552 



SEVENTH PAIR OF NERVES. 



gencies of the case. In man, the nerve at- 

 tains the maximum of general development. 



Experiment also confirms the testimony 

 afforded by the human and comparative ana- 

 tomy of the facial nerve : indeed, the results 

 afforded by this method of enquiry first led 

 Sir Charles Bell* to the discovery of its 

 function. On cutting across the trunk of 

 the nerve, he found that the whole side of 

 the face on which it was divided had com- 

 pletely lost the power of movement, while its 

 sensibility remained unimpaired. His experi- 

 ments have since been frequently repeated, 

 and invariably with the same results. 



The over-excitement of the nerve affords 

 evidence of its motor function, equally with 

 the destruction of its continuity. Thus, gal- 

 vanism of the distal extremity of the cut 

 nerve at once sets up convulsive movements 

 in the muscles to which it is distributed. 



The paralysis produced by section includes 

 all those muscles which the facial has been 

 previously described to supply ; but the 

 muscles of the jaw, which are furnished with 

 nerves from the inferior maxillary division of 

 the fifth, are still free to execute their con- 

 tractions, and hence the movements of the 

 jaw continue. But although these are still 

 carried on, yet the act of mastication as a whole 

 is rendered very imperfect ; since this not only 

 requires the apposition and trituration of the 

 teeth upon each other, but also demands 

 accessory though subordinate movements of 

 the neighbouring lips and cheek, and the 

 section of the facial nerve distributed to these 

 parts render these adjuvant movements im- 

 possible. In such cases an imperfect masti-' 

 cation may indeed be seen to take place ; but 

 the cheek and lips, having lost their contrac- 

 tility, instead of pressing in the food towards 

 the teeth, and submitting it again and again 

 to their action, allow it constantly and gra- 

 dually to accumulate in this flaccid and 

 yielding pouch ; or permit it to fall out of 

 the anterior opening in the mouth. In this 

 instance, experiment throws a reflected light 

 upon descriptive anatomy. The buccinator 

 muscle, which forms the greater part of the 

 fleshy parietes of the cheek, was previously 

 mentioned as receiving branches from both 

 the facial and inferior maxillary nerves ; and 

 were we to confide altogether in the appear- 

 ances seen in dissecting these nerves in the 

 human subject f, we might perhaps justifiably 

 regard them as sharing between them the 

 supplying of the muscle. But the paralysis 

 of the buccinator, which is always present in 

 those instances where the facial nerve has 

 suffered division, points distinctly enough to 

 the latter as at least taking the more consi- 

 derable and important part of the two ; while 



* Exposition of the Natural System of the Nerves 

 of the Human Body. London, 1824. 



f The lesser development of the facial muscles of 

 the lower animals allows the distribution of the 

 small buccal nerve to be more easily traced. Thus, 

 in the dog, the two small filaments which form it 

 pass, as is evident on dissection, almost exclusively 

 to the mucous membrane and buccal glands. 



the failure of the galvanic stimulus to affect 

 the muscle through the buccal nerve, indicates 

 that the filaments of the facial are in all 

 probability the only motor nerves which are 

 distributed to it. 



Many of those cases of paralysed facial 

 nerve, which occur in the human subject as 

 the effect of disease involving their structure, 

 approximate closely to the results obtained by 

 an artificial division of the nerve in animals ; 

 but in consequence of the much more ex- 

 pressive character of the human countenance 

 in the normal state, the deviations produced 

 are even of a more striking appearance. One 

 half of the face forms a perfect blank, its 

 muscles hanging passively from the subjacent 

 structures ; while the movements of the 

 opposite side are distorted by the absence of 

 their proper antagonist motions, and are ex- 

 aggerated in appearance by the contrast. 



In the experiments above mentioned, Sir 

 Charles Bell found that immediately on di- 

 viding the nerve the muscular aperture of the 

 nostril, which had previously been subject to 

 an alternate dilatation and contraction during 

 the periods of inspiration and expiration 

 respectively, suddenly lost this movement. 

 He has termed the portio dura the respiratory 

 nerve of the face ; since it presides over these 

 and other motions of the facial muscles, 

 which are developed independently of the 

 will, and in answer to the necessities of res- 

 piration. 



The section of the facial nerve indirectly 

 affects the sense of smell. This fact was 

 also first pointed out by Sir Charles Bell, 

 and has since been confirmed by many other 

 observers. Under these circumstances, the 

 power of discerning strong odors, as tobacco 

 and ammonia, appears to be much diminished 

 on the affected bide, although scarcely abso- 

 lutely lost. This loss of smell has been 

 ascribed to the absence of two causes greatly 

 conducive to the exercise of this faculty in 

 health. The muscular contraction of the 

 nostril which accompanied the effort of 

 snuffing effects a considerable narrowing of 

 the aperture ; and in the deep inspiration 

 which accompanies the act, the rapidity of 

 the entering current of air is thus greatly 

 augmented, and in this greater velocity is 

 implied an increased contact of the odorous 

 vapour with the sensitive surface. Besides 

 this, the direction of the current of air seems 

 to be somewhat altered ; the muscles, tending 

 much more to constrict the posterior than 

 the anterior parts of the orifice, appear to 

 direct the current more upwards or anteriorly 

 than in the ordinary inspiration. The me- 

 chanical nature of the action has been illus- 

 trated by Diday *, who has shown that dila- 

 tation of the nostrils by a glass tube, through 

 which the air may be respired, equally pre- 

 vents the perfect" exercise of the olfactory 

 sense; and Longet confirms his experiments. 



The effect of division of the portio dura on 



* Gazette Me'dicale, 1838. Me'moire sur les appa- 

 reils musculaires annexes aux organes des sens. 



