THE VESSELS of the brain are much more resistant to changes in caliber than vessels elsewhere in the body. Forbes and Cobb1 observed that when the cervical sympathetic nerve is stimulated, arteries in the skin constrict ten times as much as those in the pia. Pool, Nason and Forbes2 found that the vessels of the dura are almost eight times as active as the arteries of the pia. Schmidt and Hendrix3 observed that the vessels of the parietal cortex are much less responsive to vasomotor drugs than the vessels of the tongue and of the mylohyoid muscle. Of all the cerebral vasodilators, carbon dioxide is the most consistently effective.4 Vasodilators, such as histamine, acetylcholine and acetyl-beta-methylcholine, which have a pronounced dilating action on the extracranial vessels, are either comparatively ineffective or irregular in their influence on the intracranial vessels.3 Nicotinic acid, another strong extracranial vasodilator,