The middle ear is a cavity containing air, of a very complicated outline (see Fig. 487). The central part is that immediately within the tympanic membrane, forming the tympanic cavity, which is traversed by the chain of ossicula. From the lower and anterior part of this cavity passes the Eustachian tube down to the naso-pharynx. The upper and posterior part of the cavity communicates with the mastoid cells. The largest of these spaces lies immediately behind the tympanic cavity, of which it is almost a con tinuation backwards. This is the antrum mastoideum whose roof, along with that of the tympanic cavity, is formed by a thin layer of bone, which alone separates these cavities from the dura mater lining the middle fossa of the skull. The remaining mastoid cells communicate directly or indirectly with the antrum. In childhood the antrum is well developed, but the other cells are very imperfect, the mastoid process being small. The roofs of the tympanum and antrum are not infrequently deficient, especially in childhood, so that there are gaps in the osseous wall. The mucous membrane of the middle ear is, in most parts, very delicate, and is covered with a single layer of epithelium. In its structure and in the character of its epithelium it resembles a serous membrane. It is particularly thin over the ossicula and the membrana tympani and in the mastoid cells. In the greater part of its extent the mucous membrane covers bone, and is inseparably united with the periosteum.

Fig. 487. - Section through middle ear. A, antrum mastoideum; B, mastoid cells; C, inner surface of tympanic membrane; O, fossa for jugular vein; H, partition between tympanum and antrum; I, dura mater; L, tympanic aperture of Eustachian tube; O, pharyngeal mouth of same. (Barb).