THE MIDDLE EAR, DRUM, OR TYMPANUM H29 



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 membrane continuous with that lining the cavity of 

 the tympanum. 



The fossa incudis (Fig. 839) is placed in the posterior and inferior part of the 

 epitympanic recess. It lodges the short process of the incus. 



The pyramid (eminentia pyramidalis) (Fig. 835) is a conical eminence situated 

 immediately behind the fenestra ovalis, and in front of the vertical portion of 

 the facial canal; it is hollow in the interior, and contains the Stapedius muscle; 

 its summit projects forward toward the fenestra ovalis and presents a small aper- 

 ture which transmits the tendon of the muscle. The cavity in the pyramid is 

 prolonged into a minute canal, which communicates with the facial canal and 

 transmits the twig from the facial 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 (Fig. 839), perforated by the caroticotympanic canaliculus, which transmits 

 the tympanic branch of the internal carotid artery and the caroticotympanic 

 nerves. 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 jjie orifice of the Eustachian 

 tube are situated at the upper part of the anterior~wall ? being incompletely sepa- 

 rated from each other by a thin, delicate, horizontal plate of bone, the processus 

 cochleariformis (septum canalis musculotubarii) (Figs. 835 and 839). The canalis 

 musculotubarius is divided by this long process into the canal for the Tensor 

 tympani and the canal for the Eustachian tube. These canals run from the 

 tympanum forward, inward, and a little downward, to the angle between the 

 squamous and petrous portions of the temporal bone. 



The canal for the Tensor tympani (semicanalis m. tensoris tympani) (Figs. 835 

 and 839) 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. The bony wall of this canal is incomplete, and the osseous vacancy is 

 filled by tough connective tissue. 



The Eustachian tube (tuba auditiva [Eustachii]) (Figs. 835 and 836) is the 

 channel through which the tympanum communicates with the pharynx. Its 

 length is 36 mm. fan inch and a half), and its direction downward, inward, and 

 forward, forming an angle of about 45 degrees with the sagittal plane and one of 

 from 30 to 40 degrees with the horizontal plane. The canal for the Eustachian 

 tube (semicanalis tubae auditivae) (Fig. 836) is formed partly of bone, partly of 

 cartilage and fibrous tissue. 



The osseous portion (pars ossea tubae auditivae) is about 12 mm. (half an inch) 

 in length. It is the outer portion of the tube. It commences in the anterior w r all 

 of the tympanum, below the processus cochleariformis, and, gradually narrowing, 

 terminates at the angle of junction of the petrous and squamous portions of the 

 temporal bone, its extremity presenting a jagged margin which serves for the attach- 

 ment of the cartilaginous portion. 



The cartilaginous portion (pars cartilaginea tubae auditivae), about 2.5 cm. (an 

 inch) in length, is formed of a triangular plate of elastic fibrocartilage (cartilago 



