Although knowledge of the cerebral circulation has been increased of late years by direct microscopic study of the blood vessels of the pia mater, no similar observations on the vessels of the dura mater have been reported. Recently, Pickering1 published important observations on the mechanism of headache produced by histamine. He presented strong evidence that the headache arises from an intracranial structure which is innervated by the trigeminal nerve, and that this structure is probably the dura. Reasoning from experimental data he suggested that the sensation of pain, in this type of headache at least, may be due to the stretching of a sensitive structure lying close to the meningeal arteries. Such stretching, he added, might conceivably arise from swelling of the perivascular tissues or from widening of the arteries. These observations are the outcome of carefully controlled experiments on human subjects. They open up a promising field of