CRAM CM AM) SCAI.P 



10H3 



that freqiiont and fatal disease, intlauiinatiou of tlic iiiiddlc car : — (A) Tlir upper, 

 or ' antrum,' ]ir(sc'nt Woth in early and late life, lidrizontal in direetii.u. rlds.lv 

 adjacent to and eonimunieating with the tympanum. ( W) The lower, or vertical. 

 This group is not developed in early life. 



A. Mastoid antrum (figs. (JGU and OGl ). — This is a small chaml»er lying 

 behind the tympanum, into the ui)i>er and back i»art of which (tympanic attic) it 

 opens. Its size varies, especially with age. Present at birth, it readies its largest 

 size (that of a pea) about the third or fourth year. After this, its size usually 

 diminishes somewhat, owing to the development of the encroacliing lione around 

 it. Its roof, the tegmen antri, is merely the backward continuation of the tegmen 

 tympani. The level of this is indicated l)y the posterior root of the zygoma. 

 'The level of the lloor of the adult skull at the tegmen antri is, on an average, legs 

 than one-fourth of an inch above the njof of the external osseous meatus; in children 

 and adolescents, from one-sixteenth to one-eighth of an inch ' (Macewen), 



The outer wall of the antrum is formed V)y a plate descending from the squamous 

 bone. This is very thin in early life, but as it develops ]>y deposit from the peri- 

 osteum, the depth of the antrum from the surface increases. Macewen gives the 

 average of the depth as varying from one-eighth to three-fourths of an inch. At the 

 junction of the two parts of the outer wall of the mastoi«l cells is the masto-scjuanKisal 

 suture, often i>resent at puberty. Through tlif floor, the antrum communicates 



Fig. 661. — Sagittal Section through Roof axd Floor of Tympanic Cavity. 



ANTRUM 



FENESTRA OVALIS 

 Canal for tensor tympani muscle 



■^f^ 



^•^:§if^.^^? ^ 







OSSEOUS EUSTACHIAN TUBE 



FLOOR OF TYMPANUM 



FENESTRA ROTUNDA 

 CANAL FOR FACIAL SERVE 



MASTOID PROCESS 



with the lower or vertical cells of the mastoid. This floor is on a l<iwtr Itvtl than 

 the opening into the tympanum, and thus drainage of the antrum is ditlicult. fluid 

 finding its way more readily into the lower cells. Behind the mastoid antrum 

 and cells is the bend of the sigmoid jtart of the lateral sinus, with its short descend- 

 ing portion (fig. ObU). Macewen gives the following guides for tliis part <»f th<> 

 sinus : ' (1) A line clrawn from the parieto-S(iuamo-mastoid junction to the tip of 

 the mastoid. (2) In the aduit, a vertu-al line drawn one-lialf of an inch behind the 

 ])Osterior bony wall of the external auditory meatus, and between the levels of the 

 roof and the floor, will, in the majority of ca.'ses, indicate the ]>osition of the 

 anterior convexity of the signK)i<l sinus.' The same authority gives the following 

 directions for exploring this important vessel : 'An opening in the bone, with its 

 posterior margin touching the line drawn from the parieto-squamo-mastoicl junction 

 to the ti)) of the mastoid, and within the jiarallcls of the roof and floor of the 

 external auditory meatus, will expose the part of the sigmoid sinus most often 

 affected with septic thromliosis. ' The sinus lies more sujterticially than tlie antrum, 

 being usually f»ne-fourth of an inch, occasionally only one-half of an inch, from 

 the surface. 



The exact position of the antrum, a little above and beliind tlie external auditory 

 meatus, is rejiresented by Macewen's ' suprameatal triangle." This is a triangle 

 bounded l>v the jiosterior root of the zygoma above, the upper and posterior seg- 



