=Radical operation for cerebellar tumours.= Craniectomy may be regarded as the operation of choice in the exposure and removal of cerebellar tumours. The formation of an osteoplastic flap is contra-indicated (see p. 29). The operative procedures vary according as to whether it is desired to expose the one or both cerebellar hemispheres. Bilateral exposure, though presenting the great advantage of offering a larger field for exploration, is by far the more serious of the two operations. _Unilateral cerebellar exposure._ The patient being placed in the semi-prone or face-down position, the incision starts at the posterior border of the apex of the mastoid process, curves inwards along the superior curved line of the occipital bone to the occipital protuberance, and then passes straight down the middle line of the neck for 2 to 3 inches. If the incision is deepened at once to the bone, hæmorrhage is severe. The incision should first involve the skin and then the muscles attached to the occipital bone. Each vessel as encountered is clipped and tied. The flap must be turned down right up to the posterior margin of the foramen magnum. The flap being held aside, the pin of the trephine is placed in such a manner that the disk to be removed will correspond to the thin central portion of the cerebellar fossa. The trephine, placed low down, is directed more or less towards the anterior fontanelle. The disk being removed, the bone is freely cut away with rongeur forceps--outwards to the posterior border of the mastoid process, upwards to the curved line, inwards to near the middle line, and downwards to the posterior margin of the foramen magnum. This generally completes the first stage of the operation, for, in cerebellar tumours, it is usually advisable to complete the operation in two stages. The scalp-flap is replaced, lightly sewn into position, and the patient sent back to bed. A few days later the flap is again turned down, the dura incised, and turned down as a flap the convexity of which corresponds to but falls short of the line of the lateral sinus. The cerebellar substance is then examined and the tumour removed after the lines enumerated in dealing with cerebral tumours. Greater difficulty, however, is experienced in the attempted removal of cerebellar tumours, for the operator is working in a very confined space, and because the cerebellum tends to herniate through the adventitious hole in dura and bone. Two other factors must be taken into consideration: (1) the danger incident to all cerebellar operations of respiratory failure,[48] and (2) the friability of the cerebellar substance. Every care must be taken to avoid unnecessary damage of the brain-matter.