40 



THE SKELETOX 



tho tvnipanic bnincli of tin' .<rloH;s<i-i)liaryngeal nerve, which enters the tympanum 

 through the tympanic canaliculus. The posterior wall of the tymijanum is 

 formed l)y the mastoid jiroccss. At tlie superior and external angle of this wall an 

 opening leads into the mastoid antrum. Immediately l)elow this o])ening there 

 is a small hollow cone, the posterior pyramid ; its cavity is continuous Avith the 

 descending limh of the Fallopian ranal. One or more bony spicule often connect 

 the apex of the pyramid with the promontory. The cavity of this cone is occui)ied 

 l)y the stapedius and the tendon of the muscle emerges at the apex. 



The roof and floor converge towards the anterior extremity of the tympanum, 

 which is, in consequence, very narrow, and occupied by tAvo canals: the loner for 

 the Eustachian tube, the upper for the tensor tympani muscle. These grooves are 

 sometimes described together as the canalis musculo-tubarius. In carefully pre- 

 ])ared l)ones the upper canal is a small horizontal hollow cone (anterior pyramid), 

 12 nun. in length, which lodges the tensor tympani muscle; the apex is just in front 

 of the fenestra ovalis, and is perforated to permit the passage of the tendon of the 

 muscle. As a rule the thin Avails of the canal are damaged, and represented merely 

 l)y a thin ridge of l:)one. The posterior portion of this ridge projects into the tym- 

 l)anum, and is knoAvn as the processus cochleariformis. The thin septum 

 betAveen the canal for the tensor tympani and the tulje is pierced by a narrow canal 

 Avhich is traversed by the small deep petrosal nerve. The outer wall is occupied 

 mainly by the external auditory meatus. This opening is closed in the recent state 



Fig. 48. — The Ixxer Wall of Tymp.^xim. 



CAROTID CANAL 



Tensor Tympani 

 GROOVE FOR EUSTACHIAN 

 TUBE 

 Levator Palati 



CANAL FOR SMALL DEEP 

 PETROSAL NERVE 



Stylo-pharyngeus 



Styli 



EXTERNAL SEMICIRCULAR 



CANAL 

 MASTOID ANTRUM 



FALLOPIAN CANAL 



ITER CHORO/€ POSTERIUS 

 STYLO-MASTOIO FORAMEN 



l>y the tympanic membrane The rim of l)one to wliich tlie membrane is attached is 

 incomplete above; the defect is knoAvn as the notch of Rivinus. Anterior to this 

 notrh. in the angle betAveen theS(iuamosaland the tympanic plate, is the Glaserian 

 fissure, and the small passage Avhich transmits the chorda tympani nerve sometimes 

 called the canal of Huguier. 



The tympanic cavity may l)e divided into three parts. The part l)elow the Ica'cI 

 of the superior margin of the external auditory meatus is the tympanum proper; 

 the portion al)ove this level is the attic of the tympanum; it receives the head of 

 the malleus, the body of the incus, and leads posteriori)^ into a recess known as the 

 mastoid antrum. 



The mastoid antrum. — This is quite distinct from the mastoid cells. It is an 

 air-chamber comnumicating Avith the attic of the tynq^anum. It is sei)arated from 

 the middle cranial fossa by the ]>osterior portion of tlie tegmen tympani; the floor 

 is formed by tl>e mastoid portion of the petrosal; it commiinicatcs Avith the 

 mastoid cells. The outer Avail is formed by the squamosal beloAV the supra-mastoid 

 crest. In cliildren, the outer wall is exceedingly thin. l)ut in the adult it is of con- 

 siderable thickness. The external semicircular canal projects into tlie antrum 

 on its inner wall, and is very conspicuous in the foetus. 



A canal occasionally leads from the ma.stoid aiitram throueh the petrous bone to open in the 

 recess which indicates the position of tho floccular fossa; it is termed the petro-mastoid canal. 

 (<lruber.) 



