192 SENSE OF HEARING. 



movements of the chain of bones, but leading to no certain knowledge 

 of the precise effect on audition of such tension or relaxation. 1 In fact, 

 although the integrity of the membrane is necessary for perfect hear- 

 ing, its perforation or destruction does not induce deafness. We have 

 numerous cases of perforation from accident and otherwise, related by 

 Messrs. Valsalva, 2 Willis, 3 Riolan, 4 Flourens, and others, in which the 

 hearing continued ; and, in certain cases of deafness, the membrane is 

 actually punctured for the purpose of restoring the hearing. 



The communication of sonorous oscillations from the membrana tym- 

 pani across the cavity of the tympanum to the internal ear is effected 

 in three ways: 1st, by the air contained in the cavity of the tympanum; 

 2dly, by the chain of bones to the membrane of the foramen ovale; and 

 3dly, by the parietes of the tympanum. So that, if the membrana tym- 

 pani should be punctured or destroyed, the aerial undulations, caused by 

 a sonorous body, which enter the meatus auditorius, may extend into 

 the cavity of the tympanum, and excite corresponding oscillations in 

 the membranes of the foramen ovale, and foramen rotundum. The 

 chorda tympani composed, perhaps, wholly of a branch of the fifth 

 pair, and distributed on the interior of the membrana tympani pro- 

 bably conveys no acoustic impression to the brain. To it is owing the 

 excessive pain, which is caused by the contact of an extraneous body 

 with the membrane; and that occasioned by a loud noise, or by com- 

 pressing the air forcibly in the meatus by passing the finger suddenly 

 and strongly into the concha. 



The uses of the mastoid cells, which communicate with the middle 

 ear, are not known. It would seem, that the strength of audition is 

 in a ratio with their extent. In no animals are they more ample than 

 in birds, which are possessed of great delicacy of hearing. This effect 

 may be induced either by their enlarging the cavity of the tympanum, 

 and allowing the sonorous oscillations to come in contact with a larger 

 surface ; or by the plates which compose them being thrown into vibra- 

 tion. It has been conceived, too, that they may serve as a diverticulum 

 for the air in the middle ear, when it is subjected by the membrana 

 tympani to unusual compression. 



Sir Charles Bell, 5 with more warmth than is judicious or courteous, 

 combats the idea of the foramen rotundum receiving the undulations 

 of air. The oblique position of the membrane of the foramen with 

 regard to the membrana tympani satisfactorily, he thinks, opposes 

 this doctrine. The function which, with M. Savart, he assigns to it 

 if not accurately, at least ingeniously is the following. As the mem- 

 brane of the foramen ovale receives the vibrations from the chain of 

 ossicles, these vibrations circulate through the intricate windings of the 

 labyrinth, and are again transmitted to the air in the tympanum by 

 the foramen rotundum. The different cavities of the labyrinth being 

 filled with incompressible fluid, no such circulation, he insists, would 

 occur, provided the parts were entirely osseous. As it is, the mem- 



1 For the fancied uses of this membrane, see Haller, Element. Physio!., v. 198, Lausan., 1769. 

 3 Op. Anat. de Aure Humana, &c., Ed. J. A. Morgagni, Venet., 1740. 

 3 Oper. Ornn. Venet., 1720. 4 Enchirid. Anat., 1. iv. c. 4, Lugd. Bat., 1649. 



* Op. chat., i. 269. 



