And M. Bernard has conclusively shown that the chorda tympani is the immediate in strument of the change. He has adduced instances of paralysis from experiments, in which the facial nerve being divided above the point where this nerve diverges, the taste was constantly impaired ; while in the facial paralysis due to disease of the nerve below its origin, the sense was unaffected.
In connexion with these facts may be mentioned the motor function of the chorda tympani. It has been previously stated that, among other courses ascribed to this nerve after its union with the gustatory, Guarini has succeeded in tracing its filaments to the lingualis muscle. But in addition to this, he has adduced experimental evidence of a much more conclusive character. He found that galvanising the fifth, ninth, and facial nerves affected the muscles of the tongue in a very different manner. When the sensitive nerve was stimulated, the tongue remained without movement ; but in the case of the ninth and facial, an upward and downward movement was perceptible. When the hypo glossal was galvanised, a backward and for ward motion was added to this common movement; while on stimulating the facial nerve, the middle tract of the tongue, which had remained tranquil in the previous experi ments, was agitated in a vermiform manner. This latter movement vvas instantly anni hilated by section of the chorda tympani. The region which it occupied was that of the lingualis muscle, and to it he traced some branches of the nerve : while the upward and downward movement belonged to the stylo glossus. The cause of the affection of the taste is very imperfectly understood ; since, in the case of the tongue, it seems difficult to con nect such an impairment of the special sense with any mere loss of motion. Bernard has, however, offered such an explanation ; in which, as a preliminary, he supposes a vermi form movement like that observed to be ne cessary to taste, and that it acts by increasing the contact of the papillm of the tongue with the sapid particles. And although this is sufficiently unlikely, yet it is advisable to recollect that, unless guided by experience, we might have asserted the same improba bility in the instance of smell; while this sense has been seen to experience an equal impairment, and in a method very similar to this which Bernard has supposed :—viz. by a
diminution of effective contact between the object of the special sense and the distribution of its nerve, which contact is itself in part the result of the contractions of certain muscles, supplied by branches of the facial.
The mixed nature of the chorda tytnpani, as laid down by Morganti, may perhaps ex plain these effects in a different manner ; by suggesting that the paralysis of this nerve involves the loss of some of the sentient as well as motor filaments distributed to the tongue. And the varieties in the nature of the affection which were indicated above, per haps render this explanation a more probable one.
A connection has also been traced between the paralysis of the portio dura and an ab normal state of the soft palate :— the curtain of the palate itself being somewhat relaxed, while the uvula is drawn towards the unaf fected side. In a great number of facial palsies, however, this deviation is absent. But although materials on this point are yet somewhat few, it may be safely stated that its presence or absence varies according to the seat of the disease causing the para lysis : if above the geniculate ganglion, the deviation appears pretty constantly present ; if below, it is absent. The light which ex periment affords is somewhat uncertain and conflictinn. Mechanical irritation of the root of the fa''cial nerve in various animals has failed to excite contractions of the muscles of the palate, both with Valentin* and Hein.t The stimulus of galvanism has also acted irregularly and variably, being sometimes fol lowed by contractions, sometimes not. It is, on the whole, difficult to avoid coming to the conclusion that the facial nerve is intimately associated with the movements of the palate by its greater petrosal branch : but the actual transmission of its uninterrupted filaments through the spheno-palatine ganglion is, on anatomical grounds, exceedingly doubtful. And the experiments above mentioned, to gether with others in which Hein found that its division did not affect pre-existinn move ments from other nervous sources, rensder the term " motor nerve " clearly an inapplicable name.