1546 A MANUAL OF ANATOMY 



of the attic in front of and behind the superior ligament of the 

 malleus. The inferior pouch of the outer attic is known as the 

 pouch of Prussak. It is bounded externally by the membrana 

 fiaccida, superiorly by the external ligament of the malleus, which 

 partially separates it from the superior pouch, and inferiorly by 

 the short process of the malleus. The pouch communicates pos- 

 teriorly with the tympanic cavity by an opening which is situated 

 a little above the level of the bottom of the pouch. If fluid there- 

 fore should accumulate in Prussak's pouch, it may readily lead to 

 perforation of the membrana fiaccida or Shrapnell's membrane. 



Two other pouches are present, namely, the anterior and posterior 

 pouches of Troltsch. These lie one in front of, and the other behind, 

 the handle of the malleus, and are produced by the fold of mucous 

 membrane which invests the chorda tympani nerve. 



The tympanic mucous membrane is covered, for the most part, 

 by columnar ciliated epithelium, except over the ossicles and 

 membrana tympani, where the epithelium consists of a single layer 

 of squamous, non-ciliated cells. 



The tympanic or mastoid antrum is supplementary to the tym- 

 panum proper, or cavum tympani, behind which it is situated. ' It 

 communicates by a large irregular opening with the attic, and is 

 lined with mucous membrane, which is continuous with that of 

 the attic and cavum tympani. Opening from the antrum there 

 are the mastoid cells, which are lined with mucous membrane, 

 continuous with that of the antrum. 



The average measurements of the antrum are as follows : vertical, 

 about 9 millimetres; antero-posterior, about ii millimetres; and 

 transverse, about 8 millimetres. The roof is formed by the thin 

 tegmen tympani, which enters into the formation of the middle 

 fossa of the base of the skull, and is consequently related to the 

 temporal lobe of the cerebrum and its meninges. The genu and 

 descending limb of the sigmoid sinus lie behind the antrum, and a 

 httle farther back is the cerebellum. The facial nerve, as it 

 traverses the descending part of the aqueduct of Fallopius, lies in 

 the posterior wall of the cavum tympani, close to the inner wall 

 and in front of the mastoid antrum. This part of the nerve is on 

 a plane anterior to the mastoid process, and is nearly flush with 

 the opening of the antrum. The ampulla of the external semi- 

 circular canal of the internal ear gives rise to a slight eminence 

 on the inner i0all at its anterior part. 



The outer wall corresponds with the area of the suprameatal 

 triangle of Macewen, and is formed by the posimeatal plate of the 

 squamous portion of the temporal bone. For a description of this 

 triangle, which is the region selected for mastoidectomy, and also 

 for the mastoid cells, see the description of the Temporal' Bone. 



