SEVENTH PAIR OF NERVES. 



551 



incredulity an}' account of the unaltered pas- 

 sage of a nerve through a ganglion, as viewed 

 by the unassisted eye ; and, in the particular 

 instance before us, the microscope disproves 

 this supposition. So, also, concerning the 

 various theories of the derivation of the super- 

 ficial petrosal nerves which have been set forth 

 as based on dissections. Let it be granted that 

 there are two ganglia, the sphenopalatine 

 and the genicnlate, which are united by an 

 intervening nervous cord : in such a case, I 

 cannot see how any merely anatomical skill 

 would enable one to predicate a definite di- 

 rection as taken by the connecting nerve. 

 Indeed, any statement of this kind really 

 amounts to asserting a special direction or 

 quality of the nervous force, and to the affir- 

 mation or denial of such a view, the scalpel 

 affords no assistance. 



The unravelling of the nerves themselves, 

 even as performed in the above dissections, 

 requires, perhaps, to be received with con- 

 siderable caution ; and that natural separation 

 or simplification which is afforded by their 

 comparative anatomy must be regarded as 

 vastly increasing the value of the results 

 obtained. The chorda tympani affords a 

 good instance of the conflicting results of 

 these dissections, when unaided by this latter 

 method of inquiry. Some have considered, 

 with H. Cloquet and Longet, that, after remain- 

 ing a short time in contact with the gustatory 

 or lingual branch of the fifth, the whole of 

 this nerve passes away from it, to form one 

 of the roots of the submaxillary ganglion. 

 Others have described it as only giving a fila- 

 ment to this ganglion, and uniting itself by 

 the remainder of its bulk with the branch of 

 the fifth ; while others have failed to detect 

 any direct transition of the chorda tympani 

 into the ganglion, but, on the contrary, have 

 found the two nerves inseparably mixed up 

 below the situation of their visible junction. 

 And more recently it has been traced by 

 Guarini* to the lingualis muscle. On general 

 grounds, the first of these notions is liable to 

 much objection, since it seems singular that a 

 nerve so far removed from the facial as the 

 chorda tympani is at the base of the skull, 

 should be involved in such an accidental 

 proximity as this would make it, or should 

 run so closely to the gustatory without any 

 interchange of fibres. Again, the total pas- 

 sage of the nerve to the ganglion appears 

 very improbable, when the relative size of the 

 entering and emerging branches is considered, 

 that is, on comparing the bulk of the 

 chorda tympani with that of the two or three 

 filaments which join the ganglion, it may be 

 seen that the former is larger than all of them 

 together. This is especially the case in some 

 of the lower animals, as the dog ; in whom 

 the submaxillary ganglion and its roots from 

 the cerebral nerves are so greatly reduced in 

 size as to be scarcely visible to the naked eye, 

 while the chorda tympani continues a compa- 

 ratively large branch. But these general 



* Annali di Medicina. Maggio, 1842. 



objections will not apply to the supposition 

 of a partial connection of the chorda tympani 

 with the submaxillary ganglion, and the ques- 

 tion must accordingly still remain in doubt. 



Physiology of the seventh nerve. The func- 

 tions sustained by the auditory nerve are 

 recognised with sufficient certainty. The 

 anatomy of its distribution, its variations in 

 the different classes of the animal kingdom, 

 and the results of experiment or disease 

 affecting its structure, all unite to indicate it 

 as the nerve which, specially adapted at its 

 periphery in the organ of hearing to receive 

 impressions from the vibrations of the ex- 

 ternal air, conveys them to the brain, and by 

 exciting corresponding impressions there, 

 gives rise to the production of a sensation 

 which we term a sound. For the further 

 details of its function, reference is made to 

 the article HEARING. 



The facial nerve. It has been seen from 

 the preceding description, that the facial 

 nerve is distributed almost exclusively to 

 muscular structures ; and, although these are 

 very numerous, yet they all admit of being 

 reduced to one class, viz. the muscles of the 

 face. A further subdivision would next sepa- 

 rate them into several groups, which serve to 

 enlarge or diminish the size of the various 

 apertures by which impressions are admitted 

 to the organs of the special senses, as the 

 eye, ear, nose, and tongue. But these ori- 

 fices are also the pathways of food and air, 

 so that the muscles which regulate their size 

 have thus far an influence on the functions 

 of respiration and digestion. In man, they 

 fulfil the further purpose of organs of expres- 

 sion ; their various and complicated adjust- 

 ments conveying, for the most part, a toler- 

 able index of the passions or emotions of the 

 presiding mind. 



This prominently muscular distribution of 

 the facial would lead us to expect a priori 

 that the nerve was chiefly motor in its func- 

 tion ; and, if we turn from its anatomy in the 

 human subject to its varieties of arrangement 

 and appearance in the animal kingdom, this 

 view will be abundantly confirmed. Not 

 only is its peripheric distribution almost ex- 

 clusively muscular, and in connection with 

 the same facial set of muscles, but it also 

 experiences a development which is co-equal 

 with that of these organs, increasing with 

 their augmented development, or disappear- 

 ing with their suppression. Thus, in Fishes, 

 the facial scarcely exists as a separate nerve. 

 In Reptiles and Birds, its small size corre- 

 sponds with the bony and immobile state of 

 the face. In the Mammalia, it becomes much 

 more considerable, and both the nerve and 

 muscles experience various degrees of aug- 

 mentation. Thus, in the monkeys it attains 

 a large size, in accordance with the number 

 and magnitude of the facial muscles generally ; 

 and the trunk of the elephant, and the mus- 

 cular apparatus connected with the blowholes 

 of Cetaceans, are supplied by large branches 

 from this nerve, which here experiences a 

 partial development, to meet the special exi- 



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