When a high-velocity bullet penetrates the head, it exerts on the incompressible, semi-fluid brain an explosive (hydro-dynamic) force, which is transmitted to all points on the inner surface of the skull and leads to shattering of the bone. _Repair._--The repair of fractures of the skull is usually attended with an exceedingly small amount of callus. Except in the presence of infection, separated fragments live and become reunited, but they may unite in such a manner as to project towards the brain and, by irritating the cortical centres, cause traumatic epilepsy. In comminuted fractures, the lines of fracture remain permanently visible on the bone, but fissured fractures may leave no trace. Gaps left in the skull by injury or operation are, after a time, filled in by a fibrous membrane, which may undergo ossification from the periphery towards the centre, but unless the aperture is a small one it is seldom completely closed by bone. The new bone which forms is derived from the old bone at the margins of the opening. Permanent defects in the skull are chiefly injurious if they are accompanied by lesions of the underlying dura, such as adhesions to the brain; large gaps may cause giddiness on stooping, or on forcible expiration, as in blowing the nose or playing a wind instrument. #Varieties.#--For descriptive purposes, fractures of the vault are divided into the fissured, the punctured, the depressed, and the comminuted varieties. Clinically, however, these varieties are often combined. The practical importance of a given fracture depends upon whether it is simple or compound, rather than upon the exact nature of the damage done to the bone. Compound fractures which open the dura mater are the most serious. Simple fractures result, as a rule, from diffuse forms of violence, and are liable to spread far beyond the seat of impact. Compound fractures result from severe and localised violence--for example, the kick of a horse or the blow of a hammer--and tend to be limited more or less to the seat of impact. In gun-shot injuries, however, there are usually numerous fissures radiating from the point at which the missile enters the skull. #Fissured fractures# generally result from blows by blunt objects or from falls, and they usually extend far beyond the area struck, in most cases passing into the base. The fissure may pass through the bone vertically or obliquely, and it may implicate one or both tables. So long as the fracture is simple, it can scarcely be diagnosed except by inference from the associated symptoms of meningeal or cerebral injury. When compound, the crack in the bone can be seen and felt. It is recognised by the eye as a split in the bone, filled with red blood, which, as often as it is sponged away, oozes again into the gap. In fractures by bursting a tuft of hair may be caught between the edges of the fracture, and this adds to the difficulty of purifying the wound.