SEVENTH PAIR OF NERVES. 



of the portio dura in that larger sense, in which 

 we generally use this word of nerves ; and 

 hence the changes effected by disease afford 

 very little aid to the settlement of the question. 

 The inquiry therefore limits itself to a judg- 

 ment on the two remaining kinds of evidence : 

 firstly, the results of experiment ; and secondly, 

 the anatomical appearances. With this latter 

 means of proof, a third is intimately associated 

 in the present instance ; viz. the analogies 

 offered by the structure of the facial to 

 other nerves, of which the functions are 

 better ascertained. These analogies, where 

 present, will argue a similarity of function ; 

 and in a degree of probability varying with 

 the degree of the resemblance. 



On the supposition that the sensory fila- 

 ments are borrowed from neighbouring nerves, 

 the very numerous junctions of the facial and 

 fifth would naturally point to the latter as 

 constituting one of the most probable and 

 important sources. There are two ways of 

 instituting the question experimentally. If 

 these filaments come from this nerve, the 

 destruction of its continuity will annihilate 

 the sensibility alike of the facial and itself. 

 Again, if the portio dura be insensible until 

 joined by these branches of the fifth, irritation 

 or section of the former nerve, previous to 

 the point of junction, ought to be unattended 

 by pain. In both these methods, the fifth is 

 functionally separated from the facial ; but in 

 the second instance, the natural isolation of 

 this nerve behind the situations where the 

 fifth joins it, supplies the place of the artificial 

 isolation practised in the first. And in both 

 the continuance of sensibility would imply 

 that the portio dura possessed inherent sensi- 

 tive fibres. 



The division of the fifth nerve within the 

 skull, or close to its origin from the ence- 

 phalon, has been attended with insensibility 

 of the facial, in the hands of Magendie*, Es- 

 chricht, Lund "j", and Longet J ; and I am not 

 aware of any such experiments which have 

 contradicted their statements. The latter 

 author states that, under these circumstances, 

 the insensibility of the portio dura is perfect; 

 but Lund and Eschricht, although they seem 

 to deduce the same conclusion that he does, 

 viz. that the sensibility of the facial nerve is 

 entirely due to its anastomoses with the fifth, 

 yet, nevertheless, distinctly state that in 

 their experiments the insensibility produced 

 extended only from the ear forwards; while be- 

 hind this situation the portio dura still evinced 

 a well-marked sensibility. Apparently, Longet 

 would explain this contradiction by supposing 

 that the nerve behind the ear, which they 

 found to be still sensible, was an ascending 

 filament of the cervical plexus ; but it seems 

 very unlikely that they should confound the 

 facial trunk with so very small a twig as one 

 of these cervical branches would be. It must 



* Le9ons siir les Fouctions du Systeme Nerveux, 

 torn. ii. p. 181. 



f Dictionnaire des Sciences Medic., Journal Cum- 

 ple'm, torn. xxvi. p. -il. 



J Loc. cit. 



be observed that the results afforded by section 

 of the fifth are only valid when the whole of 

 the nerve has been divided, since in any 

 other case there is a possibility that the 

 sensibility of the facial, which remains after 

 the operation, is due to the reception of 

 filaments from the uncut branches. 



These anatomical considerations apply even 

 more forcibly to the second series of experi- 

 ments. Thus, in some of them, conclusions 

 are sought to be drawn from the observed 

 sensibility of the larger branches of the nerve 

 in the face ; but the numerous anastomoses 

 with the fifth, of which mention has pre- 

 viously been made, and especially that large 

 union with the aunculo-temporal nerve of 

 its third division, immediately in front of the 

 ear, invalidate all these results. 



Similar contradictory evidence obtains con- 

 cerning the sensibility of the facial at its 

 emergence from the skull, or behind its more 

 visible junctions with the fifth. Thus, Valen- 

 tin regards it as insensible in this place, while 

 the experiments of Longet, Morganti (and 

 probably Eschricht and Lund, as above 

 stated), induce them to maintain the opposite 

 opinion. So that, perhaps, on the whole, 

 the balance of evidence inclines towards the 

 statement that the irritation of the facial 

 nerve at the stylo-mastoid foramen is attended 

 with expressions of pain, and, therefore, that 

 the nerve is possessed of sensibility at this 

 place. 



The reception of this fact considerably 

 narrows the question ; since the only branches 

 connected with the facial above this point 

 are, the greater and lesser superficial pctrosal 

 nerves, and the auricular filament of the pneu- 

 mogastric. But Morganti has laid bare the 

 chorda tympani in the tympanum, and has 

 proved its sensibility to irritation. And this 

 nerve, it will be recollected, comes* from the 

 portio dura at a point above its junction with 

 the auricular filament; and since the latter 

 is thus not essential to the sensibility' of this 

 branch of the facial, so in all probability it is 

 not necessary to the sensibility of this trunk 

 itself. Thus the superficial petrosal nerves 

 only remain ; and many who consider one or 

 both of these to join the facial, explain the 

 sensibility of the nerve in the Fallopian canal 

 by supposing that they convey to it branches 

 of the second or third division of the fifth, 

 which pass through the spheno-palatine and 

 otic ganglia respectively. But, as has been 

 previously stated, anatomy fails to recognise 

 such a direct passage to the facial ; ami, on 

 the contrary, by showing the unequivocally 

 ganglionic nature of the genii form intumes- 

 cence, renders it highly improbable. And on 

 physiological grounds, it seems difficult to 

 imagine that -a nerve or nerves should pass 

 unchanged through two successive nervous 



* Some have supposed the chorda tympani to 

 ]>,i-s from the gustatory to the facial nerve, con- 

 veying sensitive fibres to it. But numerous argu- 

 ments, especially its anatomy and function as above 

 mentioned, combine t<> ivndrr ihis supposition quite 



untenable. 



