Fig. 92. Ivory Exostoses of the Skull.
Both frontal and parietal eminences are also sites of active and prolonged ossification, and the tumours there arising are to be explained on a like hypothesis.
Similar features are to be observed with respect to those bony tumours which develop in the aural region, the numerous centres of ossification for the periotic capsule accounting satisfactorily for their origin.
Whether originating in the region of the frontal sinus or in the aural area, the tumour naturally develops along the line of least resistance, filling up the frontal sinus and growing into the external auditory meatus and mastoid antrum.
More rarely, small exostoses develop on the inner aspect of the skull, chiefly from the frontal bone in the region of the crista galli. In some cases the inner aspect of the skull is studded with small bony tumours, more especially along the line of the superior longitudinal venous sinus.
Fig. 93. The Development of the Frontal Bone. A, Metopic suture; B, Primary centre for frontal eminence; C, Secondary centre for external angular frontal process; D, Secondary centre for trochlear fossa; E, Secondary centre for nasal spine.
These internal exostoses seldom give rise to pressure symptoms, although, according to Wilks and Moxon,[77] they may push inwards the dura mater and even lead to idiocy and epilepsy. I have seen several cases of internal exostosis development, but in all cases their discovery was accidental.