THE SENSES. 



G77 



slight arch directed upward, passes inward and a little forward 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 continuous with that of 

 the pinna, and extending over the outer part of the membrana tympani. 



Toward 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. 



Middle Ear or Tympanum. The middle ear, or tympanum (3, 

 fig. 395), 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 



Fig. 398. 



; > 



Fig 397. The incus, or anvil-bone. 1, body; 2, ridged articulation for the mallei*. A 

 eessus brevis, with 5 rough articular surface for ligament of incus; ? proceiu^ma^nus ' wfth 

 articulating surface for stapes; 7, nutrient foramen (Schwalbe ) magnus, with 



i f Fl &- 398. -The stapes, or stirrup-bone. 1, base; 2 and 3, arch; 4, head of bone which artim 

 (Icfwllbe.) Pr CeSS f ^ inCUS; 5 ' constricted W fc of neck; 6, one 'of 1 the crSm." 



flattened canal, dilated at both ends, composed partly of bone and partly 

 of elastic cartilage, and lined with mucous membrane, 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 nos- 

 trils. The cavity of the tympanum communicates posteriorly with air 

 cavities, the mastoid cells in the mastoid process of the temporal bone ; 

 bat its only opening to the external air is through the Eustachian tube 

 (4, fig. 395). The walls of the tympanum are osseous, except where aper- 

 tures 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 

 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. 



