The white matter of the cerebral hemispheres

The white matter of the cerebral hemispheres is represented by various systems of nerve fibers, among which the following are distinguished:

1. associative;
2. commissural and
3. projection.

They are considered as conducting pathways of the brain and spinal cord. Associative nerve fibers that come out of the cortex of the hemisphere (extracortical) are located within one hemisphere, connecting different functional centers. Commissural nerve fibers pass through the commissures of the brain (corpus callosum, anterior commissure). Projection nerve fibers that go from the cerebral hemisphere to its lower parts (intermediate, middle, etc.) and to the spinal cord, as well as those that follow in the opposite direction from these formations, make up the internal capsule and its radiant crown (corona radiata).

The internal capsule (capsula interna) is a thick, angled plate of white matter. It is bounded laterally by the lentiform nucleus and medially by the head of the caudate nucleus (in front) and the thalamus (in back). The internal capsule is divided into three sections. Between the caudate and lentiform nuclei is the anterior crus of the internal capsule (crus anterius capsulae internae), and between the thalamus and the lentiform nucleus is the posterior crus of the internal capsule (crus posterius capsulae internae). The junction of these two sections at an angle open laterally is the genu capsulae internae.

The internal capsule contains all the projection fibers that connect the cerebral cortex with other parts of the CNS. The genu of the internal capsule contains the fibers of the corticonuclear tract, which runs from the cortex of the precentral gyrus to the motor nuclei of the cranial nerves. The anterior part of the posterior leg, directly adjacent to the genu of the internal capsule, contains the corticospinal fibers. This motor tract, like the previous one, begins in the precentral gyrus and goes to the motor nuclei of the anterior horns of the spinal cord.

Behind the listed conducting pathways in the posterior peduncle are the thalamocortical (thalamoparietal) fibers. They are represented by the processes of the thalamic cells directed to the cortex of the postcentral gyrus. This conducting pathway contains the fibers of the conductors of all types of general sensitivity (pain, temperature, touch and pressure, proprioception). Even more behind this tract in the central sections of the posterior peduncle is the temporo-parieto-occipito-pontine tract. The fibers of this tract originate from the cells of various areas of the cortex of the occipital, parietal and temporal lobes of the hemisphere and go to the nuclei of the pons located in its anterior (basilar) part. In the posterior sections of the posterior peduncle are the auditory and visual conducting pathways. Both originate from the subcortical centers of hearing and vision and end in the corresponding cortical centers. The anterior leg of the internal capsule contains the fronto-pontine tract.

These are only the most important conducting pathways, the fibers of which pass through the internal capsule.

The fibers of the ascending pathways, diverging in different directions in the cortex of the cerebral hemispheres, form the so-called radiant crown (corona radiata). Below, the fibers of the descending pathways of the internal capsule in the form of compact bundles are directed to the peduncle of the midbrain.

The corpus callosum contains fibers (commissural pathways) that pass from one hemisphere to the other and connect the areas of the cortex belonging to the right and left hemispheres, with the purpose of uniting (coordinating) the functions of both halves of the brain into one whole. The corpus callosum is a thick, specially curved plate consisting of transverse fibers. The free upper surface of the corpus callosum, facing the longitudinal fissure of the cerebrum, has a gray cover (indusium griseum) - a thin plate of gray matter.

On a sagittal section of the brain, one can distinguish the bends and parts of the corpus callosum: the genu, continuing downwards into the beak (rostrum), and then into the terminal (end) plate (lamina terminalis). The middle part is called the trunk (truncus) of the corpus callosum. To the rear, the trunk continues into a thickened part - the splenium. The transverse fibers of the corpus callosum in each hemisphere of the cerebrum form the radiatio corporis callosi. The fibers of the anterior part of the corpus callosum - the genu - bend around the anterior part of the longitudinal fissure of the brain and connect the cortex of the frontal lobes of the right and left hemispheres. The fibers of the central part of the corpus callosum - the trunk - connect the gray matter of the parietal and temporal lobes. The ridge contains fibers that envelop the posterior part of the longitudinal fissure of the cerebrum, connecting the cortex of the occipital lobes.

Under the corpus callosum is the fornix. The fornix consists of two arcuately curved strands connected in the middle part by transverse fibers - the commissure of the fornix (comissura fornicis). The middle part is called the body of the fornix (corpus fornicis). In front and downwards it continues into a rounded paired strand - a column of the fornix (columna fornicis). The right and left columns of the fornix are directed downwards and somewhat laterally to the base of the brain, where they end in the right and left mammillary bodies. Behind, the body of the fornix also continues into a paired flat strand - the crus of the fornix (crus fornicis), fused with the lower surface of the corpus callosum. The paired crus of the fornix on the right and left sides gradually goes laterally and downwards, separates from the corpus callosum, flattens even more and fuses with the hippocampus on one side, forming the fimbria of the hippocampus (fimbria hippocampi). The other part of the fimbria is free and faces the cavity of the inferior horn of the lateral ventricle. The fimbria of the hippocampus ends in the hook, thus connecting the temporal lobe of the telencephalon with the diencephalon.

In front of the fornix in the sagittal plane is the septum pellucidum, which consists of two plates lying parallel to each other. Each plate of the septum pellucidum is stretched between the body and the column of the fornix behind, the corpus callosum above, and the genu and beak of the corpus callosum in front and below. Between the plates of the septum pellucidum is a slit-like cavity of the septum pellucidum (cavum septi pellucidi), containing a transparent fluid. The lamina of the septum pellucidum serves as the medial wall of the anterior horn of the lateral ventricle. In front of the columns of the fornix is the anterior commissure (comissura rostralis, s. anterior), the fibers of which are oriented transversely. On a sagittal section, the commissure has the shape of a small oval. The anterior part of the commissure is thin, connecting the gray matter of the olfactory triangles of both hemispheres. The larger posterior portion contains nerve fibers connecting the cortex of the anteromedial parts of the temporal lobes.

The white matter of a hemisphere includes fibers that connect different areas of the cortex within one hemisphere (associative fibers) or the cortex with the subcortical centers of a given hemisphere. Along with short associative nerve fibers, large long bundles are distinguished in the white matter, which have a longitudinal orientation and connect areas of the cerebral cortex that are far apart from each other.