494 



HEARING, VOICE AND SPEECH 



A. THE EXTERNAL EAR 



Since, owing to the feeble development of the ear muscles, the human 

 pinna cannot be turned in different directions, it is of but slight service in 

 the collection of sound waves. It has been shown also that the reflection 

 of sound waves by the pinna is of no importance (Harless, Mach). 



The external auditory meatus ought probably to be described as a means 

 of protection for the eardrum. The indirect course of the canal itself favors 

 this view, since in order to see the drum the pinna must be drawn considerably 

 upward and backward. Besides, this canal is provided with sensitive hairs 

 which together with the disagreeable odor of the earwax secreted in the canal 

 serve to prevent the entrance of insects. The passage also protects the middle 

 and internal ear from variations of temperature. 



Like every hollow space of the kind the external auditory meatus has its 

 own resonance tone, situated between c iv and a lvl (Helmholtz, Hensen). If this 

 tone be contained in a sound, naturally it will be intensified above other tones 



by the sympathetic vibration of the air 

 in the canal, but owing to its high 

 position in the scale its resonance is 

 of no great consequence from a practi- 

 cal standpoint. 



A tuning fork allowed to die out 

 until the vibrations are just impercep- 

 tible when it is held close to the ear, 

 can be heard again if the handle be 

 placed between the teeth. In this case 

 the sound is conducted in part directly 

 through the bones of the head to the 

 internal ear and in part is transmitted 

 from the bones to the eardrum and 

 propagated thence as usual through 

 the auditory ossicles. 



FIG. 196. Transverse section through the left 

 auditory canal and tympanic membrane of 

 man, enlarged four times, after Hensen. 

 The section is taken just behind the handle 

 of the hammer in a plane parallel to the 

 handle. G, external auditory meatus; C, 

 tympanic cavity; S, the stapes; H, the ham- 

 mer; a ledge projects at L, to which the 

 ligaments are attached. Between the long 

 process of the anvil and the handle of the 

 hammer the tendon of the tensor tympani 

 may be seen ; LS, ligamentum superior. 



its radii. In this way the membrane is 

 funnel with an aperture of about 125 



B. THE MIDDLE EAR 



1. Vibrations of the Eardrum. 

 The tympanic membrane or eardrum 

 is a fibrous membrane 0.1 mm. in 

 thickness, formed mainly of external 

 radial and internal circular fibers. It 

 is obliquely placed across the internal 

 end of the external auditory meatus 

 and is drawn inward at its middle by 

 the long process of the malleus which 

 is inserted into its tissue along one of 

 given the form of a shallow, irregular 

 (Fig. 196). 



1 The small Roman numerals designate octaves. 



