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II/1.6: Horizontal (axial) sections of the brain
If possible, the slices are also made in alignment with defined anatomical orientation points. The first section is made above the corpus callosum, the second in the plane of the interventricular foramina of Monro, the third in the plane of the anterior commissure, and the fourth in the plane of the nucleus ruber. Structures of the telencephalon, diencephalon, and (parts of the) mesencephalon can be studied on these slices.
It must be noted that it is difficult to assess the horizontal plane on a removed brain, and exact establishment of the section planes can only be certain in situ, with orientation points taken on the head.
In the slice taken above the corpus callosum, grey and white matter are discernible. White matter here is called semioval center because of the hemispheres’ half oval shape. It includes the radiation of the corpus callosum and the radiation of the internal capsule which is called corona radiata. The fibers of the two pathways don’t separate.
Gyri and sulci are discernible on the surface. The central sulcus of Rolando and the precentral and postcentral gyri on its sides are especially important orientation points in a clinical aspect. Due to their band-like cross-sectional appearance they are also called ’motor strip’ and ’sensory strip’. Other important orientation points are the frontal and occipital poles and the parietooccipital sulcus.
The next slice is made in the level of the interventricular foramina of Monro which are bordered by the anterior tuberculum of the thalamus from the back and the pars libera of the columna fornicis. The choroid plexus can be observed in the foramen. Behind that, the gap-like cavity of the third ventricle is seen, and the anterior horn of the lateral ventricle appears anterolaterally. Between the two anterior horns are the laminae of the septum pellucidum with the cavum septum pellucidum between them. In front of the septum and the cavum is a section of the genu of the corpus callosum, and in front of that, a double lumen indicates the pericallosal artery. The largest cross-sections of the basal ganglia and diencephalon are seen around the ventricles in this level. Thus the head of the caudate nucleus is seen beside the frontal horn, and the thalamus appears laterally from the third ventricle. In this section these are separated from the lentiform nucleus by the V-shaped internal capsule. The lentiform nucleus is well discernibly divided to the outer putamen and the inner globus pallidus which is further divided into internal and external parts. Sometimes the Y shape of the medullar lamina is seen in the thalamus, dividing it into anterior, medial and lateral nuclei.
Laterally from the putamen appears the fine external capsule, then the claustrum and the extreme capsule. Outward from that, the insular cortex is seen at the bottom of the lateral cerebral fossa with the short insular gyri. At the posterior part of the third ventricle is the habenular trigone and the pineal gland, and behind them appears the cistern of the great cerebral vein (or quadrigeminal cistern).
Posterolaterally from the thalamus is the transition of the cella media into inferior and posterior horns and the choroid glomus. At the anterolateral part of the cavity appears the oval section of the tail of the caudate nucleus, and laterally it is bordered by a fine membrane of the corpus callosum radiation which is called tapetum. Between the medial surfaces of the two parietal lobes and the occipital lobe, the uppermost point of the cerebellum’s superior vermis is also seen.
On the slice made in the level of the anterior commissure, mainly the continuation of the above structures is seen. In addition, instead of the habenular commissure, the posterior commissure appears at the posterior part of the third ventricle. In this level, the top of the superior colliculi becomes visible, and the medial geniculate body (CGM) is seen posterolaterally in the continuation of the two inferior colliculi and brachia of inferior colliculus which belong to the auditory pathway.
At the transition of the inferior and posterior horn of the lateral ventricle appears the hippocampus and the fimbria hippocampi. A more pronounced swelling called calcar avis narrows the posterior horn.
Laterally from the insula appear the sections of the lateral cerebral fossa and the covering frontal and parietal operculum. Larger vascular cross-sections in the fossa correspond to the M2 segment of the middle cerebral artery.
More below, on the slice made at the level of the nucleus ruber appears the ’shorts’ shaped section of the mesencephalon, where the legs of the shorts correspond to the basis of the midbrain, namely the two cerebral crura. These are separated posteriorly from the tegmentum of the mesencephalon by the inferior edge of the substantia nigra which is just visible here. At the border of the tegmentum and tectum appears the narrow cross-section of the cerebral aqueduct surrounded by the central grey matter. In front of the midbrain, anterolaterally from the cerebral crus is the posterior part of the optic tract as it radiates into the lateral geniculate body. The surrounding gap corresponds to the crural cistern. The lower part of the CGM also appears directly medially from the CGL.
The two mamillary bodies are also seen as smaller grey matter areas between, and in front of the two cerebral crura. The mamillo-thalamic tract (or Vicq d’Azyr band) irradiating into the mamillary bodies is seen on the front as a regular round or oval bundle.
The lateral part of the anterior commissure runs in an anteriorly convex arch, piercing the corpus striatum, between the putamen and the external part of the globus pallidus.
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