Abstract
1. In anaesthetized cats under artificial ventilation, a few milligrams of pentobarbitone sodium injected into the cerebral ventricles produced a pronounced fall in arterial blood pressure, which was central in origin and resulted from inhibition of vasomotor tone.2. Pentobarbitone sodium was more effective in lowering blood pressure when injected into the cerebral ventricles than when injected into the cisterna magna, yet the pentobarbitone sodium did not act on structures in the ventricular wall, but acted on structures reached from the subarachnoid space.3. To produce its vasodepressor effect, the pentobarbitone sodium had to pass through the foramina of Luschka into the subarachmoid space beneath the medulla oblongata and to penetrate its ventral surface in a region caudal to the trapezoid bodies and lateral to the pyramids. This was the outcome of experiments in which the pentobarbitone sodium was injected into or perfused through the cerebral ventricles with or without an outflow cannula inserted into the aqueduct or into the fourth ventricle, and of experiments in which pentobarbitone sodium solutions were applied by means of Perspex rings to this region of the exposed ventral surface of the medulla. Whereas the application of pentobarbitone sodium to this region on one side had a weak vasodepressor effect only, its application on both sides produced a pronounced fall in arterial blood pressure.4. The region where pentobarbitone acted on topical application covers the region where nerve cells are found in the marginal glia immediately under the pia mater. The possibility is discussed that these cells are the morphological substrate on which the pentobarbitone acts, that arterial blood pressure is maintained by their activity which is suppressed by the pentobarbitone sodium.