Frontal Section Through the Caudal Thalamus
At this level, the section again shows the medial nuclear group (red) (A – C1) and the lateroventral nuclear group (blue) (A – C2). Itincludes the most caudal parts of the medial nucleus. Dorsally separated from the super-ficial dorsal nucleus (A–C3) by a narrow my-elin layer, they are otherwise surrounded by the internal medullary lamina and the in-tralaminar nuclei. The nonspecific thalamic parts here reach special expansion through the centromedian nucleus (A – C4).
The most rostral nuclear portions of the pulvinar (A – C5) lie dorsally between themedial and the lateral nuclear groups. This rostral part of the pulvinar projects to the upper convolutions of the temporal lobe and is thought to receive fibers from the lateral lemniscus; it is therefore assumed tobe an integration nucleus of the acoustic system.
The ventral posterior nucleus (A – C6) is seen in the lateroventral area. The medial lemnis-cus, the spinothalamic pathways, and thesecondary trigeminal fibers terminate here. The outer portion, which receives the fibers for the limbs and the trunk, is rich in myeli-nated fibers and has fewer cells than the inner portion, which receives the fibers for the head region. The inner portion is rich in cells and has thinly myelinated fibers. It sur-rounds the centromedian nucleus ventrally and laterally; it appears as a crescent-shaped figure in the myelin-stained section and, hence, is called the semilunar nucleus (B7).
The lateral geniculate body (A – C8) lies slightly apart from the complex of the thalamus at the ventral surface of the dien-cephalon. It is indented at the base and pro-trudes laterally (lateral geniculum). It is characterized by prominent stratification into six layers of cells and five intercalating layers of fibers. The latter are formed by the fibers of the optic tract, which disperse ac-cording to a set pattern and terminate at neurons of different cell layers. The upper four of these layers are parvo-cellular, the lower two layers are magno-cellular. In the second, third, and fifth layers terminate the fibers from the retina of the ipsilateral eye (uncrossed optic nerve fibers), while those from the contralateral eye (crossed optic nerve fibers) terminate in the first, fourth, and sixth layers. Fibers from the site of visual acuity, the macula, termi-nate in the central area (A9). When the mac-ula is destroyed, the geniculate cells of this area undergo transneuronal degeneration. The lateral geniculate body is surrounded by a dense capsule of myelinated fibers. These are the dorsally and laterally emerging fibers of the optic radiation (geniculocal-carine tract).
Medially to the lateral geniculate body, the section shows the caudal portion of the me-dial geniculate body (A–C10). The reticular nucleus (AC11) forms the lateral capsule. Itwidens ventrally and also encloses the lateral geniculate body.