Extra-dural hæmorrhage may be arterial or venous in origin. In the former case the blood is derived almost exclusively from the middle meningeal artery. When of venous origin the source is usually to be found in a laceration of one of the sinuses of the brain. When blood is poured out from a torn sinus it tends to occupy, and be widely diffused in, the subdural space. A venous extra-dural extravasation is relatively of infrequent occurrence, and the pressure exercised on the brain seldom suffices to permit of the development of local or general symptoms of cerebral compression. On these grounds extra-dural hæmorrhage may be regarded as almost necessarily of arterial origin, and as derived from a torn middle meningeal artery. Middle meningeal hæmorrhage. _General considerations._ The middle meningeal artery enters the middle fossa of the skull through the foramen spinosum, and divides, after a short intracranial course, into two main terminal divisions, anterior and posterior. The _anterior_ branch passes forwards towards the anterior inferior angle of the parietal bone, then changing direction and turning upwards and backwards towards the vertex of the skull. The _posterior_ branch passes horizontally backwards--grooving the squamous portion of the temporal bone--towards the posterior inferior angle of the parietal bone. Throughout their intracranial course the main trunk and its terminal branches are embedded in the outer wall of the dura mater, except in the spheno-parietal region, where the anterior branch of the artery usually occupies a channel in the bone--one between ¹⁄₂ and 1 inch in extent. The anterior branch overlies the pre-Rolandic motor area, whilst the posterior division is related to the temporo-sphenoidal and lower parietal regions--‘silent’ areas of the brain. Middle meningeal hæmorrhage occurs most commonly in connexion with a fissured or comminuted fracture of the temporal region--the result of direct violence. A fracture, however, is not necessarily present, the hæmorrhage may take place on the side opposite to that at which the injury was received (laceration by contre-coup), and both vessels may be involved. Crisp English narrates three cases in which bilateral extravasation resulted. These more rare examples of middle meningeal hæmorrhage are more or less confined to patients suffering from marked arterial degeneration.