THE STRUCTURE OF THE HUMAN SKULL. 



L33 



squamosal is interposed between the upper half of the tympanic 

 and the upper parts of the pars petrosa and pars mastoidea, the 

 depth of the interposed squamosal being greatest posteriorly, 

 while it diminishes to nothing anteriorly. 



The upper region of the pars petrosa, however, does not 

 directly abut, by its thick mass, against the squamosal, but by 

 a thin horizontal plate, which roofs over the tympanum, the 



Fig. 56. 



Fig. 56. — Views of the petrous and tympanic portions of the right temporal bone of the 

 skull represented in Figs. 48 and 49, magnified two diameters. — A, the anterior half of 

 the bone; B, its posterior half; I.M., internal meatus; E.M, external meatus; o, 

 processus cochleariformis ; b, chamber in which the heads of the malleus and incus 

 lie ; c, groove for the tympanic membrane ; S. V.C., superior vertical semicircular 

 canal ; ex., external semicircular canal; p.c, posterior vertical semicircular canal ; 

 Co, cochlea; JP, Pyramid ; F.O,, fenestra ovalis ; IT/, canal for portia dura ; VII 1 , 

 for poitio mollis. 



