134 ON THE VERTEBRATE SKULL. 



Eustachian tube, and the antrum mastoideum, and is the tegmen 

 tympani.* 



The lower region of the pars petrosa in like manner gives off 

 a thicker and shorter plate, which forms the floor of the Eusta- 

 chian tube and the outer or inferior boundary of the carotid 

 canal, in front ; the floor of the tympanum, in the middle ; and 

 then, becoming gradually thicker, constitutes the lower boun- 

 dary of the antrum mastoideum. It is with the outer edge of 

 this inferior, or floor-plate, of the tympanum that the lower por- 

 tion of tympanic bone becomes anchylosed. The inner wall is 

 of course constituted by the outer surface of the more massive 

 part of the pars petrosa. Thus, the roof and part of the floor of 

 the tympanum are formed by the superior and inferior prolonga- 

 tions of the pars petrosa, while the outer wall of the tympanum 

 is constituted above by the squamosal, and below by the tym- 

 panic. A section taken vertically and transversely to the axis 

 of the skull through the middle of the fenestra ovalis, in the way 

 described above, shows that the squamosal limits, externally, an 

 upper chamber of the tympanum (b, Fig. 56), which is nearly as 

 deep as, and is wider than, the lower division, bounded externally 

 by the tympanic membrane and tympanic bone (Fig. 56). It is in 

 this upper chamber that the heads of the malleus and incus are 

 lodged, the handle of the one and the long process of the other, 

 only, depending into the proper tympanic cavity. Hence, in 

 looking into the tympanum from without (Fig. 55) when the ear- 

 bones are in situ, only these processes are seen, the heads of 

 both malleus and incus being hidden by the arched plate of the 

 squamosal. 



Thus, the tympanum is formed by a very complicated adjust- 

 ment of bony elements, and we shall by and by see reason to 

 believe that it is even more complex than it now appears to be, 

 inasmuch as the so-called pars petrosa will prove to be composed 

 of two distinct elements ; an inferior, opistliotic, bone, containing 

 the lower part of the cochlea, and a superior, pro-otic, sheltering 

 the greater part of the vestibule, the upper part of the cochlea, 

 the anterior vertical semicircular canal, part of the posterior 

 vertical canal., and the external semicircular canal. 



* It lios immediately beneath the letters Pr.O., Fig. 56. A. 



