576 



HEARING. 



the suppurativo process does not extend so far 

 as to destroy the stapes, the hearing is only 

 impaired; but should that bone and its attached 

 membrane sufier, then a total deafness is the 

 consequence. In one case, related by Sir A. 

 Cooper, the membrana tympani was entirely 

 destroyed on the left side, and partially so on 

 the right, yet this gentleman, if his attention 

 were excited, was capable, when in company, 

 of hearing whatever was said in the usual tone 

 of conversation, but it was remarkable that he 

 heard better with the left ear than with the 

 right, although in the former there were no 

 traces of a membrana tympani. He could not 

 hear from as great a distance as others, and he 

 stated that, in a voyage he had made to the 

 East Indies, while others, when ships were 

 hailed at sea, could catch words with accuracy, 

 his organ of hearing received only an indistinct 

 impression. His musical ear was not impaired, 

 " for he played well on the flute and had fre- 

 quently borne a part in a concert." The ex- 

 ternal ear too had acquired a considerable 

 degree of mobility under the direction of the 

 will, so that it could at pleasure be raised or 

 drawn backwards, and this motion was ob- 

 served to take place whenever the attention 

 was directed to sounds not very distinctly 

 audible. 



The Eustachian tube evidently performs a 

 two-fold office : it is the passage for the en- 

 trance of air into the tympanic cavity from the 

 throat, thus affording a provision for keeping 

 that cavity constantly full of air in order to 

 allow of the free vibration of the membranes as 

 well as of the chain of bones ; and it seems 

 obvious that the tube communicates with the 

 throat in order that the air introduced through 

 it shall have acquired the temperature of the 

 body. It likewise affords an outlet for the es- 

 cape of such sonorous undulations as do not 

 impinge upon the labyrinthic wall of the tym- 

 panum, which, were there no such communica- 

 tion with the external air, would cause an echo, 

 and in this respect it performs a function si- 

 milar to that of the masloid cells. The neces- 

 sity of such a provision as is afforded by the 

 first office which the Eustachian tube performs, 

 is manifest from the frequency of deafness re- 

 sulting from a stoppage either in the tube or at 

 its extremity. Bressa supposed that the Eus- 

 tachian tube conducted the sonorous impulses 

 excited by one's own voice from the cavity of 

 the mouth to the labyrinth ;* but the incor- 



* Reil and Autenrieth, Archiv.fur die Physiolo- 

 gic, H. viii. This was a modification of an opinion 

 expressed by Boerhaave, viz., that those sounds 

 from without which entered the mouth were conveyed 

 to the labyrinth through the Eustachian tube. An 

 English physiologist advocates the opinion that 

 some sounds are conveyed through the Eustachian 

 tube, and particularly as he supposes in the Cetacea, 

 from the great development of that tube in these 

 animals compared witb the external auditory pas- 

 sage, and the erroneous notion propagated by Home 

 that the malleus had no connexion with the tym- 

 panum, but now disproved by the careful exami- 

 nation of Professor Owen. See Fletcher's Phy- 

 siology, and Owen's Edition of Hunter's Animal 

 Economy. 



rectness of ihis notion is abundantly proved by 

 the fact that persons who labour under obstruc- 

 tion of this tube can hear their own voices 

 plainly enough, while they are deaf to those of 

 others. Moreover, if we introduce into the 

 mouth a watch or a vibrating tuning fork, care 

 being taken that they do not touch any of the 

 walls of the mouth, they are heard gradually 

 less distinctly as they are approximated to the 

 Eustachian tube; indeed when held far back 

 in the mouth they are totally inaudible. In 

 some birds the air of the tympanum finds its 

 way not only into the mastoid cells, but also 

 between the two tables of the skull, as in the 

 owl and in singing birds. The arrangement of 

 the osseous structure corresponding to the 

 diploc is exceedingly beautiful in the canary, in 

 which I have examined it. The two tables 

 seem as it were connected by very fine and nu- 

 merous bony pillars, the extremities of which 

 are attached to each table ; cells freely commu- 

 nicating with each other surround these pilhirs 

 every where, and the air from the tympanum 

 thus traverses the whole of this cellular struc- 

 ture. The superfluous sonorous undulations 

 find their way into these cells, and being re- 

 peatedly reflected from their parietes become 

 greatly weakened, so that they can exert no fur- 

 ther influence upon the hearing. 



Functions of the nerves. The nervous ap- 

 paratus connected with the organ of hearing 

 consists of the nerve which receives the sono- 

 rous impressions, and of other nerves which are 

 connected with the mechanism of the organ. 

 That the portio mollis of the seventh pair an- 

 swers to the former office, anatomy alone abund- 

 antly proves. With respect to the latter nerves 

 some few remarks seem necessary. The mus- 

 cular apparatus of the tympanic ossicles receives 

 its nerves partly from the facial and partly from 

 the otic ganglion, thus exhibiting an analogous 

 arrangement to that of the muscular structure 

 of the iris. Such an analogy renders it ex- 

 tremely probable that the actions of the muscles 

 of the ossicles are excited in a similar way to 

 that in which the iris is prompted to act. The 

 stimulus of sound conveyed to that portion of 

 the nervous centre with which it is connected, 

 excites by reflection the motor power of the 

 facial nerve, which, through its connexion di- 

 rect or indirect with the muscles of the ossicles, 

 causes them to act, and the action is in propor- 

 tion to the intensity of the sound, inasmuch as 

 the more tense the membrane of the tympanum, 

 the less will be the excursions of its vibrations ; 

 as in the iris the more intense the light, the 

 more contracted will the pupil be. It is im- 

 possible in the present state of our knowledge 

 to say what is the office of the chorda tympani, 

 or whether indeed it has any office in connexion 

 with hearing; but we may easily conceive that 

 from its connexion with the facial, an irritation 

 of it may excite that nerve. Equally ignorant 

 are we of the function of the tympanic anas- 

 tomosis. 



I shall conclude with the following brief 

 summary of the present state of our knowledge 

 respecting the functions of the several portions 

 of the organ of hearing. 



