THE TYMPANIC ANTRUM 



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their remnants, infection may reach such sinuses as the inferior petrosal. The 

 sutures normallj^ close in the second year after birth. Through the floor, the 

 antrum communicates with the lower or vertical cells of the mastoid. This floor 

 is on a lower level than the opening into the tympanum, and thus drainage of an 

 infected antrum is difficult, fluid finding its way more readily into the lower cells. 

 Behind the mastoid antrum and cells is the bend of the sigmoid part of the trans- 

 verse (lateral) sinus, with its short descending portion (fig. 1087). The average 

 distance of the sinus from the superior meatal triangle is 1 cm. (| in.). It may be 

 further back; on the other hand, it may come within 2 mm. (yV in.) from the 

 meatus, and even overlap the outer wall of the antrum. 



Fig. 1088. — The Mastoid Antrum and Cells. (Jacobson and Steward.) 

 1. Posterior root of zygoma forming the supramastoid or suprameatal crest and upper 

 part of Macewen's triangle. 2. Antrum, and in front of it, the epitympanic recess. 3. Vertical 

 cells of the mastoid. 4. Ridge on the inner wall of the tympanum, caused by the facial canal. 

 5. Fenestrse on inner wall of tympanum, indicated in shadow, 6. A deficiency present in the 

 tegmen tympani, enlarged with a small osteotrite to emphasise the thinness of the roof of the 

 antrum and tympanum. 7. Cells extending, in this case, even into the root of the zygoma. 



The exact position of the antrum, a little above and behind the external auditory meatus 

 is represented by Macewen's 'suprameatal triangle.' This is a triangle bounded by the posterior 

 root of the zygoma above, the upper and posterior segment of the bony external meatus below, 

 and an imaginary Une joining the above boundaries (fig. 1087). "Roughly speaking, if the orifice 

 of the external osseous meatus be bisected horizontally, the upper half would be on the level 

 of the mastoid antrum. If this segment be again bisected vertically, its posterior half would 

 again correspond to the junction of the antrum and middle ear, and immediately behind this 

 lies the suprameatal fossa.' (Macewen.) When opening the antrum through this triangje, 

 the operator should work forward and medially, so as to avoid the transverse sinus (fig. 1087j ; 

 while, to avoid the facial nerve (fig. 1087), he should hug the root of the zygoma and the upper 

 part of the bony meatus as closely as possible. The level of the base of the brain will be a few 

 lines above the posterior root of the zygoma (fig. 1089) and about 6 mm. (i in.) above the roof 

 of the bony meatus. (Macewen.) 



