Identify the encircled structure in middle-ear cavity:
-The sinus tympani is located on the medial wall of the tympanum between the subiculum and the ponticulus.
-It extends in a posterior direction, medial to the pyramidal eminence, stapedius muscle, and facial nerve and lateral to the posterior semicircular canal.
-The sinus is subject to great variability in the size and in posterior extent.
-The entrance of the infrapyramidal portion is likewise subject to striking variation in respect to size and shape.