EXTERNAL EAR 



601 



11 



FIG. 304. Diagrammatic view of auditory organ. (After SCHAFER.) 

 1, auditory nerve ; 2, internal auditory meatus ; 3, utricle ; 5, saccule ; 6, canalis 

 media of cochlea ; 9, vestibule containing perilymph ; 12, stapes ; 13, fenestra 

 rotunda; 19, incus; ^18, malleus; 17, membrana tympani ; 16, external auditory 

 meatus ; 14, pinna of external ear ; 23, Eustachian tube. 



In animals the junction between the pinna and meatus is so fashioned that 

 the orifice can be restricted by means of a constrictor muscle. This permits 

 the intensity of sound reaching the internal mechanism of the ear to be con- 

 trolled as light by the iris in the case of the eye. 



THE MIDDLE EAR 



This consists of a cavity hollowed out of the temporal bone, which com- 

 municates externally with the meatus, internally by two windows, one circu- 

 lar and the other oval, with the series of chambers forming the internal ear, 

 below by means of a long duct called the Eustachian canal with the throat. 

 At the junction with the meatus is a special bony ring to which is attached a 

 thin diaphragm, the membrana tympani (or drum), which completely closes 

 the orifice. 



THE MEMBRANA TYMPANI. The sound waves which pass down 

 the external meatus impinge on the drum of the ear and set this into 

 vibration. The vibrations are thence transmitted by a chain of three 

 small bones, the auditory ossicles, across the cavity of the tympanum to 

 the fluid which bathes the terminations of the auditory nerve in the 

 internal ear. Since the drum of the ear has to pick up and transmit 

 vibrations of every frequency, and to reproduce accurately in its move- 

 ment the finest variations of pressure in the course of the wave, it is 



