‘ACTION OF THE TYMPANIC APPARATUS. 407 
acts upon this bone in such a manner as to relax the tym- 
um. 
517. The use 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, 
Hemme 
eT 
SR 
k oS Ue ee 2 ) 
Fig, 204.—Verticat Secrion oF THE OrGAN oF HEARING IN May. 
The internal portions are proportionately enlarged to make them more evident: 
a, b, c, the external ear; d, entrance to the auditory canal f; ¢, e, petrous portion 
of the temporal bone, in which the internal ear is excavated; g, membrane of the 
tympanum; 4, 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 the membrana tympani are seen the fenestra ovalis and rotunda ;7k, Eusta- 
chian tube; /, vestibule; m, semicircular canals; 2, 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 
