To understand epidural and subdural hematomas — two serious consequences of head injuries — we need to know the basic anatomy of the brain and its coverings. Imagine an evil carpenter with an electric drill intent on drilling into a person’s brain. What layers would the drill encounter in its passage from the outside of the head to its destination?

The drill would pass through the skin and then the skull (braincase) before penetrating a series of three membranes comprising the meninges. In sequence, the three membranes are the dura mater (Latin for “tough mother”), the arachnoid mater (cobwebby mother) and the pia mater (tender mother) and then finally the brain itself.

Epidural and subdural hematomas are alike in that they are masses of clotted blood (hematomas) caused by head trauma and deposited outside the brain but inside the skull. However, they differ in their locations relative to the dura mater. An epidural hematoma lies outside (on top of) the dura mater, while a subdural hematoma lies inside (beneath) the dura mater and outside the arachnoid mater. Thus, the locations of the two kinds of hematoma are encoded in their names — “epi” is Greek for “upon” and “sub” is Latin for “below.” A third kind of hematoma caused by head injuries is traumatic intracerebral hemorrhage. These occur within the brain tissue itself and are no less serious than those outside the brain, but are not the subject of the current essay.

Epidural and subdural hematomas are produced by ruptures of different blood vessels. Epidural hematomas are usually caused by bleeding from an artery that nourishes the meninges known as the middle meningeal artery, while subdural hematomas are usually due to bleeding from veins that drain blood away from the surface of the brain.