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The purpose of this paper is to describe a new technic for observing and measuring the changing diameters of the blood vessels of the piaarachnoid, together with fluctuations in intracranial and intravascular pressures. Of the many ways devised in the past for studying the factors which operate to control the blood supply of the brain, two general procedures have been found most useful. First, the recording of pressures within the skull and within arteries and veins in divers situations and under varying experimental conditions, and second, the direct observation of blood vessels on the surface of the brain through an opening in the skull. Each method has its advantages and its limitations. Up to the present time, the method of measurement and comparison of pressures has yielded more important data than that of direct observation. The technic of the latter, however, has never been perfected, nor have its possibilities been
HENRY S. FORBES (Tue,) studied this question.