Section 7 – With this section, we have moved anterior from the brainstem to the thalamus.
Two prominent fiber bundles are the medial lemniscus, carrying sensory fibers to the somatosensory thalamus and the optic tract (laterally), carrying visual sensory fibers from the retina to the visual thalamus, slightly posterior to this level. In the midline, the mammillothalamic tract, slightly dorsal to the mammillary bodies, is cut in cross section, as is the fasciculus retroflexus. This slightly more dorsal tract carries fibers from the habenula nucleus (“epithalamus”) to the interpeduncular nucleus (see Section 6). The cerebral hemispheres overlie the thalamus, and are interconnected by fibers running between them in the corpus callosum. Between the cerebral hemispheres and the thalamus is the C-shaped hippocampal formation. At this level, the cerebral aqueduct runs into the third ventricle, in the midline, and segments of the C-shaped lateral ventricle are also apparent.
This section has many of the features of Section 7. The mammilothalamic tract forms a compact circle of fibers cut in cross section. Laterally, lining the ventral third of the brainstem, is the optic tract. Lateral still and slightly dorsal to this is the internal capsule, largely consisting of descending cortical output fibers. The paired habenula nuclei are evident in the dorsal midline, as elongated ovals. At the ventral base, they are lined by fibers of the fasciculus retroflexus. Underlying the cerebral hemispheres is the caudate nucleus and ventral to this, parts of the amygdala. The amygdala forms an approximate triangle, bounded by the external capsule laterally and the internal capsule medially. The hippocampus (dentate gyrus, CA3, CA2, and CA1) is restricted to its dorsal limb. The cerebral hemispheres ventrally are outlined by the lateral olfactory tract.
This section is at the anterior pole of the thalamus and anterior to the hippocampal formation. In the dorsal midline, the main body of the fornix lies under the corpus callosum. The fornix carries hippocampal fibers, mainly to the septal nuclei and mammillary bodies. It is one of the C-shaped structures, and is thus often cut twice in cross section. The columns of the fornix appear, as smaller bundles, in the mid-ventral midline. Below the fornix is the stria medullaris thalami, mainly carrying fibers from the septum and other structures to the habenula. The internal and external capsules can be identified. The optic chiasm outlines the ventral base of the diencephalon.