228 PHYSIOLOGY. 



auditorius externus ; 6, the tympanum ; c, the malleus ; d, the incus ; 

 c, the os orbiculare ; /, the stapes ; g, the semicircular canals ; h, the 



cochlea ; i, the meatus auditorius internus ; k t the eustachian tube. 

 / 



5. Membrane of the Drum, stretched across the inner end 

 of the auditory tube, is a membrane, called by anatomists the 

 membrane of the tympanum, (membrana tympani,) or drum of 

 the ear. It is somewhat oval in shape, but hollowed out, or 

 depressed at the centre, where, as we shall presently see, it 

 is fastened to the end of a small bone. This membrane is 

 tense like a drum-head, thin, and transparent. Its use is to 

 convey the vibrations of the atmosphere to the expansion of 

 the optic nerve in the internal ear. It does not seem entire- 

 ly essential to the function of hearing, as the hearing some- 

 times remains after it is destroyed. When it is ruptured, a 

 person can force the air out of his ear ; but while it remains 

 entire, it is impossible for insects or other bodies to get into 

 the drum, as is generally believed. The drum is capable of 

 being rendered more or less tense, and thus of moderating 

 the intensity of vibrations transmitted to it. 



6. The Drum. What is called the drum, or tympanum, 

 is an irregular cylindrical cavity, separated from the external 

 passage by the membrane of the drum. It contains the little 

 bones of the ear, the openings to the labyrinth and other 

 parts of the organ. Anteriorly a passage leads from this 

 cavity, forwards and downwards to the throat, called the 

 Eustachian tube. Posteriorly there are several openings into 

 osseous cells, called the mastoid cells, which are situated in 

 that projecting process of bone immediately back of the ear. 

 This cavity then is filled with air. 



7 Eustachian Tube.-^-l have stated that this tube leads from 

 the drum into the mouth, and serves for the introduction of 

 air into the internal ear. The necessity of such a contri- 

 vance is obvious from the fact, that sounds cannot be trans- 

 mitted through a vacuum, as is shown in the common ex- 

 periment of ringing a bell in an exhausted receiver, when 

 no sound is made. So in a drum, there is an air-hole, else 



