CHAPTEK V 



THE SENSE OF HEARING 



CONTENTS. 1. The organ of hearing. 2. Functions of the external ear. 3. 

 Functions of the tympanic apparatus (tympanic membrane and chain of ossicles). 

 4. Functions of internal ear muscles (tensor tympani and stapedius). 5. Functions 

 of tympanic cavity, Eustachian tube and fenestra rotunda (cochleae). 6. Structure 

 of organ of Corti, and dissimilar vibratory properties of the rods, basilar mem- 

 brane, and tec to rial membrane. 7. Compound tones, noises, simple tones, and 

 differences of pitch and strength. 8. Limits of the perceptive capacity for tones, 

 and faculty of discriminating between different tones. 9. Timbre or quality of 

 simple and compound tones. 10. Acoustic phenomena perceived on the simultaneous 

 production of several tones. 11. Theory of perception of simple and compound 

 tones. 12. Consonance and dissonance of tones, musical chords. 13. Rising and 

 falling phases of auditory sensation ; auditory fatigue. Entotic and subjective 

 auditory sensations and hallucinations. 14. Binaural audition and localisation of 

 sounds. Bibliography. 



WE have already seen that from both the morphological and the 

 physiological point of view the Internal Ear has two distinct 

 portions, one of which is innervated by the vestibular, the other 

 by the cochlear branch of the eighth cerebral nerve, and that 

 they represent two distinct sense-organs (see Vol. III. p. 405). 



The peripheral organ of hearing is the Labyrinth (cochlea) with 

 the terminations of the cochlear nerve, to which alone the name of 

 auditory nerve should be applied. The Vestibular organs (saccule, 

 utricle, and semicircular canals) may, as we have seen, be ex- 

 tirpated in birds and mammals without causing any perceptible 

 depreciation of hearing; the destruction of the cochlea, on the 

 contrary, produces deafness. 



The vestibular organs which regulate the tone of the muscles 

 reflexly by means of sub-conscious impulses (p. Ill) are phylo- 

 genetically a stage in evolution from primitive cutaneous sensi- 

 bility ; the cochlear organ, which subserves auditory sensations, 

 represents a much later stage in evolution it is absent in fishes, 

 first appears in amphibia and reptiles, increases in birds, and 

 finds its maximal perfection in mammals. 



The adequate stimulus of auditory sensations consists in the 

 vibrations of elastic bodies, within certain limits of frequency and 

 intensity. These vibrations are transmitted through the air to 



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