CHAP. XIX.] OKGANS OF SPECIAL SENSE. 229 



be higher in the middle than at either end. It is lined by a 

 prolongation of the skin, which in the outer half of the canal is 

 very thick and not at all sensitive, and in the inner half is thin 

 and highly sensitive. Near the orifice the skin is furnished 

 with a few hairs, and further inwards, with modified sweat- 

 glands, the ceruminous glands, which secrete a yellow, pasty 

 substance, resembling wax. 



The middle ear or tympanum is a small, irregularly flattened 

 cavity, situated in the petrous portion of the temporal bone, 

 and lined with mucous membrane. It is separated from the 



FIG. 134. SEMI-DIAGRAMMATIC SECTION THROUGH THE RIGHT EAR. M, concha ; 

 G, the external auditory canal ; T, tympanic, or drum-membrane ; P, tympanum, 

 or middle ear; o, oval window; r, round window. Extending from T to o is seen 

 the chain of the tympanic bones ; R, Eustachian tube ; V, B, S, bony labyrinth; V, 

 vestibule; B, semicircular canal; S, cochlea; 6, I, v, membranous labyrinth in semi- 

 circular canal and in vestibule. A, auditory nerve dividing into branches for vesti- 

 bule, semicircular canal, and cochlea. 



external auditory canal by the drum membrane (membrana 

 tympani)) and from the internal ear by a bony wall in which 

 there are two small openings covered with membrane, the 

 oval window or fenestra ovalis, and the round window orfenes- 

 tra rotunda. The cavity of the middle ear is so small that 

 probably five or six drops of water would completely fill it. 

 It communicates below with the pharynx bj r the small passage 

 called the Eustachian tube, through which air enters the cavity 

 and serves to keep the atmospheric pressure equal on each 

 of the drum-membrane. The middle ear also communi- 



