568 HANDBOOK OF PHYSIOLOGY. 



and (3) the internal ear. The two first are only accessory to the third 

 or internal ear, which contains the essential parts of an organ of hearing. 

 The accompanying figure shows very well the relation of these divisions 

 one to the other (Fig. 378). 



(1) External Ear. The external ear consists of the pinna or auri- 

 cle, and the external auditory canal or meatus. 



The principal parts of the pinna (Fig. 379) are two prominent rims 

 inclosed one within the other (helix and antihelix), and inclosing a cen- 

 tral hollow named the concha; in front of the concha, a prominence di- 

 rected 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 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 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 a peculiar yellow substance called ceru- 

 men, or ear-wax. 



(2.) Middle Ear or Tympanum. The middle ear, or tympanum 

 (3, Fig. 378), is separated by the membrana 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, composed partly of bone and partly 

 of cartilage, and lined with mucous membrane, which forms a commu- 

 nication 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 external air is through the Eustachian tube (4, Fig. 

 378). The walls of the tympanum are osseous, except where apertures 

 in them are closed with membrane, as at the fenestra rotunda, and fe- 

 nestra ovalis, and at the outer part where the bone is replaced by the 

 membrana tympani. The cavity of the tympanum is lined with mucous 

 membrane, 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 Membrana 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 cutaneous 

 lining of the auditory canal, its inner surface with part of the ciliated 

 mucous membrane of the tympanum. 



