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Show 156 ON 'l'HE STRUCTUHE OF THE SKULL. but it soon sends a curved process downwards behind the auditory meatus and between the tympanic ring and the periotic bones. In the footal skull represented in Fig. en, D, it is obvious that this process corresponds with the JJfargo tym-panicus or post-auditory proces. of the adult temporal bone; and the manner in which the hinder end of the pro-otic ossification is fitted in between it and the representative of the ascending part of the posterior root of the zygoma is very well shown. The tympanic bone is at first a delicate ring, interrupted above, and with tapering ends, which approach one another very closely. The anterior end is thicker than the posterior, however, exhibiting a sort of flange, or internal process, which corresponds in position with the middle root. of tho zygmna, and eventually unites with it. The lower arched part of the tympanic ring becomes anchylosed with the floor of the tympanum, while its posterior and upper end unite with the squamosal. In the process of os ification thuR commen ed and advancing in the footal cranium, certain centres, at fir t di t inct, unite, and become hard to distingui h from one another even before birth. At this period a considerable interval of cartilage separates the basi-occipital from the ba ·i-sphenoid ; but the latter has, as at a, Fig. 62, A, become firmly united with the pre phenoid, though traces of the original separation, and rmnains of the primitive cartilage, are readily discernible. The ex-occipitals are still di tinct from the upra- and basioccipital, and the ali phenoid are only ·uturally united with the lingulm sphenoidales, which are still large in comparison with the basi-sphenoid, though they very early unite with it. The orbito-sphenoid and the presphenoid are completely anchylosed together by the superior root of the former, but the inferior root of the orbito- phenoid, or middle clinoid process, abuts against the basi-sphenoid. (Fig. 63.) In the temporal bone-the pro-otic, opi thotic, and epiotic are indistingui hably united into the p ars petrosa and pars mastoidea. The latter and the squama. al nre firmly united, but the petro-squamo al sut-tu·e between the tegmen tympani of the forn1er and the squamo..,al bone i,' obvjous. rl,he tym- THE DEVELOPMENT OP Til F: HUMAN SKULT1. ]57 panic bone, still little more than a mere ring, is firmly anchylosed with the squamosal and with the opisthotic portion of the pars petrosa, but the indication of the primitive distinctne s of the two latter can be readily traced. (Fig. 62, C.) Fig. 62. vf1. ·v.s.c. ! A Fig. 62.-The human cranium at birth.-A, vertical and loncritudinal section of th b 1 half of tl1 e c1· am· um; B , upper, anrl C, under, view of t"h' e sam<e preparation. e asa · It is only after birth, and with the gradual advance towards adult years, that the spheno-occipital and the spheno-ethmoid sync~on~roses are obliterated, and the vomer becoming anchy~ osea With the ethmoid, the whole cranio-facial axis is fused 1nto one bone, to which the ex:-occipitals and supra-occipital, |