VITAL PROPERTIES OF TUE FIFTH PAIR OF discussion of the vital properties of the fifth nerve the writer pro poses may be fitly arranged under the following heads: 1. its sensibility ; 2. its influence upon the faculties of sensation and volition, as also upon the ordinary sensibility of the parts to which it is distributed ; 3. its relation to the special senses and connection with the function of nutrition.
1. Scrisibilily.—Numerous experiments per formed and repeated by different physiologists have established the fact, that the fifth nerve enjoys exquisite sensibility. Bell appears to have been the first who directed attention particularly to this point : in his paper, pub lished in the Philosophical Transactions for the year 1821, it is stated that, touching the su perior maxillary branch of the fifth nerve, when exposed in an ass, " gave acute pain." In tile first of Mayo's experiments upon the fifth nerve, published in his Commentaries in 1822, it was also found that " on pinching the opposite extremities" (those connected with the brain) " of the infmorbital and inferior maxillary nerves in an ass, the animal struggled violently as at the moment of dividing the nerves : these latter results uniformly attend the division of the nerves above-mentioned, and of that branch of the fifth which joins the portio dura."t Similar results were obtained by the writer last quoted from experiments of the same description upon the dog and the rabbit, and upon the pigeon, in regard to the first division of the fifth. Ile also found " that on pinching the gustatory nerves in living rabbits pain was evinced." Magendie carried the inquiry farther, and in the fourth volume' of the Journal of Physiology, has related an experiment in which he exposed the fifth nerve within the cranium in the rabbit and dog, and found that the slightest touch produced signs of acute sensibility. From the preceding facts we infer that the ganglionic portion of the nerve at least is exquisitely sensitive, and that it is endowed with sen sibility through its entire extent : further, the experiment of Magendie indicates that the sen sibility of the nerve is proper and independent of the influence of other nerves, he having ex perimented upon it at a point prior to its junction with any other.
With regard to the non-ganglionic portion of the nerve, our data are at present altogether analogical : it is so situated that satisfactory experiments upon it separately are hardly to be accomplished, so that we are left to infer of it as probable what has been ascertained of other non-ganglionic nervous cords, viz. the anterior roots of the spinal nerves. The question in regard to the functions of the different portions of the spinal nerves has been inquired into by Magendie, by whom the endowments of both sets of roots have been tested in various modes, and who has inferred that the anterior roots are not devoid of sen sibility, and if they be sensitive it is probable that the lesser packet of the fifth is sensitive also.t 2. Influence of the fifth nerve upon sensation and volition.--it is hardly necessary to remark that this point has been the subject of much dispute, as well with regard to the fact itself as to the relative claims of the several inquirers to whom we are indebted for the investigation of the matter: however, physiologists now seem to be generally agreed that the nerve is one of compound function, being subservient to both the faculties of sensation and volition, and that the faculty of sensation is dependent upon its ganglionic, that of voluntary motion upon its non-ganglionic portion, and that it thus resembles the spinal nerves. That the
nerve is one of compound function, and sub servient to the two faculties, was announced by Bell in the paper already alluded to. lie there distinguishes the nerves into two classes ; one original and symmetrical, the other super added and irregular. To the former class he refers the spinal nerves, the suboccipital, and the fifth nerve, and assigns to them the fol lowing characters, namely, they hive all double origins ; they have all ganglia on one of their roots ; they go out laterally to certain divisions of the body ; they do not interfere to unite the divisions of the frame ; they are all mus cular nerves, ordering the voluntary mations of the frame ; they are all exquisitely sensible, and the source of the common sensibility if the surfirces of the body : to it he refers the nerves of the spine, the suboecipital, and the AIM nerve.I It has been already stated that he had ascertained by experiment that the fifth nerve was exquisitely sensitive ; that it is the source of the sensibility of flit parts to which it is distributed, he has also determined, for in allusion to the fifth he says, " if the nerve of this original class be divided, the skin and common substance is deprived of sensibility;" and " by an experiment made on the 16th of March, it was found that, on cutting the infra-orbitary branch of the fifth on the left side, the sensibility of that side was completely destroyed."t The experiments of Bell were repeated by Magendie, and a similar result, so far as regards the sentient properties of the fifth, obtained, as mentioned in the Journal de Physiologie, Octobre 1821. A similar result has been obtained also by Mayo in his experiments upon the fifth nerve, as detailed in his Commentaries for August 1822, more than a year after the publication of Bell's paper. In his first experiment the infra-orbital and inferior maxillary branches were divided on either side in an ass, where they emerge from their canals, and the sensibility of the lips seemed to be destroyed : and, in a second experiment, the frontal nerve was divided on one side of the forehead of an ass, when the neighbouring surface appeared to lose its sen sibility : the same effect was produced by the division of that branch of the fifth which joins the portio dura, inasmuch as the cheek loses sensation upon its division. From these ex periments Mayo concluded that the facial branches of the fifth are nerves of sensation. The experiments upon the influence of the nerve on sensation have been carried still further by Magendie ; he divided the nerves within the cranium, where they lie against the cavernous sinus, and also between the pons Varolii and the petrous portion of the temporal bone, and in both instances he obtained the same result with regard to the sensibility of the parts*to which the nerves are distributed, viz. total loss of sensibility on one or both sides of the face,according as one or both nerves were divided ; this extended not only to the integuments as in the former trials, but also to the lining membrane of the nostrils, to the conjunctiva, to the tongue and the interior of the mouth. The effect upon the nostril was so remarkable that the most active effluvia, even those of ammonia and acetic acid, pro duced no impression upon it: in like manner neither piercing instruments nor ammonia ex cited any sensation when applied to the con junctiva, and the tongue was insensible to the action of sapid bodies at its anterior part.