The thalamus consists chiefly of gray substance, but its upper surface is covered by a layer of white substance, named the stratum zonale, and its lateral surface by a similar layer termed the lateral medullary lamina. Its gray substance is incompletely subdivided into three parts—anterior, medial, and lateral—by a white layer, the medial medullary lamina. The anterior part comprises the anterior tubercle, the medial part lies next the lateral wall of the third ventricle while the lateral and largest part is interposed between the medullary laminæ and includes the pulvinar. The lateral part is traversed by numerous fibers which radiate from the thalamus into the internal capsule, and pass through the latter to the cerebral cortex. These three parts are built up of numerous nuclei, the connections of many of which are imperfectly known.
The thalamus may be regarded as a large ganglionic mass in which the ascending tracts of the tegmentum and a considerable proportion of the fibers of the optic tract end, and from the cells of which numerous fibers take origin, and radiate to almost every part of the cerebral cortex. The lemniscus, together with the other longitudinal strands of the tegmentum, enters its ventral part: the thalamomammillary fasciculus, from the corpus mammillare, enters in its anterior tubercle, while many of the fibers of the optic tract terminate in its posterior end. The thalamus also receives numerous fibers from the cells of the cerebral cortex. The fibers that arise from the cells of the thalamus form four principal groups or stalks: those of the anterior stalk pass through the frontal part of the internal capsule to the frontal lobe; the fibers of the posterior stalk arise in the pulvinar and are conveyed through the occipital part of the internal capsule to the occipital lobe; the fibers of the inferior stalk leave the under and medial surfaces of the thalamus, and pass beneath the lentiform nucleus to the temporal lobe and insula; those of the parietal stalk pass from the lateral nucleus of the thalamus to the parietal lobe. Fibers also extend from the thalamus into the corpus striatum—those destined for the caudate nucleus leave the lateral surface, and those for the lentiform nucleus, the inferior surface of the thalamus.