854 THE ORGANS OF SPECIAL SENSE. 



The fenestra ovalis is a reniform opening leading from the tympanum into the 

 vestibule ; its long diameter is directed horizontally, and its convex border is 

 upward. The opening in the recent state is occupied by the base of the stapes, 

 which is connected to the margin of the foramen by an annular ligament. 



The fenestra rotunda is an aperture placed at the bottom of a funnel-shaped 

 depression leading into the cochlea. It is situated below and rather behind the 

 fenestra ovalis, from which it is separated by a rounded elevation, the promontory ; 

 it is closed in the recent state by a membrane (membrana tympani secundaria, 

 Scarpa). This membrane is concave toward the tympanum, convex toward the 

 cochlea. It consists of three layers : the external, or mucous, derived from the 

 mucous lining of the tympanum ; the internal, from the lining membrane of the 

 cochlea ; and an intermediate, or fibrous layer. 



The promontory is a rounded hollow prominence, formed by the projection out- 

 ward of the first turn of the cochlea ; it is placed between the fenestra;, and is 

 furrowed on its surface by three small grooves, which lodge branches of the tym- 

 panic plexus. A minute spicule of bone frequently connects the promontory to the 

 pyramid. 



The rounded eminence of the aquceductus Fallopii, the prominence of the bonv 

 canal in which the facial nerve is contained, traverses the inner wall of the tympa- 

 num above the fenestra ovalis, and behind that opening curves nearly vertically 

 downward along the posterior wall. 



The posterior wall of the tympanum (paries mastoidea) is wider above than 

 below, and presents for examination the 



Opening of the antrum. Pyramid. 



The opening of the antrum is a large irregular aperture, which extends back- 

 ward from the epitympanic recess and leads into a considerable air space, the antrum 

 mastoideum (see page 68). The antrum communicates with large irregular cavities 

 contained in the interior of the mastoid process, the mastoid air-cells. These 

 cavities vary considerably in number, size, and form ; they are lined by mucous mem- 

 brane continuous with that lining the cavity of the tympanum. 



The pyramid is a conical eminence situated immediately behind the fenestra 

 ovalis, and in front of the vertical portion of the eminence above described ; it is 

 hollow in the interior, and contains the Stapedius muscle; its summit projects 

 forward toward the fenestra ovalis, and presents a small aperture which transmits 

 the tendon of the muscle. The cavity in the pyramid is prolonged into a minute 

 canal, which communicates with the aqureductus Fallopii and transmits the nerve 

 which supplies the Stapedius. 



The anterior wall of the tympanum (paries carotica) is wider above than below; 

 it corresponds with the carotid canal, from which it is separated by a thin plate of 

 bone, perforated by the tympanic branch of the internal carotid artery. It presents 

 for examination the 



Canal for the Tensor tympani. Orifice of the Eustachian tube. 



The processus cochleariformis. 



The orifice of the canal for the Tensor tympani and the orifice of the Eustachian 

 tube are situated at the upper part of the anterior wall, being separated from each 

 other by a thin, delicate, horizontal plate of bone, the processus cochleariformis. 

 These canals run from the tympanum forward, inward, and a little downward, to the 

 retiring angle between the squamous and petrous portions of the temporal bone. 



The canal for the Tensor tympani is the superior and the smaller of the two ; it 

 is rounded and lies beneath the forward prolongation of the tegmen tympani. It 

 extends on to the inner wall of the tympanum and ends immediately above the fenestra 

 ovalis. The processus cochleariformis passes backward below this part of the canal, 

 forming its outer wall and floor ; it expands above the anterior extremity of the 

 fenestra ovalis and terminates by curving outward so as to form a pulley over which 

 the tendon passes. 



