CHAPTEK LIV 



HEARING 



Anatomy of the Ear. 



THE Organ of Hearing (tig. 539) is divided into three parts, (1) the 

 external, (2) the middle, and (3) the internal ear. The two first are 

 only accessory to the third or internal ear, which contains the 

 essential parts of the organ of hearing. 



External Ear. The external ear consists of the pinna and the 

 external auditory meatus. 



The principal parts of the pinna are two prominent rims enclosed 

 one within the other (helix and antihelix), and enclosing a central 

 hollow named the concha ; in front of the concha is a prominence 

 directed backwards, the tragus, and opposite to this one directed 

 forwards, the antitragus. From the concha, the auditory canal, with 

 a slight arch directed upwards, passes inwards and a little forwards 

 to the membrana tympani, to which it thus serves to convey the 

 vibrating air. Its outer part consists of fibre-cartilage continued 

 from the concha, its inner part of bone. Both are lined by skin con- 

 tinuous with that of the pinna ; the skin also extends over the outer 

 surface of the membrana tympani. Towards the outer part of the 

 canal are fine hairs and sebaceous glands, while deeper in the canal 

 are small glands, resembling the sweat-glands in structure, which 

 secrete the cerumen or wax of the ear. 



Middle Ear or Tympanum. The middle ear, or tympanum or 

 drum (3, fig. 539), is separated by the membrana tympani from the 

 external auditory meatus. It is a cavity in the temporal bone, 

 opening through its anterior and inner wall into the Eustachian tube, 

 a cylindriform flattened canal, dilated at both ends, composed partly 

 of bone and partly of elastic cartilage, and lined with mucous mem- 

 brane, which forms a communication between the tympanum and the 

 pharynx. It opens into the cavity of the pharynx just behind the 

 posterior aperture of the nostrils. The cavity of the tympanum 

 communicates posteriorly with air-cavities, the mastoid cells in the 

 mastoid process of the temporal bone ; but its only opening to the 



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