30-J 



FIFTH PAIR OF NERVES. 



by Dr. Yelloly, as liable on turning her eyes 

 aside to drop glasses, plates, &c. which she 

 held in safety so long as she looked at them ; " 

 but that the absence of motion in the lips on 

 the division of the fifth is due to the loss of 

 sensation merely, and not of voluntary power, 

 is positively proved by the effect of the division 

 of the portio dura on the two sides, an experi- 

 ment performed for the first time by Mayo : in 

 it the voluntary motion of the lips is altogether 

 lost, while sensation continues unaffected,* and 

 hence the division of the fifth cannot deprive 

 them of voluntary power, but only of sensation. 

 The explanation of Mayo has been admitted 

 and adopted by Bell himself in his " Exposi- 

 tion," 1824, in which he has added to the 

 detail of his experiment, as already related, 

 the following note : " what I attributed to the 

 effect of the loss of motion by the division of 

 the fifth, was in fact produced by loss of sen- 

 sation ; " and he corroborates this by the case 

 of a gentleman in whom loss of sensation in 

 the lip had been produced by extraction of a 

 tooth. " On putting a tumbler of water to his 

 lips, he said, ' Why, you have given me a 

 broken glass:' he thought that he put half a 

 glass to his lips, because the lip had been de- 

 prived of sensation in one half of its extent ; 

 he retained the power of moving the lip, but 

 not of feeling with the lip." The last particu- 

 lar noted is of great value, as demonstrating 

 satisfactorily the separation of the two faculties, 

 and, taken in connexion with anatomical con- 

 siderations, renders it necessary to refer them 

 to separate sources. It is manifest, then, that 

 the circumstance of the animal not taking up 

 the food by means of the lips, after the divi- 

 sion of the fifth nerve, is not proof that it had 

 lost the voluntary muscular power of them, 

 but only that it did exert it, not having been, 

 as it were, apprised of the necessity of doing 

 so. It is also stated by Bell, that on the 

 division of the nerve upon one side, " the side 

 of the lip was observed to hang low, and it 

 was dragged to the other side." This result 

 also is objected to by Mayo, first, as contrary 

 to his observation, for in his first experiment, 

 after the division of the infra-orbital and inferior 

 maxillary nerves, " the lips did not lose their 

 tone or customary apposition to each other and 

 to the teeth;" and secondly, as being the 

 effect of an extensive division of the muscular 

 fibres, a cause quite adequate certainly to 

 explain the fall of the lip, independent of the 

 influence of the nerves. The difficulty, there- 

 fore, which these circumstances appear at first 

 to present is removed, and we are left to deter- 



* It will be satisfactory to those interested in 

 this question to know, that the result of Mayo's 

 experiment has received full confirmation from 

 those of others; and first from Shaw, who has 

 bestowed so much labour to establish the respiratory 

 connexion of the portio dura. In the Medical and 

 Physical Journal for December, 1822, he writes, 

 " immediately on cutting the nerve (the portio 

 dura) on both sides, the lips became so paralyzed 

 that the animal could no longer use them in raising 

 its food." The same result has been obtained by 

 Mr. Broughton in experiments upon the horse, as 

 detailed in the same Journal, June, 1823. 



mine the question by other means, and they 

 are abundantly furnished from other sources. 

 In the first place, the division of the nerves 

 completely destroys the sensation of the parts 

 to which they are distributed, without pro- 

 ducing any effect upon the tone or contractile 

 power of those parts, nor does irritation of the 

 divided nerves excite muscular contractions. 

 Secondly, were these nerves the source of the 

 voluntary powers of the parts they supply, the 

 division of every other nerve must fail to affect 

 that power while the former remain entire; 

 but Mayo, in several instances, divided the 

 portio dura alone on both sides, and the 

 result was, that " the lips immediately fell 

 away from the teeth, and hung flaccid," and 

 could not be used by the animal to take hold 

 of food, and consequently had lost all volun- 

 tary power; while, " when the extremity, 

 nearest the lips, of either divided nerve was 

 pinched, the muscles of the lips and nostrils 

 on that side were convulsed." Bell doubtless 

 asserts that after the division of the portio dura 

 nerve on one side, the animal " ate without 

 the slightest impediment;" to this Mayo 

 objects that " the experiment is inconclusive, 

 because the nerve was not divided on both 

 sides;" but in truth the experiment is quite 

 conclusive, for though the animal can eat, and 

 without impediment, his eating is far from 

 perfect, and the imperfection is not the less 

 obvious because confined to one side. 



When an animal which has had the portio 

 dura divided upon one side only takes food, 

 the lips remain motionless upon that side; and 

 when it masticates, the lips continue in the 

 same state, while on the other side they ac- 

 tively co-operate, the food and saliva escaping 

 on the side at which the nerve has been cut, 

 and on the other being confined within the 

 mouth. Now, if any action of the lips be 

 voluntary, it is assuredly that by which they 

 co-operate in the prehension and mastication 

 of food ; and since no action of their muscles 

 can be excited by irritation of the branches of 

 the fifth nerve, while such action can be ex- 

 cited by that of the portio dura, and all volun- 

 tary action is destroyed by the division of that 

 nerve, but one inference remains, that of Mayo 

 already adverted to, viz. that those branches 

 of the fifth in question possess no influence 

 upon the voluntary faculty of the muscles ; 

 that they are exclusively sentient ; and that the 

 contractile power of the muscles of the face, 

 whether voluntary or involuntary, is to be attri- 

 buted to another source. 



After what has been stated, we must admit 

 that Mayo has been the first expressly to an- 

 nounce that the function of these nerves is 

 restricted to sensation. Beyond that, however, 

 he has not gone, in reference to the question 

 of sensation, in the publication alluded to, 

 though it must be admitted that little remained 

 to be added in order to complete the conclu- 

 sion, that the ganglionic portion of the nerve is 

 exclusively sentient. At the same time he 

 inferred, " from the preceding anatomical 

 details," viz. their exclusive distribution to 

 muscles, " that other branches of the third 



