In previous communications,1 Weiss, Lennox, Robb and Ellis have presented evidence which indicates that the minute cerebral blood vessels of man respond with dilatation to histamine. A quantitative comparison reveals a marked sensitivity of the cerebral blood vessels to this substance. Histamine base administered intravenously in doses of from 0.001 to 0.003 mg. produces a dilatation of the arterioles. So small an amount of histamine may be present in the circulating blood under physiologic or pathologic conditions.
The evidence presented previously regarding the dilatation of the cerebral vessels in man was based on: (1) Simultaneous studies of the arterial and venous blood pressures, the cerebrospinal fluid pressure and the degree of the pulsatile expansion of this fluid. These observations revealed that histamine causes a marked increase in the pulsatile expansion and the pressure of the cerebrospinal fluid, without change in the arterial or venous blood pressures. Such a change