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II/1.5: Sectional anatomy of the brain
For anatomical orientation, the most common cuts are in the frontal (coronal), horizontal (axial), and midsaggital plane, and Flechsig’s oblique cut is also used. Here we only introduce the sections which are necessary for the understanding of anatomical structure arrangement. Naturally, imaging procedures in the clinical practice use significantly more slices. There are various possibilities of anatomical section preparation, and we adjust the cut planes specifically to the well identificable superficial structures. Structures are therefore described in thicker (6-10 mm) slices, but the topography of the individual structures – following the order below – practically corresponds with the interpretation of thinner slices used in radiology.
On the grounds of the above, in the frontal plane we are going to present slices bordered by planes that go through the genu of the corpus callosum, the lamina terminalis, the infundibular stalk, the mamillary bodies and the lateral geniculate nucleus.
In the central part of the first slice the genu of the corpus callosum is seen from the front, and around it – representing the anterior part of the radiation of corpus callosum – commissural fibers of the forceps minor run toward the cortex. The two vessels appearing under the corpus callosum are the anterior cerebral arteries after the origin of anterior communicating artery, and their continuation is also seen above the corpus callosum. These two sections together are called pericallosal artery.
From behind, the anterior ends of the frontal horns are just seen, with deeper fibers of the genu or even the septum pellucidum between them. Lateral wall is consisted here only of the anterior part of the head of the caudate nucleus.
On the outer surface of the slice, sulci and gyri of the three cerebral surfaces are well identificable. On the base, the olfactory tract is seen in the olfactory sulcus.
On the frontal surface of the second slice, the genu is seen above the frontal ventricular horns, and below them is the rostrum of corpus callosum, and below the latter is the subcallosal area. Between them are the two laminae septi pellucidi and they encompass the cavum septi pellucidi (its dilated form is called Verga’s ventricle). In this level, anterior communicating artery may already appear under the rostrum. Laterally, fine grey matter bands bridging the anterior part of the internal capsule connect the head of the caudate nucleus with the outer part of the lentiform nucleus which is called putamen. These two grey nuclei together constitute the striatum. Laterally from the striatum are the external capsule, the claustrum, and the extreme capsule. Frontal pole appears laterally and below. This is separated from the frontal lobe by the lateral cerebral fissure. This part of the frontal lobe corresponds to the opercular part of the inferior frontal gyrus. In the bottom of the fissure is the insular lobe and M1 and M2 segments and branches of the middle cerebral artery may also be seen.
The terminal lamina with the pars libera of the two columnae fornicis atop is usually seen on the posterior surface of the slice. The anterior commissure connecting the olfactory lobes runs horizontally in between. The columnae and the commissure frame the triangular recess. Below them is the optic recess followed by the optic chiasm. So the anterior part of the third cerebral ventricle cavity is also seen here. Depending on its pathway, the internal carotid artery and its bifurcation may also appear on the sides of the chiasma. The area above corresponds to the anterior perforated substance. On the sides, the grey nuclei are completed by the globus pallidus which is located medially from the putamen. The globus pallidus has well discernible internal and external parts. The lateral cerebral fossa widens vertically, and the temporal operculum appears under the frontal operculum.
On the front of the third slice, the tuber cinereum and the infundibular recess is seen with the optic tracts on the side. The fine anterior choroid artery and the posterior communicating artery are usually unrecognizable. The drop-shaped cavity of the anterior horn of the lateral ventricle changes here, and cella media appears which is flattened from above downward and widens horizontally. Interventricular foramens including choroid plexuses that turn to the top of the third cerebral ventricle are seen at the transition of the two ventricle parts. The anterior nuclear group of the thalamus is seen under the vascular plexus. The lateral wall of the cella media is constituted by the body of the caudate nucleus, and between that and the dorsal surface of the thalamus, appears the stria terminalis including the thalamostriate vein. Between the two cellae media the body of the fornix is seen with the choroid plexus in its cavity. The parts of the lentiform nucleus are well discernible. The corpus amygdaloideum is seen as an irregular ovel structure in the white matter behind the temporal pole. The anterior part of the parahippocampal gyrus appears medially from the above. Gyri and sulci on the temporal lobe’s convexity and base are also discernible.
On the fourth slice, the two mammilary bodies are seen at the bottom of the third cerebral ventricle. The ventricle is divided by the massa intermedia (interthalamic adhesion) above them. The medullar lamina of the thalamus may appear on the section, and it helps to distinguish the medial and lateral nuclei of the thalamus. The subthalamic nucleus (Luys’ body) appears below the thalamus and beside it the internal capsule turns into the cerebral crus as a thick bundle. The continuation of the optic tracts is seen laterally from these. The cellae media of the lateral ventricles are more flattened and the cross-section of the body of the caudate nucleus is also smaller. Between the two cellae media the body of the fornix pass into the anterior ends of the crura of the fornix which are connected by the commissure of the fornix (Lyra Davidis, psalterium). Otherwise, the section doesn’t substantially change. The cross-section of the lentiform nucleus is somewhat larger, and its parts are well discernible. The claustrum also gets thicker.
If the fifth slice is made in the level of the two lateral geniculate bodies (CGL), then the posterior part of the third ventricule burst upon our view. Depending on the slice plane, the posterior commissure may be seen with the habenular commissure above. The ventricle is bordered by the more unified thalami, and the crura of the fornix get quite flattened above them. The small, round cross section of the tail of the caudate nucleus is seen at the side of the cella media. Of the lentiform nucleus, usually the posterior part of the putamen is seen. The two red nuclei (nucleus ruber) appear below the thalami, and below them is the substancia nigra. The latter constitutes the border between the tegmentum and the basis of the midbrain, and it belongs to the basis. The two cerebral crura frame a small pit called interpeduncular fossa. The substance of the crura and the pons seems to intermingle. The hippocampus appears in the lower horn of the lateral ventricle beside the brainstem, in the mesial part of the temporal lobe, above the parahippocampal gyrus. The fimbria of the hippocampus and a part of the choroid plexus is seen on the top of the hippocampus. Above and medially from the inferior ventricular horn, the jockey’s hat shaped cross-section of the CGL is seen, and laterally from that is the end of the tail of the caudate nucleus which turns here from the cella media. The collateral sulcus which runs on the basis, causes a swelling from below (collateral eminence).
On the back surface of the slice, we can usually see the transition between the fimbria of the hippocampus and the crus of the fornix, and the transition between the inferior horn and the cella media above. The thickened part of the choroid plexus in the latter is called choroid glomus. The pineal gland may appear behind the third cerebral ventricle surrounded by the cistern of the great cerebral vein with a strong venous network belonging to its collection area.
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