The gross anatomy of the middle cerebral artery has been described by many authors. Early workers include Duret,1 Heubner,2 Beevor,3 and Ayre.4 More recent contributions have been made by Kaplan,5,6 Shellshear,7 and Alexander.8 Lazorthes9 has published a monograph on the subject of cerebral vasculature. This paper presents a continuing study of the areas of irrigation of individual vessels of the circle of Willis. Method The method used is a modification of that described by Ostrowski, Webster, and Gurdjian10 in their study of the proximal anterior cerebral artery. Twenty human brains were utilized in this study. The brain is obtained at autopsy and perfused with isotonic saline via polyethylene catheters placed in both the internal carotid arteries as well as the vertebral-basilar system. The perfusion is continued until the returning fluid is cleared of all traces of blood. The brain is then