674 THE SENSE OF HEARING. 



outer part consists of fibro-cartilage continued from the 

 concha ; its inner part of bone. Both are lined by skin 

 continuous with that of the pinna, and extending over the 

 outer part of the memlrana 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 a peculiar 

 yellow substance called cerumen, or ear-wax. 



The middle ear, or tympanum (3, fig. 189) is separated by 

 the memlrana tympani from the external auditory canal. 

 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, com- 

 posed partly of bone and partly of cartilage, lined with 

 mucous membrane, and forming 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 mas- 

 toid process of the temporal bone ; but its only opening 

 to the external air is through the Eustachian tube (4, fig. 

 189). The walls of the tympanum are osseous, except 

 where apertures in them are closed with membrane as at 

 the fenestra rotunda, and fenestra ovalis, and at the outer 

 part where the bone is replaced by the memlrana tympani. 

 The cavity of the tympanum is lined with mucous mem- 

 brane, the epithelium of which is ciliated and continuous 

 with that of the pharynx. It contains a chain of small 

 bones (ossicula auditus), which extends from the membrana 

 tympani to the fenestra ovalis. 



The memlrana tympani is placed in a slanting direction 

 at the bottom of the external auditory canal, its plane 

 being at an angle of about 45 with the lower wall of the 

 canal. It is formed chiefly of a tough and tense fibrous 

 membrane, the edges of which are set in a bony groove ; 

 its outer surface is covered with a continuation of the 



