The trephine is applied in such a manner that its circle falls well below the lower level of the lateral sinus and behind the mastoid process. After removal of the disk, the dura mater is cautiously opened by crucial incision and the evacuating instrument introduced, the cerebellum being explored in the forward and slightly inward direction--towards the posterior aspect of the petrous bone. The abscess should be reached within 1¹⁄₂ inches from the opening in the bone. A drainage tube is introduced, stitched in position, in the manner described when dealing with temporo-sphenoidal abscess. This tube should be shortened daily and dispensed with after seven to ten days. _Trephining for frontal abscess._ An abscess of the frontal lobe is generally situated so far back, and connected with the source of the trouble (the frontal sinus) by so long a ‘stalk’ of infection, that considerable difficulty may be experienced in attempting drainage from the frontal or nasal regions. Moreover, operations conducted through the frontal region are apt to result in considerable deformity. The operation may therefore be conducted with advantage from the temporo-frontal region. In this situation, the bone is thin, the operation can be conducted between the split fibres of the temporal muscle, excellent drainage is supplied, and the scar is inconspicuous. For a general description of the intermusculo-temporal operation, the reader is referred to p. 121. In this instance, the field of operation is shifted further forwards, otherwise the details are very similar. The skin incision commences above and in front of the external angular frontal process, curves along the temporal crest, and terminates well in front of the ear. The temporal fascia is turned down and the muscle split in the general direction of its fibres, and well retracted on either side. The trephine is applied, the dura mater incised, and the abscess evacuated and drained in the manner previously described. The tube may be removed after seven to ten days.