PHYSIOLOGY OF THE INTERNAL EAR. 197 



acquaintance with them is limited to this ; that they contain the final 

 expansions of the auditory nerve ; and that it is within them, that the 

 nerve receives its impressions from the oscillations of sonorous bodies. 



It has been observed, that sonorous vibrations may reach the nerve 

 by the bony parietes, and that the ticking of a watch held between the 

 teeth is, in this way, heard. A blow upon the head is distinctly aud- 

 ible; and Ingrassias 1 relates the case of a person, who had become 

 deaf in consequence of obstruction of the meatus auditorius externus, 

 and yet could hear the sound of a guitar by placing the handle between 

 his teeth, or by making a communication between his teeth and the 

 instrument by means of a metallic or other rod. The physician has 

 recourse to a plan of this kind for detecting if a case of deafness be 

 dependent upon obstructed Eustachian tube; upon some affection of the 

 meatus auditorius externus ; or upon insensibility of the auditory nerve, 

 or of the part of the brain where the sensation is accomplished. If the 

 latter be the fact, the ticking of a watch, applied to the teeth, will not 

 be audible, and the case will necessarily be of a hopeless character. 

 If, on the other hand, the sound be perceived, the attention of the 

 physician may be directed, with well-founded expectation of success, to 

 the physical parts of the organ, or to those concerned in the transmis- 

 sion of vibrations. Frequently, it will happen, in such cases, that the 

 Eustachian tube is impervious, and properly directed efforts may succeed 

 in removing the obstruction; or, if this be impracticable, temporary, if 

 not permanent, relief may be obtained by puncturing the membrana 

 tympani, and allowing the aerial undulations, in this way, to reach the 

 middle and internal ear. 



Lastly; as regards the precise nerve ofliearing. In this sense, we 

 have the distinction between the nerve of general, and that of special 

 sensibility, more clearly perceptible. The experiments of M. Magendie 2 

 have shown, that the portio mollis of the seventh pair is a nerve of 

 special sensibility; that it may be cut, pricked, or torn, without ex- 

 hibiting any general sensibility, and is inservient to the sense of 

 hearing only. The same experiments demonstrate, that it cannot act 

 unless the fifth or nerve of general sensibility be in a state of integrity. 

 If the latter be divided within the cranium, hearing is always enfeebled, 

 and frequently destroyed. The experiments of M. Flourens, 3 to which 

 allusion has been made, led him to infer that the rupture of the cochlea 

 was of less consequence than that of the semicircular canals. Lacera- 

 tion of the nerve, distributed to the vestibule, enfeebled the hearing, 

 and its total destruction was followed by irreparable deafness. For 

 these, and other reasons afforded by comparative anatomy, M. Lepelle- 

 tier 4 infers, that, in the higher organisms, the vestibule and its nerve 

 constitute the essential organ of impression; the other parts being super- 

 added to perfect the apparatus. 



An interesting case of malformation has been related by Professor 



1 De Ossibus, p. 7. Also, Boerhaave, Praelectiones, iv. 415, and Haller, Element. Physiol. 

 torn. v. p. 253, Lausann., 1763. 

 a Precis, &c., 2de edit., i. 114. 



3 Experiences sur le Systeme Nerveux, p. 42, Paris, 1825, or 2de edit., Paris, 1842. 



4 Traite de Physiologic Medicale et Philosophique, iii. 143, Paris, 1832. 



