834 THE OEGANS OF SENSE. 



Paries Mastoidea. The mastoid or posterior wall presents, from above down- 

 wards : (1) a rounded or triangular opening, extending backwards from the 

 recessus epitympanicus and leading into the tympanic antrum (Fig. 709) ; (2) a 

 depression, the fossa incudis, situated in the postero-inferior part of the recessus epi- 

 tympanicus (Fig. 710), for the reception of the end of the short crus of the incus ; 

 (3) a minute conical bony projection, the eminentia pyramidalis (Fig. 709), the 

 summit of which is perforated by a round aperture for the passage of the tendon of 

 the stapedius muscle. This aperture is continued downwards and backwards as a 

 canal in front of the facial canal, and frequently opens, by a minute orifice, on the 

 base of the skull in front of the stylo-mastoid foramen ; it communicates with the 

 facial canal by one or two small foramina, which transmit the vessels and nerve 

 to the stapedius muscle ; a minute spicule of bone often extends from the eminentia 

 pyramidalis to the promontory on the labyrinthic wall of the tympanum ; (4) a small 

 aperture, the apertura tympanica canaliculi chordae (Fig. 710), which is situated close 

 to the posterior edge of the membrana tynipani, nearly on a level with the upper 

 end of the manubriuni mallei ; (5) a rounded eminence, the prominentia styloidea, is 

 sometimes seen below the last, and is caused by the upward and forward prolongation 

 of the styloid process. 



Paries Carotica. The carotid or anterior wall is narrowed in its transverse 

 diameter by the approximation of the lateral and medial boundaries of the tympanic 

 cavity, and in its vertical diameter by the descent of the roof and the ascent of the 

 carotid canal. It presents (Fig. 709) two parallel semicanals, one above the other 

 separated by a thin lamella of bone, the septum canalis musculotubarii (O.T. 

 processus cochleariformis). These run forwards on the lateral wall of the carotid 

 canal and open in the angle between the squama and the petrous part of the 

 temporal bone. The higher and smaller of the two is termed the semicanalis m. 

 tensoris tympani, and lies immediately below the tegmen tympani. It has a diameter 

 of about 2 mm., and extends on to the medial wall of the tympanic cavity above 

 the anterior part of the fenestra vestibuli. The lower and larger semicanal gradu- 

 ally increases in size from before backwards, and is named the semicanalis tubas 

 auditivse. It forms the bony part of the auditory tube and opens .on the carotid 

 wall of the tympanic cavity opposite the orifice leading into the tympanic antrum. 

 Below the orifice of the auditory tube the anterior part of the tympanic cavity 

 is separated from the ascending portion of the carotid canal by a thin plate of 

 bone in which there are sometimes gaps or deficiencies. It is perforated by the 

 carotico- tympanic canal, which transmits the carotico- tympanic nerve from the 

 sympathetic plexus of the carotid artery to the tympanic plexus. The auditory 

 tube is described on p. 837. 



Paries Membranacea. The membranous or lateral wall is formed almost en- 

 tirely by the membrana tympani (Fig. 710), which closes the medial end of the 

 external acoustic meatus, and is fixed throughout the greater part of its circum- 

 ference in a groove, the sulcus tympanicus. The bony ring containing this sulcus is 

 deficient superiorly, where it exhibits a distinct notch, the notch of Eivinus. On a 

 level with the upper edge of the membrane, and in front of the ring of bone in 

 which it is fixed, is the medial end of the petrotympanic fissure. This transmits the 

 tympanic branch of the internal maxillary artery, and lodges the anterior process 

 and anterior ligament of the malleus. Close to the medial end of the fissure is 

 the iter chordae anterius through which the chorda tympani nerve leaves the 

 tympanic cavity. 



Membrana Tympani. The tympanic membrane is an elliptical disc, its greatest 

 diameter, 9 to 10 mm., being directed from above, downwards and forwards, whilst 

 its least diameter is from 8 to 9 mm. It is placed very obliquely, forming an 

 angle of about 55 with the lower and anterior walls of the external acoustic 

 meatus ; it is said to be more oblique in cretins and deaf mutes, and more 

 perpendicular in musicians. 



The circumference of that portion of the membrane which is fixed in the sulcus 

 tympanicus is considerably thickened, and is named the annulus fibrocartilagineus. 

 It is prolonged from the anterior and posterior extremities of the notch of Kivinus 

 to the processus lateralis of the malleus, in the form of two ligamentous bands, the 



