#Mechanism.#--When the skull is broken by _direct_ violence, the fracture takes place at the seat of impact, and its extent varies with the nature of the impinging object and the degree of violence exerted. If, for example, a pointed instrument, such as a bayonet, a foil, or a spike, is forcibly driven against the skull, the weapon simply crashes through the bone, disintegrating it at the point of entrance, and cracking or splintering it for a variable, but limited, distance beyond. On the other hand, when the head is struck by a "blunt" object--for example, a batten falling from a height--the force is applied over a wider area and the elastic skull bends before it. If the limits of its elasticity are not exceeded, the bone recoils into its normal position when the force ceases to act; but if the bone is bent beyond the point from which it can recoil, a fracture takes place--"_fracture by bending_." The bone gives way over a wide area, the affected portion may be comminuted, and one or more of the fragments may remain depressed below the level of the rest of the skull. Cracks and fissures spread widely in different directions--often (70 to 75 per cent.) extending into the base. In almost all fractures of the vault the inner table splinters over a wider area than the outer, partly because it is more brittle and is not supported from within, but also because the diffusion of the force as it passes inwards affects a wider area. If a bullet traverses the cranial cavity the inner table is more widely shattered at the aperture of entrance, and the outer table at the aperture of exit. Von Bergmann reported thirty cases in which the inner table alone was fractured by a blow on the head. Fractures by _indirect_ violence--that is, fractures in which the bone breaks at a point other than the seat of impact--are almost always due to violence inflicted with a blunt object, and acting over a wide area--such, for example, as when the head strikes the pavement. Much discussion has taken place as to the method of their production. It has been shown that when the skull is depressed at one point by a force impinging on it, it bulges at another, so that its whole contour is altered. But the elasticity of the bone varies at different parts of the skull, owing to differences in thickness and in structure. If, therefore, the part which is depressed--that is, the part directly struck--happens to be less elastic than the part which bulges, it gives way, and a fracture by "bending" results; but if the bulging part is the less elastic, it bursts outwards--_fracture by_ "_bursting_." The term "fracture by _contre-coup_" has been incorrectly applied to such fractures when the area of bulging happens to be opposite to the seat of impact. _Contre-coup_, properly so-called, is only possible in a perfectly spherical body, which, of course, the skull is not.