The dispute scarcely involves the function 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 experhnent ; 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 reseniblance.
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 continnity 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 vvithin 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 t, and Longett ; 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 satne 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 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 auriculo-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 petrosaI nerves, and the auricular tilament 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 ; and, on the contrary, by showing the unequivocally ganglionic nature of the genuform 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 centres, of which they form such large and important roots : while, allowing them to be affected in their functions, we are at least not justified in calling them " sensitive branches of the fifth." By this elision of one sensitive anastomosis after another, sensibility still remaining, we have been led, in a retrograde course, to the ganglion at the hiatus Fallopii : at and above this point the evidence afforded by experi ment fails us.