Studies on man and lower animals have shown that the cranial blood vessels are pain-sensitive structures, capable under special circumstances of giving rise to headache.1The distention of cerebral arteries is primarily responsible for the headache produced by certain chemical agents, of which histamine is representative;2the headache associated with septicemia and fever is in the same category.3It has been shown that the afferent fibers from the pain-sensitive cerebral arteries above the tentorium enter the brain stem primarily through the fifth cranial nerve, whereas the fibers from the arteries below the tentorium enter the nervous system chiefly through the upper cervical nerves.4The distention of certain branches of the external carotid arteries is chiefly responsible for migraine headache and the headache associated with arterial hypertension.5 Thus, data about the great variety of headaches resulting from distention of cranial arteries are specific, but there is