634 



THE EAK. 



The membrane closing the fenestra rotunda — the secondary membrane 

 of the tipnpanum (Scarpa) — is rather concave towards the tympanic 

 cavity, and is composed of three strata like the membrana tympani ; 

 the middle layer being fibrous, and the outer and inner derived from 

 the membranes lining the cavities between which it is interposed, viz., 

 the tympanum and the cochlea. 



The posterior wall of the tympanum presents at its upper part one 

 larger (fig. 44 G, 1), and several smaller openings, which lead into irre- 

 gular cavities, the mastoid cells, in the substance of the mastoid process 

 of the temporal bone. These cells communicate, for the most part, 

 freely with one another, and are lined by a thin mucous membrane 

 continuous with that of the tympanum. Behind the fenestra ovalis, 

 and directed upwards, is a small conical eminence, called the prjramid, 

 or eminentia papillaris (fig, 447, 12). Its apex is pierced by a foramen, 

 throuo-h which the tendon of the stapedius muscle emerges from a canal 

 which turns downwards in the posterior wall of the tympanum, and 

 joins obliquely the descending part of the aqueduct of Fallopius. 



The anterior extremity of the tympanum is narrowed by the gradual 

 descent of the roof, and is continued into the Eustachian orifice 

 (fig. 447). Of the two compartments of this orifice, the lower, lined 

 with mucous membrane, forms the commencement of the Eustachian 

 tube ; the upper, about half an inch long, lodges the tensor tympani 

 muscle, and opens into the tympanum immediately in front of the 

 fenestra ovalis, surrounded by the expanded and everted end of the 

 cochleariform process (fig. 446, 7), which separates it from the lower 

 compartment. 



Fig. U1. 



Fig. 447. — Antero-posterior Section of the Temporal Bone, showing the Innee 

 Wall of the Tympanum, with the Eustachian Tube and Small Bones in the 

 Recent State (from Arnold). 



1, styloid process ; 2, mastoid process ; 3, the upper part of the petrous bone ; 4, 

 pharyngeal end of the Eustachian tube ; 5, its cartilage ; 6, its mucous surface ; 7, carotid 

 canal; 8, fenestra rotunda; 9, malleus; 10, incus; 11. stapes; 12, pyramid and 

 stapedius muscle ; above 9, and behind 10, the suspensory ligament of the malleus and 

 the posterior ligament of the incus are also seen. 



_ The Eustacliian tube is a canal, formed partly of bone, partly of car- 

 tilage and membrane, which leads from the cavity of the tympanum to 



