NERVES OF SPECIAL SENSE. AUDITORY AND GUSTATORY. 



343 



[Fig. 101. 



apparatus; so that, the decussation of the motor fibres in the pyramids being 

 accompanied by a decussation of the optic nerves, the same effect is produced 

 as if neither decussated, which last is the case with Invertebrated animals in 

 general. 



446. The functions of the Auditory nerve, or Portio Mollis of the Seventh, 

 are easily determined, by anatomical examination of its distribution, and by 

 observation of pathological phenomena, to be analogous to those of the two 

 preceding, Atrophy or lesion of the trunk destroys the sense of Hearing; 

 whilst irritation of it produces auditory sensations, but does not occasion pain. 

 From experiments made upon the 

 nerve before it leaves the cranial 

 cavity, it appears satisfactorily as- 

 certained, that this nerve is not 

 endowed either with common sen- 

 sibility, or with the power of di- 

 rectly stimulating muscular move- 

 ment. Nor can any obvious reflex 

 actions be executed by irritation of 

 this nerve; but it seems neverthe- 

 less by no means improbable, that 

 the muscles which regulate the ten- 

 sion of the tympanum, are called 

 into action by impressions made 

 upon it and reflected through the 

 auditory ganglion, in the same man- 



ner as the diameter of the pupil is A view of , he origin and distribution of the Portio 



regulated through the Optic nerve. Mollis of the Seventh pair or Auditory Nerve ; 1, the 



- It has been attempted by Flotl- medulla oblongata; 2, the pens Varolii; 3. 4. the crura 



rens to show, that the division of cerebelli of the right side; 5,^6 eighth pairof nerves; 



,, . i- i i_ i 6, the ninth pair; 7, the auditory nerve distributed to 



the Auditory nerve, which proceeds the coc hlea and labyrinth; 8, th ? sixth pair of nerves 5 



to the Semi-Circular Canals, has 9, the portio dura of the seventh pair; 10, the fourth 



functions altogether different from pair; n, the fifth pair.] 



that portion which supplies the Ves- 



tibule and Cochlea. This inference, however, is grounded only upon the 



movements exhibited by animals, in which these nerves are irritated ; which 



movements are capable of a different explanation ( 432). 



a. It is interesting to remark, that microscopic examination of the structure of the Audi- 

 tory nerve clearly indicates its intermediate character between the nerves of special sensa- 

 tion issuing from the anterior part of the cranium (namely, the Optic and Olfactory), and 

 those whose function is to minister, either to common sensation, or to that of Taste, which 

 approaches nearly to it, (namely, the Fifth pair and the Glosio-pliaryngeal,) which issue 

 from the posterior part of the Encephalon, and are more nearly analogous to the Spinal 

 nerves. The primitive fibres are not so soft as those of the Olfactive, nor so slender as 

 those of the Optic ; and they are softer than those of the Glosso-pharyngeal. Moreover, the 

 Auditory nerve forms a plexus with the Facial, to which there is no analogy in the Optic 

 and Olfactive nerves, but to which a similar one exists in the Glosso-pharyngeal. This 

 intermediate structural character is interesting, when we compare it with the intermediate 

 character of the function; for the impressions made upon the sense of Hearing are pro- 

 duced through vibrations of a material fluid, instead of being, as in the case of Sight, the 

 result of changes so subtle as to be almost inscrutable to our means of research, or, as in 

 the case of Taste and Touch, being produced by the direct contact of the substance which 

 rise to the sensation. 



447. The nerves which minister to the sense of Taste, as already men- 

 tioned, are destitute of the peculiarities which distinguish the preceding; 

 being no other than certain branches of ordinary afferent nerves, the Fifth 

 Pair and Glosso-pharyngeal, the peculiar endowments of which seem to 



