300 



FIFTH PAIR OF .NERVES. 



he had ascertained by experiment that the fifth 

 nerve was exquisitely sensitive ; that it is the 

 source of the sensibility of the 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." f 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 Physiologic, 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. 



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 



* Philosophical Transactions, 1821, p. 405. 

 t Ibid. p. 417. 



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. He 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 he 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 of 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 he regarded them as being all alike 

 nerves of compound function, nerves both 

 of voluntary motion and sensation ; and, such 

 being the case, either that he 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 



