750 PHYSIOLOGY. 



hearing is perfectly normal in those persons from whom the external 

 ear has been removed by accident or otherwise. 



The external auditory meatus and canal extend from the concha 

 of the auricle to the tympanum. The canal is composed partly of 

 cartilage and partly of bone; the bony portion belongs to the tem- 

 poral bone. The canal is lined by skin, which contains modified 

 sebaceous and sudoriferous glands. By the glands is secreted the 

 cerumen, or earwax. 



The internal end of the auditory canal is bounded by an ellipsoid 

 structure which is composed of three layers of tissue : the tympanic 

 membran-e,- 



Fig. 307. Diagram of the External Surface of the Left Tympanic 

 Membrane. (HENSEN.) 



a, Head of malleus. &, Incus, e, Joint between malleus and incus. Between 

 c and d is the flaccid portion of the membrane, ax, Axis of rotation of ossicles. 

 The urnbo is the deeply shaded part. 



Function of the External Ear. Sound-vibrations strike the ex- 

 ternal ear, some of which go directly into the external auditory 

 meatus. The irregularity of the surface of the pinna permits us to 

 judge more correctly of the direction and the intensity of sound. If 

 these irregularities are filled up with wax, while the meatus is left 

 open, the intensity of sound is diminished, and it is more difficult to 

 judge of the direction. In the external meatus the waves of sound 

 undergo a series of reflections, which conduct them to the membrana 

 tympani. By reason of the obliquity and curves of the membrana 

 tympani, the sound-waves strike it in a nearly perpendicular direc- 

 tion. The external ear has for its function the collection and trans- 

 mission of sounds to the membrana tympani. The horse is con- 



