The ventral thalamus consists of the hypothalamus and the subthalamus. The nuclear masses of the ventral thalamus form an ancient and unique ensemble of structures which, through their close union with the basal parts of the forebrain on the one hand and with the tegmental region of the midbrain on the other, may be regarded as closely interrelated anatomicophysiologic systems. The mechanism of the hypothalamus is believed to deal with the integration of the internal organic environment with the various experiences of the organism through the medium of the forebrain and the supra-optic decussations; that of the subthalamus, with the generalized primitive motor reactions through the tegmentum and the reticular formation. From the region of the forebrain the olfactory impulses are brought into the hypothalamus by way of the functionally old preoptic region, and from the hypothalamus they are brought into functional relation with the dorsal thalamus and tegmental region