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Show V34 ON THE VEll'J'li;BRATE SKULL. E t l :Jns ac 1·w n tube , and the antrttfJm rnastoideun~, and is the te<n~ men tympani.* . . . The lower region of the pars petrosa In hke manner g1 ves off a thicker and shorter plate, which forms the f1oor of the Eustachian tube and the outer or inferior boundary of the carotid canal, in front.; the floor of the tympanum, in tho middle; and then, becoming gradually thicker, constitutes th lower boundary of the antrttt?n rnastoideum. It is with the outer edge of this inferior, or floor-plate, of the tympanum that the lower portion of tympanic bone becomes anchy losed. Tho inner wall is of course constituted by the outer surface of the more massiro part of the pars pet,rosa. Thus, the roof and part of the floor of the tympanum are formed by the superior and inferior prolongations of the pars petrosa, while the outer wall of the tyn1panun1 is constituted above by the squamosal, and below by the tympanic. A section taken vertically and transversely to the axis of the skull through the 1niddle of the f enestra ovalis, in tho way described above, shows that the squamosallin1its, externally, an upper chamber of the tympanum (b, Fig. 56), which is nearly as deep as, and is wider than, the lower di vi ion, bounded externally by the tympanic membrane a·nd tympanic bone (Fig. 5G). 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 tyn1panic cavity. Hence, in looking into the tympanum fro1n without (Fig. 55) when the earbones are in situ, only these processes are con, the heads of both ma1leus and incus being hidden by the arched plate of the sguan1osal. Thus, the tympanum is formed by a very con1plicated adjustment of bony elen1ents, 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, opisthotic, bone, containing .the lower part of the cochlea, and a superior, p"r·o-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 sen1icircular canal. * It lies immediately ben ath the lr ttr rs Pr.O .. ~"'i g-. 5G, A. 'l'HE STRUO'l,URE OF 'l'IIE HUMAN Sl\ULL. 135 Behind t~1e p~steri?r boundary of the squamosal, constituted by t~e two diverging lines above described (Fig. 55), lies all that portwn of the temporal bone 'vhich is known as t11e t 'd B pars mas-oz .e~. ut, as I shall have occasion to demonstrate, when ex-plainmg the Inode of development of the tem1)oral bone t} · t 'd . . . ' us pa:s .m_as oz ea ~s, In reality, made up of extensions of two of the primitive constituents of the pars petrosa and of a third 1 t tl · · T ' e emen le erwtw. he posterior margin of the squamosal, as abov~ ~escnbed, may. be s.aid .roughly to form two sides of a parallelo- 5ram. Tl~e third side Is the thick part of the upper edge of the pars masto~dea, corresponding with the termination of the upper and anterwr surface of the pars petrosa on the inner side of the ~one. If a fourth side is made by an imaginary line connect~ ng the ends of the others, the bony surface whieh lies above and 1~ front of the l~ne will, as nearly as possible, belong to the pro-ohc element, while that which lies below and behind 't · 1 d. h . I ' In-c u m g t e mastOid process, appertains to the epiotic. On the other. hand: a certain amount of the pars mastoidea internal to the d1gastnc groove belongs to the opisthotic. |
