When of the ivory type and growing from the flat bones of the skull, but so situated that no marked deformity or pressure symptoms are likely to ensue, they may be left alone, but when definitely progressive and situated in accessible regions, they should be removed. Their exposure is carried out by the formation of a suitable scalp-flap--designed as far as possible so as to be subsequently hidden by the hairy scalp--and the exostosis removed by the application to its base of a Gigli saw. This method is greatly superior to the older procedures whereby the tumour was chiselled away with hammer and gouge. Occasionally the tumour is so dense and presents so wide a basal attachment that it becomes necessary to attack from a more distant line, cutting out a trench, deepened to the diploic tissue, circumferentially around the tumour and levering away the central mass. When the tumour extends more deeply, involving nearly the whole thickness of the skull, it may be removed by the application of a small trephine immediately to one side of the tumour, followed by the use of de Vilbiss forceps circumferentially around the main mass, thus freeing it from its surroundings. The resultant gap in the skull may be protected by one or other of those measures enumerated in Chapter VI. =Frontal and mastoid= exostoses often necessitate formidable operations insomuch as their size and anatomical relations present considerable difficulties (see Fig. 94). The =indications for operation in the case of aural exostoses= are as follows[78]:-- 1. If there is middle-ear suppuration and signs of retention of pus. 2. When the pressure of the exostosis produces pain which cannot otherwise be relieved. 3. If the exostosis nearly blocks up the meatus of both ears, and there is prospect of each side becoming completely blocked in the near future. Here operation is carried out on the worst side. 4. If the meatus is nearly blocked by the exostosis and the patient going to a country where he cannot be within easy reach of a competent medical man. The =indications for operation in the case of a frontal exostosis= are as follows:-- 1. When the exostosis interferes with the actions of the ocular muscles, causes proptosis and threatens the integrity of the globe. 2. When associated with pain which cannot otherwise be relieved. 3. When the exostosis leads to blockage of the accessory sinuses of the nose, more especially when such blockage is associated with pus pent up within. 4. When very unsightly. The accompanying figure illustrates the deformity and dangers associated with large frontal exostoses. The tumour developed from the inner angle of the orbital cavity, pushing the globe forwards and outwards, with diplopia and severe neuralgia.