FIFTH PAIR OF NERVES. 



301 



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 gangl ionic, 

 and muscular action upon its non-ganglionic 

 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 

 nia.ri/liin/ nerve is one both of sensation and 

 voluntary motion to the lip (see the preceding 

 page): to this conclusion Magendiewas 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 theTesult 

 which we have obtained agrees perfectly with 

 that which we have just related, with the e.r- 

 ct'i>/ion always of the influence of the section 

 of the infra-orbital n/ton 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 dura 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, w;is 

 divided on either side : in the fourth experi- 

 ment, the under lip at first appeared to fall 

 away from the teeth; at times the lip^ w in- 

 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 dr.iwn 

 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 while, 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 tlie 

 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 them, and to supply the place 

 of that which has been lost: a person afflicted 

 with anxsthesia is described in a case quoted 



