Direct observation of the pial vessels has been carried out for over 100 years.* A window was used by Ravina20 in 1811. He inserted a short cylinder of wood into a trephine hole in a dog's skull. The top of the cylinder was closed by a watch glass cemented in situ. Donders,6 in 1859, devised an airtight window and made the observation that when the nose and mouth of an animal were obstructed for as short a time as 10 seconds, dilatation of many small vessels became evident. Many other experimentalists used this method of study of the pial circulation. Thus, skull windows as a means of investigation of brain circulation were carried out by Berlin,2 in 1851; Kussmaul and Tenner,15 in 1857; Leiden,17 in 1866; Riegel and Jolly,21 in 1871; Elder,8 in 1897; Lewin,18 in 1920, and Lee,15 in 1925.
GURDJIAN ES, WEBSTER JE, MARTIN FA, THOMAS LM. Cinephotomicrography of the Pial Circulation: A Study of Factors Influencing Vascular Caliber; Preliminary Report. AMA Arch NeurPsych. 1958;80(4):418–435. doi:https://doi.org/10.1001/archneurpsyc.1958.02340100018004
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