1368 



SUKFACE AND SURGICAL ANATOMY. 



concavity of the membrane (umbo), situated a little below its centre (Fig. 1071); passing 

 downwards and forwards from the umbo is the triangular cone of reflected light, to which 

 too much importance must not be attached, since its appearances vary considerably in 

 healthy ears. Normally, the long crus of the incus is but faintly visible, and still less 

 so are the promontory and fenestra cochleae; in the condition of obstruction of the 

 auditory tube (Eustachian), however, in which the membrane is indrawn, these structures, 

 along with the folds of the drum-head, become more distinct. 



In performing the operation of paracentesis of the tympanic membrane the postero- 

 inferior quadrant is the site chosen for making the puncture, as, in addition to providing 

 good drainage, it is farthest removed from important structures, especially the chorda 

 tympani nerve. 



In order to understand the clinical importance of the parts seen through the trans- 

 lucent membrane, it is necessary to study the relative position of the structure of the 

 " mesotympanum" that is to say, that part of the tympanum which lies opposite the 

 tympanic membrane. If the tympanic plate and the tympanic membrane be carefully 

 removed so as to leave the ossicles and chorda tympani nerve in position, it will be seen 



Roof of tympanic antrum 



Middle cranial fossa 



Tympanic antrum 



Groove for transverse sinus 



Portion of lateral semi- 

 circular canal 



Elevation caused by 

 canalis facialis 



Groove for 

 middle menin-^ 

 geal artery 



Head of 



malleus 



Body of incus 

 Canal for tensor tympani muscle 



Carotid canal 

 Membrana tympani 



Styloid process 



Facial nerve 



Posterior margin of jugular foramen 



FIG. 1074. SECTION THROUGH PETROUS PORTION OF TEMPORAL BONE OF ADULT. 

 Showing the relation of the tympanum to the middle and posterior fossae of the skull. 



that the head of the malleus and the body and short crus of the incus are altogether 

 above the tympanic membrane, and that they occupy the tympanic attic or epitympanic 

 recess (Fig. 1074). At the junction of the two upper quadrants of the membrane is the handle 

 of the malleus, which is directed downwards, backwards, and medially. The lateral process 

 of the malleus is directed laterally a little below the deepest part of the roof of the 

 osseous external acoustic canal. Opposite the postero-superior quadrant are the long 

 crus of the incus, which descends behind and almost parallel to the handle of the 

 malleus, and the stapes, which is directed medially and slightly backwards to the fenestra 

 vestibuli. The chorda tympani nerve runs from behind forwards between the lateral surface 

 of the superior part of the long crus of the incus and the medial surface of the neck of 

 the malleus. At the deepest part of the roof of the osseous canal, above the chorda 

 tympani nerve and the lateral process of the malleus, is a notch (notch of Rivinus}, which 

 is occupied by the flaccid and highest portion of the membrana tympani (ShrapnelVs 

 membrane}. Opposite the postero-inferior quadrant of the drum-head is the promontory 

 caused by the first part of the cochlea, below and behind which is the fenestra cochleae. 

 Opposite the antero-superior quadrant are the processus cochleariformis, the tendon of the 

 tensor tympani, and the passage leading towards the auditory tube. 



The labyrinthine wall of the tympanic cavity is related to the internal ear. The 

 tegmental wall is separated from the middle fossa of the skull and the under surface of the 

 temporal lobe of the brain by the tegmen tympani a thin plate of bone, which is continued 



