158 



ON THE STRUCTURE OF THE SKULL. 



the alisphenoids and orbito-sphenoids, add themselves by a 

 similar obliteration of the primitive separations. By addition 

 of bony matter to its free margin, more especially to that 

 of its lower part, the tympanic bone becomes converted into 

 the gutter-like external auditory meatus. The epiotic grows 

 out, inferiorly, into the mastoid process. The cavity beneath 

 the bony arch in which the superior vertical semicircular canal 

 is lodged, at first filled only by a plug of dura mater, becomes 

 obliterated by bone. 



Fig. 63. 



JP.S. 



■gXi! '■■ 



\ s — ~^ ' ■■ '- "' 



B.S. 



BS^i 



B 



Fig. 6.3. — The basi-sphenoid and presphenoid, with the orbito-sphenoids of a human skull at 

 birth. — A, viewed from above; B, from the right side; C, from below. T.S., tuber- 

 culum sellce ; L.S., IAngula sphenoidalis ; a, basi-presphenoidal synchondrosis. 



The basi-sphenoid acquires larger dimensions in proportion 

 to the lingulse sphenoidales, and the posterior clinoid processes, 

 at first cartilaginous, become completely ossified. The bones of 

 Bert in unite with the under-surface of the presphenoid, and the 

 latter becomes almost obliterated, or converted into a mere 

 vertical lamina of bone, by the extension of the olfactory 

 chambers backwards to give rise to the sphenoidal sinuses. 



The lateral masses of the ethmoid become anchylosed with 

 the lamina perpendicularis, and form one bone — the ethmoid of 

 human anatomy. 



