Horizontal section of hemispheres photographic view

In this photograph, the brain has been sectioned in the horizontal plane. From the dorsolateral view (the small figure on the upper left), the level of the section is just above the lateral fissure and at a slight angle downward from front to back. Using the medial view of the brain (the figure on the upper right), the plane of section goes through the anterior horn of the lateral ventricle, the thalamus and the occipital lobe.

This brain section exposes the white matter of the hemispheres, the basal ganglia, and parts of the ventricular system. Understanding this particular depiction of the brain is vital to the study of the forebrain. The structures seen in this view are also of immeasurable importance clinically, and this view is most commonly used in neu-roimaging studies, both CT and MRI (shown in Figure 28A and Figure 28B).

The basal ganglia are present when the brain is sectioned at this level (see Figure 25). The head of the caudate nucleus protrudes into the anterior horn of the lateral ventricle (seen in the CT, Figure 28A). The lentiform nucleus, shaped somewhat like a lens, is demarcated by white matter. Since the putamen and caudate neurons are identical, therefore, the two nuclei have the same grayish coloration. The globus pallidus is functionally different and contains many more fibers, and therefore is lighter in color. Depending upon the level of the section, it is sometimes possible (in this case on both sides) to see the two subdivisions of the globus pallidus, the internal and external segments (see Figure 24).

The white matter medial to the lentiform nucleus is the internal capsule (see Figure 26 and Figure 73). It is divisible into an anterior limb and a posterior limb and genu. The anterior limb separates the lentiform nucleus from the head of the caudate nucleus. The posterior limb of the internal capsule separates the lentiform nucleus from the thalamus. Some strands of gray matter located within the internal capsule represent the strands of gray matter between the caudate and the putamen (as shown in Figure 23). The base of the "V" is called the genu.

The anterior horn of the lateral ventricle is cut through its lowermost part and is seen in this photograph as a small cavity (see Figure 20A). The plane of the section has passed through the connection between the lateral ventricles and the third ventricle, the foramina of Monro (see Figure 20B). The section has also passed through the lateral ventricle as it curves into the temporal lobe to become the inferior horn of the lateral ventricle, the area called the atrium or trigone (better seen on the left side of this photograph; see Figure 20A and Figure 25). The choroid plexus of the lateral ventricle, which follows the inner curvature of the ventricle, is present on both sides (not labeled; see Figure 20A).

The section is somewhat asymmetrical in that the posterior horn of the lateral ventricle is fully present in the occipital lobe on the left side and not on the right side of the photograph. On the right side, a group of fibers is seen streaming toward the posterior pole, and these represent the visual fibers, called the optic radiation (discussed with Figure 41A and Figure 41B). The small size of the tail of the caudate nucleus alongside the lateral ventricle can be appreciated (see Figure 23 and Figure 25).

The third ventricle is situated between the thalamus of both sides (see Figure 9). The pineal is seen attached to the back end of the ventricle. A bit of the cerebellar vermis is visible posteriorly, behind the thalamus and between the occipital lobes.

Clinical Aspect

This is the plane of view that would be used to look for small bleeds, called lacunes, in the posterior limb of the internal capsule (discussed with Figure 62). The major ascending sensory tracts and the descending motor tracts from the cerebral cortex are found in the posterior limb.

Brain Horizontal SectionLateral Ventricle Anterior Horn

Putamen

Globus pallidus (external segment)

Globus pallidus (internal segment)

Corpus callosum

Lateral ventricle (anterior horn)

Caudate nucleus (head) Fornix

Internal capsule (anterior limb) Foramen of Monro

Putamen

Globus pallidus (external segment)

Globus pallidus (internal segment)

Lentiform nucleus

Internal capsule (posterior limb) 3rd ventricle Caudate nucleus (tail) Pineal

Lateral ventricle (atrium) Optic radiation

Lateral ventricle (occipital horn)

F = Frontal lobe T = Temporal lobe O = Occipital lobe

Th = Thalamus C = Cerebellum (vermis)

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