_Opening the dura mater._ The membrane can be opened either by crucial incision or by flap formation. The latter method is to be preferred. All meningeal vessels that cross the line proposed for dural section must be underrun on either side of that line. The dural incision should always fall short of the margins of the gap in the skull by at least ¹⁄₂ inch, in order to allow of accurate approximation at the termination of the operation. The following points with respect to the opening of the dura mater, though already enumerated in Chapter II, should be noted. The membrane is lightly incised with the scalpel, and, so soon as the pia-arachnoid is exposed, the section completed with the blunt-pointed scissors, or on a grooved director. The dural flap is then turned down and the brain laid bare. It is most essential that every precaution should be taken to avoid injury to superficial cerebral veins--the cortex is probably under high tension, firmly compressed against the dura mater and bulging out forcibly so soon as tension is relieved. _Examination of the brain._ In the event of the exposure of the tumour, its removal can at once be attempted. If, however, the tumour be subcortical in position, its position and boundaries may be estimated by electrical stimulation, palpation and exploration. _Electrical stimulation_ will evidence whether the area exposed corresponds to the symptoms evinced by the patient. With respect to the actual technique, one pole is placed on the patient’s extremity--it matters not which, though preferably on the homo-lateral side--the other over the exposed brain. The current should be just strong enough to contract exposed muscle--some of the fibres of the temporal muscle are generally available for the purpose. If there is much pia-arachnoid œdema, some of the fluid should be evacuated--by gentle scratching of the membranes--and the bare brain stimulated. In the event of complete degeneration of the pyramidal tracts there is little or no response to such stimulation. Under other circumstances the results are quite definite. _Palpation_ may reveal the nature of the tumour, whether fluid or solid. _Exploration of the brain_ should only be undertaken in the light of a reasonably certain diagnosis, and every precaution must be taken to avoid needless damage to the cerebral substance. The exploration should invariably be preceded by incision with the brain-knife or scalpel, introduced in such a manner as to avoid injury to all visible vessels and directed at right angles to the surface of the brain, so as to cause the least possible damage to the corona radiata, &c. _Extirpation of the tumour._ The proportion of brain tumours surgically removable is small, and even when the tumour is fully exposed considerable experience is required in estimating the possibility of removal.