That part of the diencephalon that lies wedged between the thalamus on the dorsal side and the cerebral peduncle ventrally, lateral to the dorsal half of the hypothalamus from which it cannot be sharply delineated. It is composed of the subthalamic nucleus (corpus luysi), the zona incerta, and the fields of Forel; laterally it expands in a winglike fashion into the reticular nucleus of the thalamus; caudally it is continuous with the midbrain tegmentum. SYN: ventral thalamus.
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sub·thal·a·mus (səb-thalґə-məs) [TA] a large ovoid mass in the anterior part of the diencephalon interposed between the dorsal thalamus, hypothalamus, and tegmentum of the mesencephalon; it contains the nuclei of the lateral and medial geniculate bodies, the subthalamic and reticular nuclei, the zona incerta, and the Forel fields. Some authorities consider this to be part of the thalamus and refer to it as the ventral thalamus.Medical dictionary. 2011.