CHAPTER XXVI. 

 THE SENSE OF HEARING. 



The physiologic mechanism involved in the sense of hearing in- 

 cludes 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 portions (Fig. 309). 



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

 auditory 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 pre- 

 sents, a little below the center, a deep depression the concha. 



The external auditory canal extends from the concha inward for 

 a distance of from 25 to 30 mm. It is directed at first upward, for- 

 ward, inward, and then 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, 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 



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