Surface injuries are central to forensic medicine. The pattern of injury provides information on the mechanism of causation of both external and internal injuries. Photographs of the injuries should be made in colour. Injuries should be photographed with and without a measuring scale and at right angles, not obliquely. Injuries may alter in appearance in the post-mortem period. Abrasions may dry out and appear more prominent, although they will not significantly change in size, unlike bruises, which may increase in size and prominence. A pure abrasion is an injury involving only the epidermis. Abrasions heal without scarring. An abrasion may result from a tangential movement of skin in comparison with an abrading material, or by direct imprint. Bruises are caused by trauma to the skin that ruptures blood vessels and blood escapes into the tissues. Particularly in deeper tissues, bruises are often called contusions.