THE MIDDLE EA^fc ^ 685 



sebum,, desquamated epithelial cells and occasional fine hairs, together 

 with foreign particles of a very varied sort. 



In the bony portion of the meatus the corium or derma is firmly 

 adherent to the periosteum of the bone, and all the layers of the skin 

 are much reduced in thickness. The scanty hairs are very fine, and, 

 with the glands, are continued inward to the tympanic membrane only 

 in the superior portion of the wall of the canal. Papillse are present 

 as far as the margin of the tympanic membrane. Upon the surface of 

 this membrane, which closes the inner end of the external acoustic 

 meatus and separates it from the cavity of the middle ear, the skin is 

 reduced to an extremely thin cutaneous coat, devoid of hairs, glands, 

 and papilla. 



THE MIDDLE EAR 



GENERAL CONSIDERATIONS 



The middle ear or tympanum ( 'ear-drum' ) is an irregular cavity, 

 broad above and behind, narrow below and in front, which lies just 

 within the external acoustic meatus. Its outer wall is largely formed 

 by the tympanic or drum membrane, its inner by the osseous wall of the 

 internal ear. 



The contour of the tympanum is very irregular, its cavity being 

 encroached upon by numerous bony elevations which are most pro- 

 nounced on its internal wall. Externally the tympanic membrane is 

 attached to a bony and fibrocartilaginous ring, the annulus tympanicus, 

 which projects somewhat into the tympanic cavity. In front, the orifice 

 of the auditory (Eustachian) tube is marked by a slight cartilaginous 

 projection near the floor of the cavity. 



Above and behind, the tympanic cavity is prolonged into a deep 

 recess, the epitympanic cavity, in the upper part of whose posterior 

 wall are the orifices of the mastoid cells. The upper portion of the 

 cavity contains the rounded heads of the malleus and incus, the two 

 largest of the auditory ossicles. The internal wall of the tympanum 

 presents anteriorly a bulging prominence which is known as the promon- 

 tory, and which indicates the position of the first or broadest turn of 

 the spiral canal of the cochlea. Beneath this prominence is a recess 

 leading to a bony 'window/ the fenestra cochleae (or fenestra rotunda), 

 which, in life, is closed by a delicate membrane, the secondary tympanic 

 membrane. Behind the promontory and at a slightly higher level a 



