These may be either _external_, affecting the integuments; or _internal_, affecting the brain substance, &c. In the latter, as a rule, there are signs of external violence. An ecchymosed tumour of the scalp may impart a _sensation of crepitation_ to the finger, and may thus be mistaken for a fracture of the skull. The tumour may also pulsate if any large vessel be near it, giving one the idea that the pulsations are due to the movements of the brain. A large wound without fracture points to a more or less oblique blow, a small wound to direct violence. A blow with a heavy blunt weapon may make a clean incised wound, and often in these cases the seat of the bruise does not correspond with the centre of the cut. Dr. Ogston mentions the case of a young lady on whom a cricket ball inflicted a wound across the forehead, immediately above, and of the length of, one of the eyebrows, which he could not distinguish from a wound by a cutting instrument. All injuries to the head are more or less severe and dangerous, and great care is required in forming a prognosis with regard to the ultimate effect of an injury to the head. Inflammation of the brain does not, as a rule, supervene for about a week after the accident, and patients should not be considered safe from danger till two or three weeks after. Be it remembered also that in some cases the inflammatory action may proceed insidiously for some months without giving any distinct evidence of its presence till close upon a fatal termination. Scalp wounds are dangerous, from erysipelas, &c. They should be examined as to their extent, form, depth, and position. _Concussion_ of the brain may arise from falls on the nates, or from blows on the head. The face becomes pale, the pupils contracted, the pulse weak and small, the extremities cold, the respiration scarcely perceptible, and the sphincters relaxed. The tendency to death is from syncope. Reaction may then occur: the pulse quickens; the skin is hot and dry; there is great confusion of thought, from which the patient ultimately recovers; vomiting is present in most cases. Concussion often passes into compression, due to hæmorrhage from the lacerated cerebral vessels. Concussion and compression differ in this: in the former, the effects are instantaneous; in the latter, a short time elapses before the symptoms make their appearance; and these become more and more marked, whereas in concussion they gradually pass off. It is often a difficult matter to distinguish the effects of compression from those common to drunkenness or narcotic poisoning. The odour of the breath and the history of the case will assist in forming an opinion. Concussion of the brain may prove fatal without either fracture of the skull, effusion of blood within the cranium, or any other change being observed on dissection, death being caused by the shock given to the whole nervous organ, which, being unrelieved, speedily lapses into annihilation of function.