The Relationship Between the Blood Pressure and the Tonic Regulation of the Pial Arteries

IN experiments on cats it was demonstrated that a decrease of blood pressure was followed by a dilatation of the pial arteries (Fog, 1934, 1937 ; Forbes, Nason, and Wortman, 1937) and that, conversely, a rise of arterial pressure caused a constriction of these vessels (Fog, 1938a). These vasomotor reactions were found to be independent of the procedures by which the fluctuations of pressure were produced. Stimulation of the sinus nerve or of the peripheral end of the cut vagus nerve or a diminution of blood volume had the same effect on the diameter of the pial arteries, provided that the same decrease in blood pressure occurred. Also the reactions during arterial hypertension were identical whether the rise in pressure was effected by the administration of adrenalin intravenously, stimulation of the splanchnic nerve, or increasing the blood volume. In this connection it is important to note that adrenalin was shown to have no direct chemical action upon the pial arteries of less than 5O,u in diameter. On the other hand, vessels of larger than 100 in diameter reacted to the hormone by distinct constriction, which, however, was not as great as that usually found in other vascular areas, e.g. the skin or the splanchnic area