ACTION OF THE TYMPANIC APPARATUS. 



407 



acts upon this bone in such a manner as to relax the tym- 

 panum. 



517. The us of this apparatus is evidently to receive the 

 sonorous vibrations from the air, and to transmit them to the 

 membrane forming the entrance to the internal or essential 

 part of the organ of hearing ; in such a manner, that the 

 sonorous vibrations excited in the latter may be much more 

 powerful than they would be if the air acted immediately 

 upon it. The usual state of the membrane of the tympanum 

 appears to be rather lax or slack ; and when in this condition, 



Je h g q r f s b 



Fig. 204. VERTICAL SECTION OP THE ORGAN OP HEARING IN MAN. 



The internal portions are proportionately enlarged to make them more evident: 

 a, b, c, the external ear ; d, entrance to the auditory canal /; e, e, petrous portion 

 of the temporal bone, in which the internal ear is excavated ; g, membrane of the 

 tympanum ; h, cavity of the tympanum, the chain of bones being removed ; 

 , openings from the cavity into the cells j excavated in the bone ; on the side 

 opposite themembrana tympani are seen the fenestra ovalis and rotunda;"*, Eusta- 

 chian tube ; I, vestibule ; m, semicircular canals ; n, cochlea ; o, auditory nerve ; 

 p, canal by which the carotid artery enters the skull ; g, part of the glenoid fossa 

 which receives the head of the lower jaw ; r, styloid process of the temporal bone. 



it vibrates in accordance with grave or deep tones. By the 

 action of a small muscle lodged within the Eustachian tube, it 



