The indirect forms of injury are less readily explained. Their frequency cannot be disputed. I drew attention to the subject in my Hunterian Lectures in 1904, there stating that indirect injury to the brain was more common than the direct form. My opinion is confirmed by Phelps,[32] who states that injury by indirect violence is of almost constant occurrence. Three theories have been advanced in explanation:-- 1. ‘Their production has been ascribed to the change in form that is suffered by a skull in virtue of its elasticity when subjected to violence which causes distortion of the brain to the point of bursting.’ 2. ‘In the displacement of the cerebro-spinal fluid by the consecutive depression and bulging that is believed to follow a blow on the head; a momentary vacuum is formed at either axis of force, and the vessels of the brain and meninges rupture from lack of support.’ Neither of these theories are tenable, insomuch as they are based on an acceptance of the ‘bursting and compression’ theories (see p. 74)--theories which, in my opinion, it is impossible to accept. 3. The ‘contre-coup’ theory. According to this theory the injury is due to sudden and violent displacement of the brain against the opposing osseous barrier. This theory has been opposed by many--notably by Helferich--on the ground that the brain is said to completely fill the cranial cavity, and that ‘shaking’ from side to side is impossible. This theory affords, however, the most satisfactory explanation of contralateral laceration. The following points may be advanced in its favour:-- (_a_) The frontal and temporo-sphenoidal lobes are more liable to contusion and laceration than any other parts of the brain, both regions possessing certain anatomical relations accounting for their ready injury. The frontal pole is related to the angle of junction between the horizontal and vertical plates of the frontal bone, the temporo-sphenoidal lobe to the cul-de-sac, at the anterior part of the middle fossa, that is overhung by the wings of the sphenoid bone. (_b_) The lacerated area is almost invariably situated at that part of the brain which lies opposite to the region struck in the line of the transmitted force. Furthermore, brain lesions are most prevalent when the accident results from a fall from a height, a class of accident in which ‘shaking’ of the brain is most likely to occur, and least common when the basal fracture is dependent on a bilateral compression force. (_c_) The statements that ‘shaking’ cannot take place, and that the soft cerebral substance cannot be driven forcibly and violently against the opposing bony barrier, so as to be locally bruised and torn, are incorrect. That such violent displacement of the brain can and does occur is proved by the following case:--