[Illustration: FIG. 186.--Relations of the Middle Meningeal Artery and Lateral Sinus to the surface as indicated by Chiene's Lines. (After Cunningham.)] It is probable that the size of the hæmorrhage depends on the nature, extent, and severity of the injury to the head. The recoil of the skull after the blow separates the dura from the bone, and if the meningeal artery is lacerated or punctured, blood is effused into the space thus formed (Fig. 187). A localised blow therefore results in a small area of separation and a correspondingly small clot; while a diffuse blow is followed by more extensive lesions. It is believed that, once the dura is partly separated, the force of the blood poured out from the lacerated artery is--on the principle of the hydraulic press--sufficient to continue the separation. [Illustration: FIG. 187.--Extra-Dural Clot resulting from hæmorrhage from the Middle Meningeal Artery.] _Clinical Features._--The typical characteristics of middle meningeal hæmorrhage are met with only when the bleeding takes place between the dura and the bone. Under these conditions the symptoms of concussion are usually most prominent at first, and those of compression only ensue after a varying interval, during which the patient as a rule regains consciousness. In some cases, indeed, he is able to continue his work, or to walk home or to hospital, before any evidence of intra-cranial mischief manifests itself. This "lucid interval" helps to distinguish the symptoms due to middle meningeal hæmorrhage from those of laceration of the brain substance, as in the latter the symptoms of concussion merge directly into those of compression. Lumbar puncture may aid in the differential diagnosis between extra-and intra-dural hæmorrhage, as blood is present in the fluid withdrawn in the latter, but not in the former. A few hours after the accident the patient experiences severe pain in the head, and he usually vomits repeatedly. For a time he is restless and noisy, but gradually becomes drowsy, and the stupor increases more or less rapidly until coma supervenes. The pulse usually becomes slow and full. The respiration is rapid (30 to 50), and becomes greatly embarrassed and stertorous. The temperature progressively rises, and before death may reach 106° F., or even higher. Monoplegia, usually beginning in the face or arm on the side opposite to the lesion, gradually comes on, and is followed by hemiplegia, from pressure on the motor areas, underlying the clot. The condition of the pupils is so variable as to have no diagnostic value; but if both are widely dilated and irresponsive to light, the prognosis is grave. Death usually ensues in from twenty-four to forty-eight hours, unless the pressure within the skull is relieved by operation; even after removal of the clot death may ensue if the brain has been lacerated, or if there is hæmorrhage at the base.