the Mode in which it administers to the Perception of Sound. 129 



absent, or its connexion with the labyrinth is destroyed, that 

 the membrane of the round aperture, when in its normal posi- 

 tion, will bulge out into the tympanic cavity, — such bulging out 

 resulting, in the first of the cases now spoken of, it may be, from 

 the united action of both the muscles of the tympanum, while 

 in the latter it must be due to the operation of the stapedius 

 alone. 



Such being the case, a condensed wave which became inci- 

 dent upon the ear under such circumstances would be stopped 

 by the membrana tympani, if that membrane were perfect; 

 or if it were absent, the condensed air pressing upon the stapes 

 could have no operation to force it further into the labyrinth, 

 that bone, through the operation of the stapedius muscle, being- 

 supposed to have been already driven as far into the labyrinth 

 as the shape of the aperture, or the liquid in the labyrinth, 

 would allow. 



The manner of the suppression of condensed waves, when 

 the tympanal membrane is destroyed, thus readily appears. The 

 mode of operation of rarefied waves under similar circumstances, 

 or when, the membrana tympani being present, there is dis- 

 connexion in the chain of bones, is a matter of greater de- 

 licacy. 



In this case, to produce that combined action of the stapes 

 and cochlear membrane which in the perfect ear has been 

 shown to be essential in order to occasion the perception of 

 sound, we must have, when a rarefied wave is incident, a va- 

 riation in the external pressure on the two fenestra?. Such a 

 variation of pressure I conceive would necessarily arise from the 

 different positions which the two apertures into the labyrinth 

 occupy with respect to the meatus externus, the base of the 

 stapes being nearly centrically opposite, and in a plane parallel 

 to the position which would be occupied by the tympanal mem- 

 brane if the latter were present"*, while the cochlear membrane 



fetrier a plus de tendance a tomber dans le vestibule que dans la caisse du 

 tympan " (vol. ii. p. 680). 



" La paroi externe de la cavite du vestibule . . . presente l'orifice de la 

 fenetre ovale ; mais cet orifice est si parfaitement comble par la base de 

 Petri ei', que cette circonstance ne trouble Paspect lisse et egal de cette 

 paroi" (vol. ii. p. 691). See also Henle's Handbuch §c. vol. ii. p. 758. 



* For the foregoing statement I rely on the general tenor of the ac- 

 counts I have read upon the subject, and on observation of preparations 

 of the part in the dry bone which I have had an opportunity of examining 

 in the Museum of the Royal College of Surgeons in London. As confir- 

 matory, so far as they go, I would refer to the plates in Cruveilhier, vol. ii. 

 pp. 669 and 693 (given also in Dr. Henle's Handbuch der systema- 

 tischen Anatomie des Menschen, vol. ii. pp. 731, 760), and to that in Dr. 

 Allen's paper in the ' Lancet ' for January 16, 1869. 



Phil. Mag. S. 4, Vol. 38. No. 253. Aug. 1869. K 



