CHAPTER XXVIII. 

 THE SENSE OF HEARING. 



The physiologic mechanism involved in the sense of hearing includes the 

 ear, the auditory nerve, its cortical connections, and nerve-cells in the cortex 

 of the temporal lobe. 



Peripheral excitation of this mechanism develops nerve impulses which, 

 transmitted to the cortex, evoke the sensation of sound and its varying 

 qualities intensity, pitch, and timbre. 



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

 Corti, is the impact of atmospheric undulations 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. 336). 



The external ear consists of the pinna or auricle and the external audi- 

 tory canal. The pinna is composed of a thin layer of cartilage which presents 

 a series of elevations and depressions. It is attached by fibrous tissue to the 

 outer edge of the auditory canal and covered by a layer of skin continuous 

 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 extends from the concha inward for a dis- 

 tance 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 con- 

 formation the perspiratory glands. They secrete cerumen or ear-wax. 



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 communica- 

 tion 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 

 membrane covered with ciliated epithelium. Near the middle of its course 

 the tube is contracted, though expanded at either extremity (Fig. 336). It 



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