512 



PHYSIOLOGY 



The external auditory meatus in man is about one inch long and directed 

 forwards, inwards, and slightly upwards. Its general function, other than 

 as a mere conductor of the sound vaves, is to protect the delicate vibrating 

 membrana tympani which closes its inner end. Opening on the skin of the 

 meatus are special sebaceous glands which secrete a yellow wax (cerumen) 

 with bitter taste and peculiar odour. The wax not only protects the cuticle 

 of the ear and the membrana tympani from drying, but, together with the 

 hairs at the orifice of the meatus, serves to repel insects and prevent their 

 entering. By the length of the meatus moreover the drum is protected from 

 draughts and its temperature is maintained constant. 



11 



FIG. 248. 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. 



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 movement the finest variations of pressure in the course of the wave, 

 it is essential that it should be devoid of any periodicity, i.e. a tendency to 

 vibrate at a certain frequency. If such periodicity were present the ear 

 would pick out and magnify some particular overtone present in the com- 

 pound tones reaching the ear and magnify it to the exclusion of the other 



