_When there is osseous deficiency and when the gap is filled in with fibrous tissue_, adherent to the margins of the gap and probably to the dura or brain as well, it is essential that this fibrous tissue should be freely removed. All evidence goes to substantiate the statement that meningeal irritation is the main cause of the fits, &c. The removal of this tissue is best carried out by beginning at the most promising part of the gap, detaching the tissue adherent to the most prominent portion of the bone, and exposing the underlying dura mater. The central mass of scar tissue should then be seized with forceps, lifted up and detached as completely as possible, both from the margins of the gap and from the underlying dura or brain. In the more complicated cases, where the scar tissue is adherent to dura and brain, the most careful dissection is required, and in many instances it is necessary to include that part of the dura mater which is incorporated with the scar. The cerebral substance should be carefully protected (see below). =The treatment of the dura mater.= When the dura has not been injured during the process of exposure, pulsating freely, and presenting a normal appearance, preparations can be made to close in or protect the gap in the skull. When the membrane bulges outwards, and when, in other respects, the indications point to an increase of intradural pressure, the dura must be incised in a crucial manner, and further investigation carried out in the search for a subdural hæmatoma, arachnoid cyst, or other lesion. A hæmatoma is washed out and drained, an arachnoid cyst shelled out or treated by excision of the parietal wall (see p. 204). When the dura is thickened, matted, and adherent to the brain, it is picked up at the least adherent part and carefully dissected away, exposing the pia-arachnoid region. The surface of the brain being so brought into view, the scalpel and dissecting forceps may be required to remove all tags and shreds of matted tissues, this process being continued till a reasonably healthy region has been brought into view. =The treatment of a cortical scar.= There is no reason why a superficial scar should not be freely removed, but, unfortunately, this procedure is necessarily followed by the formation of another scar, at least as extensive as the original fibroid condition. The removal of cortical scars has therefore justifiably fallen into disrepute, and most surgeons content themselves with an exposure of the pia-arachnoid region, the actual cerebral substance being left intact. All bleeding must be arrested, blood extravasation merely favouring the formation of fresh adhesions.