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HANDBOOK OF PHVSIOLOGV 



NEUROPHYSIOLOGY I 



formation necessary for the central nervous system to 

 achieve such discriminations. A third is the mecha- 

 nism by which the mechanical forces of acoustic 

 energy excite the nerve impulses in the sense organ. 



Significance of Bitemporal Localion oj Ears 



The location of the inner ears within the temporal 

 bone of the skull gives them excellent mechanical 

 protection. Certain details of anatomical architecture 

 seem to give special acoustic isolation from the inter- 

 nal sounds of the body, including the sound of one's 

 own voice (22). The location at the sides of the head 

 provides an acoustic baffle between the two ears that 

 insures the differences in intensity of sound waves and 

 in times of arrival that are the basis of the sensing of 

 direction of the incoming waves. The location in the 

 head also allows the use of scanning movements of 

 the whole head, which, in the human, replace the 

 separate movements of large external ears. Acous- 

 tically, the human auricle is an organ of little sig- 

 nificance. 



GENERAL PLAN OF EAR 



The external ear (fig. i) includes the canal which 

 extends diagonally inward about 27 mm (in man) to 

 the tympanic membrane. This partition, however, 



belongs to the middle ear or 'ear drum.' The middle 

 ear is air-filled and is periodically ventilated for 

 equalization of air pressure by opening of the audi- 

 tory (Eustachian) tube. The latter connects with the 

 nasopharynx. A chain of three small bones, the ossi- 

 cles, in the middle ear form a system of mechanical 

 levers that connect the outer tympanic membrane 

 with a smaller inner opening, the oval window, into 

 the inner ear. A second opening from middle to inner 

 ear, the round window, is closed by the flexible 

 round-window membrane. As we shall see, the chief 

 acoustic function of the middle ear is to provide an 

 impedance match between the air of the external ear 

 canal and the fluid that fills the inner ear and thus 

 to deliver acoustic energy efficiently to the inner ear 

 where the sensory cells are located. 



The auditory portion of the inner ear is a spirally 

 coiled canal, called the cochlea because of its snail- 

 like shape, within the temporal bone. It is fluid-filled 

 and it is divided along nearly its entire length by a 

 partition. This partition is actually a tube, the coch- 

 lear duct, which contains the sense organ proper, 

 namely the organ of Corti, and its accessory structure, 

 the tectorial membrane. 



The coiled tube that contains the organ of Corti is 

 roughly triangular in cross section (fig. 2). One side is 

 formed by the fibrous, elastic basilar membrane, 

 which extends from the inner bony core of the coch- 

 lea, the modiolus, to the spiral ligament which lines 



Auricle: 



Cartilage 



SCMICIRCULAR 

 MaLLELUS CANALS 



VeSTIBULE 



VeSTlBULAR N 



Facial n 



Cochlear m 



Internal 

 auditory 

 canal 



Cochlea 



Mastoid tip 



Cross section 

 OF eustachian tube 



FIG. I. In this semidiagrammatic drawing of the ear, the cochlea has been turned slightly from 

 its normal orientation to show its coils more clearly. The opening for nerves through the bone to 

 the brain cavity of the skull is quite diagrammatic. The muscles of the middle car are omitted. 

 [From Davis (2).] 



