In all cases of head injury, the questions that dominate the whole clinical outlook are, whether the brain is directly damaged or not, and whether it is likely to become the seat of infection. It is impossible to consider separately in their clinical aspects injuries of the cranium and injuries of the brain. It seldom happens that one is seriously damaged without the other suffering to a greater or less extent. Sometimes the skull suffers comparatively little, while the brain is severely damaged, but it is rare for a serious injury to the bone to be unaccompanied by definite brain lesions. In any case it is the damage to the brain, however slight, that gives to the injury its clinical importance. It is an old and a true saying that "no injury of the head is so trivial as to be despised or so serious as to be despaired of." Injuries at first sight apparently slight may prove fatal from hæmorrhage or infection; on the other hand, recovery has followed injuries of great severity--for example, the famous "American crowbar case," in which a bar of iron three and a half feet long and one and a half inches thick passed through the head, and yet the patient recovered. It is convenient to consider the injuries of the brain before those of the skull. TRAUMATIC LESIONS OF THE BRAIN It is probable that in all cases of injury to the head in which a patient loses consciousness, there is some definite damage to the cerebral tissue. This takes the form of a greater or less degree of contusion or laceration, and the lesions are usually most severe and dangerous when the skull is fractured and fragments are driven in upon the brain, but they may exist--indeed they may be very extensive--in the absence of fracture. Several degrees are recognised. (1) Numerous minute _petechial hæmorrhages_ may be found widely scattered throughout the brain substance, as a result of a diffused blow on the head, which has shaken up the brain and caused symptoms of cerebral shock or "concussion." We have found, on microscopic examination in such cases, in addition to these small extravasations, collections of colloid bodies, patches of miliary sclerosis, and chromatolysis and vacuolation of nerve-cells.[3] [3] Miles, _Laboratory Reports, Royal College of Physicians, Edinburgh_, vol. iv.