156 ON THE STRUCTURE 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 foetal skull represented in Fig. 61, D, 

 it is obvious that this process corresponds with the Margo tym- 

 panicus or post-auditory process 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, w r hich 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 the zygoma, and 

 eventually unites with it. The lower arched part of the tym- 

 panic ring becomes anchylosed with the floor of the tympanum, 

 while its posterior and upper end unites with the squamosal. 



In the process of ossification thus commenced and advancing 

 in the foetal cranium, certain centres, at first distinct, unite, and 

 become hard to distinguish from one another even before birth. 



At this period a considerable interval of cartilage separates 

 the basi-occipital from the basi-sphenoid ; but the latter has, 

 as at a, Fig. 62, A, become firmly united with the presphenoid, 

 though traces of the original separation, and remains of the 

 primitive cartilage, are readily discernible. 



The ex-occipitals are still distinct from the supra- and basi- 

 occipital, and the alisphenoids are only suturally united with 

 the lingiilee splienoidales, 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-sphenoid, or middle clinoid process, 

 abuts against the basi-sphenoid. (Fig. 63.) 



In the temporal bone — the pro-otic, opisthotic, and epiotic 

 are indistinguishably united into the pars petrosa and pars 

 mastoidea. The latter and the squamosal are firmly united, 

 but the petro-squamosal suture between the tegmen tympani 

 of the former and the squamosal bone is obvious. The tym- 



