The study of the structure of the central nervous system has beenapproached in various ways. The method that forms the basis of this presentation was described in a previous paper,1 in which the segmental and suprascgmental systems of the hindbrain and midbrain were fullydiscussed and illustrated. The method takes into account the sequence of development which the data and methods of embryology and comparative anatomy reveal. By combining this perspective with a survey of the facts of neurophysiology and of experimental and pathologic neuroanatomy, a dynamic approach to the study of the central nervous system is provided.2 It is our purpose to present the mechanisms of the forebrain in terms of this analysis, with the various systems translated into three dimensions by means of an accurate plasteline reconstruction, made from a complete series of Weigert-stained cross-sections of a human adult brain. The apparatus of the forebrain, in