T HE source of bleeding in subdural haematoma commonly is thought to be venous in origin, al though there are well recognised exceptions to such a statement, such as the subdural clots associated with a ruptured aneurysm or those associated with contre-coup injury to the brain. Cases in which the bleeding arises from a single, small rent in a cortical artery are becoming recognised with increasing frequency. One hundred cases of subdural haematoma have been admit ted to the Neurosurgical Service in London, Ontario; in 11 of these the bleeding definitely was arterial in origin, spurt ing from a small rent in a surface cortical artery, without evidence of surrounding brain injury. These cases form the basis of this presentation. This arterial source of bleeding has received little emphasis in the past, and, with a few exceptions, the descriptions have been of postmortem specimens from untrea ted cases. Hey 1 reported the case of a football player who was struck on the head by a ball and died r hours later. At autopsy the fatal subdural haemorrhage apparent ly arose from a torn artery near the vertex of the hemisphere. Werkgartner 8 described a 71-year-old man who fell and exhibited signs of cerebral compression in ~ hours. Dea th occurred 3 days later and histologic examination revealed the source of the fatal subdural haemorrhage to be a ruptured ar tery on the right temporal lobe. Stevenson, ~ in a review of 80 cases of chronic subdural haematoma treated surgically in the Toronto General Hospital, cited ~ cases (1~ days and ~ months following injury) in which the haemorrhage apparent ly resulted from a pin-point opening in an ar tery lying on the surface of the brain. * Read before the American Academy of Neurological Surgery, Boston, Yfassachusetts. October 1960. He postulated t rauma as the cause, the artery having been adherent th rough the arachnoid to the dura mater. On the other hand, Scott 5 felt in his case of chronic subdural haematoma in a 66-year-old clergyman, that the bleeding from a pin-point opening in the left Sylvian ar te ry was spontaneous and not the result of t rauma. Jaeger, 2 in his discussion of Scott 's paper, ment ioned that he had uncovered a spur t ing arterial rupture after wiping away a large clot from a Sylvian artery in a case of vascular hypertension. In a s tudy of 10~ cases of fatal subdural haemorrhage Vance 7 found 6 undoubted instances in which rup ture of a surface cerebral ar tery was the cause of the bleeding. Five were acute subdural clots and the initial loss of consciousness was profound and persistent in only 1 case, t ransient in another and absent in ~ cases. In the 5th case as well as in the case of a subacute clot the patients were admit ted in a presumed alcoholic stupor with no history of injury. Dividing these cases into two groups, he presumed that in ~ cases, arterial branches were present in communica t ing or bridging vessels from the subarachnoid space to the dura mater and were ruptured during the t rauma which probably caused oscillation of the brain. He had examined a few communicaring vascular stalks and had found that while some contain only veins, others contain both arteries and veins. In the remaining 4 cases, the arterial lesion was te rmed a fire hose rupture, as it occurred in arteries on the lateral cerebral surface which possess small arterial twigs coining off the outer wall of the vessel at right angles just under the arachnoid. He felt tha t a point of weakness was present which was ill-fitted to wi ths tand any increase of pressure within the vascular lumen. He considered tha t the brain was