latter must he ad mitted to come, if not quite, at least very near to the general opinion, but it is altogether at variance with the former, and one is rather at fault for the author's precise meaning. Refe rence to later writings, however, leaves no doubt upon that point. In the conjoint work of Desmoulins and Magendie (1825) upon the nervous system of the vertebrata, besides other similar passages, will be found the following : " La einquieme paire, par ses branches nasales dans les mammiferes, et par ses branches pro pres a la eavite pre-oculaire des trigonoeephales et des serpents a sonnettes, est done l'organe de l'odorat."T Notwithstanding the weight of Magendie's authority, a careful review of the matter will not permit us to assent to this con clusion, and compels us to avow not only that it is not proved, but that the premises justify a contrary one. In the first place it is not war rantable to call the effluvia of ammonia or acetic acid odours : they are no more odours than the fumes of muriatic or nitric acid ; and, though aware of the objection, he still calls them odeurs fortes, and bases his inference upon their operation. But he says the objec tion does not apply to oil of lavender or the animal oil of Dippel: this, however, is but an assumption at variance with fact; in the human subject these agents may act feebly upon the sensibility of the membrane of the nostrils, and may not appear to possess irritating properties; but this will not prove that they act similarly upon animals, whose organ of smell is more sensitive than that of man, and accordingly Dr. Eschricht,§ who combats the opinion of Magendie, has found that, on application to the nostrils of those animals upon which the experiments of Magendie have been performed, they produce all the same effects which am monia or nitric acid does. In the second place his experiment of presenting food to a dog, whose olfactories had been destroyed, enclosed in paper, and in which the animal undid the paper, upon his own showing not only does not justify his inference, but, so far as it reaches, proves the contrary. To establish his position the animal must have discovered the food by smell, without knowing that it was in the paper ; but it is manifest, from Magendie's own relation, that when the animal undid the paper, it knew, or was led by some circum stance to expect the food to be in it; but that when it was not already aware or in expecta tion that the food was near it, it did not dis cover it. To the writer it seems that the na tural inference from the experiment, as related, is that the animal'sproper sense of smell de pended upon the olfactory nerves, inasmuch as it did not display fair evidence of its presence after their destruction, and that the sensibility displayed by the membrane of the nostrils after the destruction of these nerves, and dependent upon the fifth, has reference only to those im pressions which are objects of tactile or general sensation, but not of the special sense.
At the same time, however, that we express our dissent from Magendie with regard to the nervous connexion of the proper sense of smell, it must be admitted that his researches posi tively indicate a distinction between the media of perception in the case of different agents operating on the olfactory organ, which it has been too much the habit to regard as pro ducing their impressions all through the olfac tory nerves: they have gone a considerable way in demonstrating the separation of those media; a result which is made complete by the conti nuance of the simple sense after the loss of the influence of the fifth nerve consequent upon disease : further, they indicate that sensations derived through the organ of smell are less simple than they are usually accounted ; that they may be, and probably are for the most part, compound, resulting from the combina tion of impressions made upon the two senses thus shewn to be enjoyed by the organ.
Magendie's view has been adopted, and an endeavour made to corroborate and establish it by Desmoulins in Reflexions' upon a case communicated by Beclard, and published in the fifth volume of the Journal of Physiology. The case is that of a patient, in whom the olfactory nerves and their bulbs were de stroyed by the growth of a tubercular disease from the anterior lobes of the brain ; " yet he took snuff with pleasure, appeared to distin guish its different qualities, and was affected disagreeably by the smell of the suppuration of an abscess with which one of his neighbours was afflicted." From this case, from that of Serves, related elsewhere, and the experiments of Magendie viewed in connexion, Desmoulins has adopted the opinion that " the nerves and lobes called olfactory are alien to the sense of smell, or at all events co-operate so little in it, that the sense continues to be exerted without them ; that, on the contrary, this sense resides essentially in the branches of the fifth pair, which are distributed to the nostrils." Serres' case has been discussed elsewhere ; that of Beclard appears at first unanswerable ; but how will it appear after the qualification by which it is followed has been perused ? " I owe it to truth," he says, " to add that these last statements were not collected till after the dissection, and that they were gathered from the patients of the ward." Such an admission manifestly destroys the value of the case : evi dence obtained only after the individual's death, so little marked as during his lifetime to have been overlooked, and relating to a question at once so obscure and delicate, can hardly fail to be imperfect ; but admitting that the patient did relish and distinguish between different kinds of snuff, and that he was disagreeably affected by his neighbour's ailment, what then? The chief property of common, if not of every snuff, is pungency and not odour, and the per ception of pungency is not the function of the olfactory nerve; and one may be as disagree ably affected by a disgusting sight as by a dis gusting smell, and the patients of the ward not make any distinction between the senses af fected, until taught by the inquiries made that it must have been that of smell. And if the case just quoted prove the existence in the organ of smell of a sensibility to the impres sion of volatile agents independent of the ol factory, and conferred by the fifth nerve, the existence of another equally independent of the latter is satisfactorily established by the continuance of smell in those cases in which the faculty conferred by the fifth nerve has been lost through disease. This may be seen from reference to the case furnished by Bedard, and advanced by the very advocates of Magendie's doctrine in support of it; but the fact is still more strongly established by the case some time since published by Mr. Bishop, in which, though the fifth nerve was completely destroyed by the pressure of a tumour within the cra nium, and both the ordinary and tactile sensi bility of the same side of the face and its cavi ties was in consequence altogether lost, the sense of smell continued unimpaired. In a case of disease of the fifth nerve which the writer has witnessed, the patient did acknow ledge the perception of certain odoriferous agents; but judging from it alone, he could not say that smell was not impaired; on the contrary it seemed very much so, inasmuch as the patient denied at first any perception of the impression of several agents accounted odorous, and when he did say that he smelt these, it was not of himself, nor until he had been particularly questioned, and then he said it was up in his head' that lie felt the sensa tion, and positive must take precedence of nega tive evidence. Further, it is very likely that in the case of sensations, themselves neither dis agreeable nor acute, the vividness of which may depend very much upon association with other and more acute ones, the former may be disregarded where the latter have been lost, and hence the rashness of inferring that brutes have lost certain faculties, because in the course of experiments they do not by the exercise of these give evidence of their existence. The fact of the absence of olfactory nerves in the Cetacca as established by Cuvicr, has also led some to the conclusion that the proper faculty of smell may be capable of being transferred at least to the fifth; but until the faculty has been proved to exist in such case, the inference is manifestly not warranted by the premises.