CHAPTER XXX 

 THE SENSE OF HEARING 



The sense of hearing is the sense by which the intensity, pitch and 

 quality or timbre of atmospheric vibrations, the objective cause of sound 

 are perceived. 



The physiologic mechanism involved in the sense of hearing includes the 

 ear, the acoustic nerve, the acoustic tract (the lateral fillet or lemniscus), the 

 acoustic radiation, and nerve-cells in the cortex of the temporal lobe. 



Peripheral stimulation of this mechanism develops nerve impulses which, 

 transmitted to the cerebral cortex, evoke the sensation of sound and its vary- 

 ing qualities intensity, pitch, and timbre. 



The specific physiologic stimulus to the terminal organ, the organ of 

 Corti, is the impact of atmospheric pulsations of varying energy and rapidity. 



THE PHYSIOLOGIC ANATOMY OF THE EAR 



The ear, the organ of hearing, is lodged within the petrous portion of 

 the temporal bone. It may, for convenience of description, be divided into 

 three portions: viz., the external, the middle, and the internal portion 

 (Fig. 3i4). 



The External Ear. The external ear consists of the pinna or auricle 

 and the external auditory canal. 



The Pinna. The pinna is composed of a thin layer of cartilage which 

 presents a series of elevations and depressions. It is attached by fibrous tis- 

 sue to the outer edge of the auditory canal and covered by a layer of skin con- 

 tinuous with that covering adjacent structures. The general shape of the 

 pinna is concave. Its anterior surface presents, a little below the center, a 

 deep depression the concha. 



The External Auditory Canal. The external auditory canal extends from 

 the concha inward for a distance of from 25 to 30 mm. It is directed at first 

 upward, forward, inward, and then somewhat downward to its termination. 

 It is composed partly of bone and partly of cartilage and lined by a reflection 

 of the skin covering the pinna. At the external portion of the canal the skin 

 contains a number of tubular glands, the ceruminous glands, which resemble 

 in their conformation the perspiratory glands. They secrete cerumen or ear- 

 wax. 



The Middle Ear. The middle ear, or tympanum, is an irregularly 

 shaped cavity hollowed out of the temporal bone and situated between the 

 external auditory canal and the internal ear. It is narrow from side to side, 

 though wider above than below. It is relatively long in its antero-posterior 

 and vertical diameters. The upper portion is known as the attic. The 

 middle ear is in communication posteriorly with the mastoid cells, anteriorly 

 with the pharynx through the Eustachian tube. 



The Eustachian Tube. The passageway between the tympanic cavity and 

 the naso-pharynx is known from its discoverer as the Eustachian tube. It 

 is composed internally of bone, externally of cartilage, and is lined by mucous 



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