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. [Illustration: 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. =Clinical characteristics.= Exostoses vary greatly both in size and consistency. Some are densely hard--ivory exostoses--others possess a covering of compact bone, whilst their interior is made up of cancellous tissue continuous with that of the bone from which they arise. The denser variety seldom attain any considerable size, but the less compact, growing in the direction of least resistance, often attain such dimensions as to be both unsightly and dangerous. Thus, a frontal exostosis may invade the frontal air sinus and grow into the orbital cavity, obliterating the sinus, interfering greatly with ocular movements, causing protrusion of the globe and even destruction of the eye. An aural exostosis may block up the external auditory meatus, compress the facial nerve, and lead to the development of a mastoid empyema. It might also be added that there are a few cases on record in which a frontal exostosis, by reason of extensive inward growth, has produced cerebral symptoms--general compression and intellectual deterioration. =Treatment.= In considering the question of treatment, it must be accepted that, although of slow growth, some of these exostoses are definitely progressive, tending to interfere with the character and functions of the region with which they are anatomically situated. There is also reason to believe that those secondary changes--sarcomatous, myxomatous, &c.--which are occasionally observed in the exostoses of long bones are also liable to develop in those cranially situated. The question of treatment hinges, therefore, to a large extent on the nature and position of the tumour.