Under these names are included two very different conditions, one of them a true tumour such as occurs in the brain. This is excessively rare, and most cases of so-called sebaceous tumour are due to nothing more than accumulated and inspissated products of inflammation, namely pus, epithelium, and debris. These collections may attain a considerable size and extend into the auditory canal. There may be a deposition of crystals of cholestearine giving a glistening appearance to the mass.

Fig. 490. - Section showing effects of prolonged otorrhoea. o, enlarged antrum mastoideum; b, mastoid cells replaced by dense bone like ivory; c, tympanic cavity. The patient (a man) died of purulent meningitis. (Barr).