1042 ORGANS OF THE SENSES AND THE COMMON INTEGUMENT 



I 



The promontory (promontorium) is a rounded hollow prominence, formed by the 

 projection outward of the first turn of the cochlea; it is placed between the fenestrae, 

 and is furrowed on its surface by small grooves, for the lodgement of branches of the 

 tympanic plexus. A minute spicule of bone frequently connects the promontory 

 to the pyramidal eminence. 



The prominence of the facial canal (prominentia canalis facialis; prominence of 

 aqueduct of Fallopius) indicates the position of the bony canal in which the 

 facial nerve is contained; this canal traverses the labyrinthic wall of the tympanic 

 cavity above the fenestra vestibuli, and behind that opening curves nearly 

 vertically downward along the mastoid wall. 



The mastoid or posterior wall (paries mastoidea) is wider above than below, and 

 presents for examination the entrance to the tympanic antrum, the pyramidal eminence, 

 and the fossa incudis. 



The entrance to the antrum is a large irregular aperture, which leads backward 

 from the epitympanic recess into a considerable air space, named the tympanic 

 or mastoid antrum (see page 142). The antrum communicates behind and below 

 with the mastoid air cells, which vary considerably in number, size, and form; 

 the antrum and mastoid air cells are lined by mucous membrane, continuous with 

 that lining the tympanic cavity. On the medial wall of the entrance to the antrum 

 is a rounded eminence, situated above and behind the prominence of the facial 

 canal; it corresponds with the position of the ampullated ends of the superior and 

 lateral semicircular canals. 



The pyramidal eminence (eminentia pyramidalis; pyramid] is situated immedi- 

 ately behind the fenestra vestibuli, and in front of the vertical portion of the facial 

 canal; it is hollow, and contains the Stapedius muscle; its summit projects forward 

 toward the fenestra vestibuli, and is pierced by a small aperture which transmits 

 the tendon of the muscle. The cavity in the pyramidal eminence is prolonged 

 downward and backward in front of the facial canal, and communicates with it 

 by a minute aperture which transmits a twig from the facial nerve to the Stapedius 

 muscle. 



The fossa incudis is a small depression in the lower and back part of the epi- 

 tympanic recess; it lodges the short cms of the incus. 



The Carotid or Anterior Wall (paries carotica) is wider above than below; it corre- 

 sponds 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, and by the deep 

 petrosal nerve which connects the sympathetic plexus on the internal carotid 

 artery with the tympanic plexus on the promontory. At the upper part of the 

 anterior wall are the orifice of the semicanal for the Tensor tympani muscle and 

 the tympanic orifice of the auditory tube, separated from each other by a thin 

 horizontal plate of bone, the septum canalis musculotubarii. These canals run from 

 the tympanic cavity forward and downward to the retiring angle between the 

 squama and the petrous portion of the temporal bone. 



The semicanal for the Tensor -tympani (semicanalis m. tensoris tympani) is the 

 superior and the smaller of the two; it is cylindrical and lies beneath the tegmen 

 tympani. It extends on to the labyrinthic wall of the tympanic cavity and ends 

 immediately above the fenestra vestibuli. 



The septum canalis musculotubarii (processus cochleariformis] passes backward 

 below this semicanal, forming its lateral wall and floor; it expands above the ante- 

 rior end of the fenestra vestibuli and terminates there by curving laterally so as 

 to form a pulley over which the tendon of the muscle passes. 



The auditory tube (tuba auditiva; Eustachian tube] is the channel through which 

 the tympanic cavity communicates with the nasal part of the pharynx. Its length 

 is about 36 mm., and its direction is downward, forward, and medialward, forming 

 an angle of about 45 degrees with the sagittal plane and one of from 30 to 40 degrees 



