The osteoplastic flap is by no means easy to frame, the external table tending to split up during the process of separation; moreover, its formation is confined to the upper-occipital and parietal regions of the skull, regions where the two tables are separated by diploic tissue. In the temporal and cerebellar regions this method cannot be adopted, and in the frontal region the deformity and scarring act as a bar to such operative procedures. In suitable cases, however, good results may be obtained by this method. Asepsis is essential to success. Bone-flaps derived from some other bone of the patient (e. g. the tibia), or from the bones of a freshly-killed animal (e. g. the scapula of a dog), seldom yield satisfactory results. The gap in the skull is first exposed, the margins refreshed, and the size and shape estimated. The bone-graft is then cut from the other bone, suited in every respect to the deficiency, inserted in the gap and sewn into position. The graft, however, seldom retains its vitality, being invaded by granulation tissue, and converted, in the course of time, into dense fibrous tissue. Furthermore, in the event of the slightest failure of aseptic technique, it acts as foreign body and must be removed. The attendant difficulties and the frequent failures of these auto- and hetero-transplantations of normal bone lead to the utilization of decalcified, calcined, and boiled bone-plates. Between these there is very little to choose, for, whether due to the absence of calcium salts or the destruction of bone-cells, they merely act as scaffolding media for the formation of fibrous tissue and are more or less completely absorbed. _(b) Plates of some foreign material._ The following method has been found to give the most satisfactory results. Plates of pure annealed silver are utilized, ⁹⁄₁₀₀₀ inch in thickness (No. 2 Birmingham metal gauge). The plates are light and of fair malleability. Supposing now that it is required to protect a deficiency in the vault--irregular in shape, round or oval, it matters not--the maximum antero-posterior and vertical diameters are measured and a piece of silver cut out, which is about ¹⁄₂ inch greater in both diameters. The convexity of the skull in the region of the gap is estimated and the plate hammered into corresponding shape. This is readily carried out by placing the plate on a heavy leaden base and hammering to the required convexity. The convexity is regulated by the site of application of the blows, the heavier being applied to the central or apical portion of the plate, the weaker to the periphery. A rough general convexity is so produced. The edges of the plate are smoothed with the file, and a few holes bored to allow of the escape of any blood or cerebro-spinal fluid that might tend to collect beneath.