THE CKANIUM. 1369 



[ anteriorly to form the roof of the osseous portion of the auditory tube, while posteriorly 

 |j it roofs over the tympanic antrum. Laterally the tegmen is limited by the petro- 

 squamous suture, which may remain unossified for some years after birth, thus 

 affording a channel along which pyogenic infection may spread from the middle ear to the 

 meninges and brain. Infection may also spread along the small veins which convey blood 

 from the tympanum to the superior petrosal and transverse sinuses. 



The jugular wall of the tympanum is formed mainly by the bone forming the jugular 

 fossa, which is occupied by the bulb of the internal jugular vein. When the transverse sinus 

 is large and unusually far forward the bulb is likewise large, and the fossa, which is con- 

 sequently deeper, may arch up into the jugular wall of the tympanic cavity, from which it 

 may be separated merely by a thin and translucent plate of bone which occasionally shows 

 an osseous deficienc} 1 -. In cases where this condition existed the jugular bulb has been 

 wounded in the operation of paracentesis of the tympanic membrane. 



Anteriorly the tympanic cavity leads into the auditory tube, which brings it into 

 communication with the nasal part of the pharynx. In the child the auditory tube is 

 shorter, wider, and more horizontal than in the adult, hence inflammations are more 

 liable to spread along it to the tympanum. 



Above the level of the membrana tympani is the epitympanic recess, which communi- 

 cates posteriorly by means of a triangular opening (aditus ad antrum) with the tympanic 

 antrum ; the base of the triangle, directed upwards, is formed by the tegmen tympani ; 

 its apex, directed downwards, is formed by the meeting of the medial and lateral walls. 

 The opening will admit an instrument half a cm. in diameter. The epitympanic recess 

 contains from before backwards the head of the malleus, the body and short crus of 

 the incus, the latter projecting backwards into the aditus. When these structures 

 are covered with inflamed mucous membrane or granulations, drainage from the 

 tympanic antrum into the tympanum proper is interfered with. The boundaries of the 

 aditus, important surgically, are as follows : superiorly, the tegmen tympani ; medially, an 

 eminence of compact bone, containing the lateral semicircular canal, inferior and anterior to 

 which is a second smaller prominence, corresponding to that portion of the facial canal 

 which curves immediately above and behind the fenestra vestibuli. The wall of the facial 

 canal is here thin and not infrequently deficient, in which case inflammation may readily 

 spread from the tympanum to the facial nerve. The lateral wall of the aditus is formed by 

 the deepest part of the upper and lateral wall of the osseous external -acoustic meatus. 



The posterior wall of the tympanum, below the aditus ad antrum, is formed by diploic 

 bone which contains the descending portion of the facial canal. 



The tympanic antrum is to be considered, developmentally as well as anatomically, 

 as an extension upwards and posteriorly of the tympanum (Fig. 1073). Its ana- 

 tomy and relations will be best understood by studying it in the child, in whom it 

 is relatively larger than in the adult. Situated above and posterior to the tympanic 

 cavity proper, its lateral wall is formed by a triangular plate of bone which descends, 

 behind the external acoustic process, from the squamous portion. Posteriorly, 

 this triangular plate is separated from the petro-mastoid element by the squamo- 

 mastoid suture, which overlies the posterior part of the antrum and transmits 

 small veins to the surface. The suture does not become completely ossified until 

 a year or two after birth, and remains of it may frequently be detected in the 

 adult bone. The anterior and superior portion of the triangular plate turns medially 

 at an angle to form the upper and posterior wall of the rudimentary osseous canal, 

 as well as the floor of the epitympanic recess. 



In the adult the lateral wall of the tympanic antrum is formed by a plate of bone, 

 from \ to | in. in thickness, which occupies the triangular and somewhat depressed 

 area between the ridge extending, posteriorly and slightly upwards, from the 

 posterior root of the zygoma (supra -mastoid portion of temporal crest), and 

 the superior and posterior quadrant of the osseous external acoustic meatus; upon 

 the latter is the supra-meatal spine, immediately posterior to which, upon the floor 

 of the above triangle, is a crescentic depression, the fossa mastoidea. The lateral 

 wall of the antrum is felt through the skin as a slight depression immediately 

 behind the auricle, and immediately inferior to the ridge formed by the supra-mastoid 

 crest ; below the depression is the prominence corresponding to the insertion of 

 the sterno-mastoid muscle. Trautmann has pointed out, however, that the supra- 

 mastoid crest, which varies considerably in its obliquity, is sometimes situated a 

 little above the level of the roof of the antrum, and that it is safer, therefore, to 



