WHITE MATTER OF THE CEREBRAL HEMISPHERE All information entering or leaving the cerebral cortex or connecting one part of the cortex with another must pass through the subcortical white matter. In general, the white matter core of the hemisphere contains association fibers, commissural fibers, and projection fibers. Association Fibers The association fibers interconnect various areas of cortex within the same hemisphere. These may be short association fibers that connect the cortices of adjacent gyri or long association fibers that interconnect more distant areas of cortex (Fig. 16-13). Important examples of the latter are the cingulum located internal to the cingulate gyrus and continuing into the parahippocampal gyrus, the inferior longitudinal fasciculus (temporal-occipital interconnections), and the uncinate fasciculus (frontal-temporal interconnections). The superior longitudinal fasciculus, located in the core of the hemisphere, interconnects frontal, parietal, and occipital cortices, whereas the arcuate fasciculus interconnects frontal and temporal lobes (Fig. 16-13). In the white matter of the temporal lobe, fibers passing between the frontal and occipital areas make up the inferior frontooccipital fasciculus. The claustrum, a thin layer of neuron cell bodies located internal to the insular cortex, is sandwiched between two small association bundles (Fig. 16-12). The external capsule is insinuated between the claustrum and putamen, and the extreme capsule is located between the claustrum and the insular cortex. Commissural Fibers: The Corpus Callosum In general, commissural fibers interconnect corresponding structures on either side of the neuraxis. The largest bundle of commissural fibers is the corpus callosum (Figs. 16-2 and 16-4). This huge bundle is located superior to the diencephalon and forms the roof of much of the lateral ventricles. The corpus callosum consists of, from rostral to caudal, a rostrum, genu, body (also called trunk), and splenium (Figs. 16-4 and 16-5B). Many of the fibers passing through the genu arch rostrally to interconnect the frontal lobes; these form the minor (or frontal) forceps. The fibers interconnecting the occipital lobes loop through the splenium of the corpus callosum, forming the major (or occipital) forceps. The tapetum, which is located in the lateral wall of the atrium and posterior horn of the lateral ventricle, is also composed of fiber bundles that cross in the splenium. Lesions of the corpus callosum, whether the result of surgery or spontaneous, disconnect one hemisphere from the other. About 80% of patients who have severe generalized seizures experience significant relief after section of the anterior 75% of the corpus callosum, largely sparing the splenium. Spontaneous lesions of the corpus callosum may result from vascular infarct, tumor (such as oligodendroglioma), or necrosis or demyelination (as in Marchiafava-Bignami disease). It is recognized that some of these patients may have a disconnection syndrome. Smaller commissural bundles include the anterior commissure and the hippocampal commissure (Fig. 16-4). In sagittal views, the anterior commissure is located caudal to the rostrum of the corpus callosum but rostral to the main part of the fornix. This bundle interconnects various parts of the frontal and temporal lobes. The hippocampal commissure is formed by fibers that originate in the hippocampal formations and cross the midline as a thin layer inferior to the splenium of the corpus callosum. The posterior commissure and the habenular commissure are small fiber bundles traversing the midline that connect caudal parts of the diencephalon (Figs. 16-1 and 16-4). The former crosses the midline at the base of the pineal gland and just posterior (dorsal) to the cerebral aqueduct. The latter is a small fascicle running along the upper aspect of the posterior commissure and interconnecting the habenular nuclei. Projection Fibers: The Internal Capsule The projection fibers of the hemispheres include both the axons that originate outside the telencephalon and project to the cerebral cortex (corticopetal) and the axons that arise from cerebral cortical cells and project to downstream targets (corticofugal). A prime example of the former are projections from the thalamus to the cerebral cortex (thalamocortical fibers); examples of the latter are corticospinal, corticopontine, and corticothalamic fibers. Projection fibers are organized into a large, compact bundle called the internal capsule (Fig. 16-14), which has intimate structural associations with the diencephalon and basal nuclei. 

Consequently, to divide the internal capsule into its constituent parts, reference must be made to these adjacent cell groups. In an axial plane through the hemisphere, the internal capsule appears as a prominent V-shaped structure with the V pointing medially (Figs. 16-14 and 16-15). It is divided into three parts: (1) an anterior limb insinuated between the head of the caudate nucleus and the lenticular nucleus, (2) a posterior limb located between the dorsal thalamus and the lenticular nucleus, and (3) a genu located at the intersection of the anterior and posterior limbs, which is located approximately at the level of the interventricular foramen (Fig. 16-14). The anterior limb of the internal capsule contains thalamocortical-corticothalamic fibers (collectively called the anterior thalamic radiations) that interconnect the dorsomedial and anterior thalamic nuclei with areas of the frontal lobe and the cingulate gyrus. Frontopontine fibers, especially those from the prefrontal areas, also pass through this structure. The genu of the internal capsule contains corticonuclear fibers that arise in the frontal cortex just rostral to the precentral sulcus and from the precentral gyrus (primary motor cortex) and project to the motor nuclei of cranial nerves. Lesions of these fibers give rise to motor deficits of cranial nerves, most notably deficits related to the facial and hypoglossal nerves. The posterior limb of the internal capsule is larger and more complex (Fig. 16-14). It is sometimes divided into a thalamolenticular part (located between the thalamus and the lenticular nucleus), a sublenticular part (fibers passing ventral to the lenticular nucleus), and a retrolenticular part (fibers located caudal to the lenticular nucleus). However, contemporary terminology and common usage refer to the thalamolenticular part as the posterior limb, the sublenticular part as the sublenticular limb, and the retrolenticular part as the retrolenticular limb. This terminology is considerably less cumbersome and much easier to remember and is the convention followed here (Figs. 16-14 and 16-15). By this scheme, the internal capsule consists of five parts: anterior limb, genu, posterior limb, sublenticular limb, and retrolenticular limb. The major fiber populations passing through the posterior, sublenticular, and retrolenticular limbs of the internal capsule are summarized in Figure 16-14. Included in the posterior limb are corticospinal fibers arising from the motor cortex and projecting to the contralateral spinal cord and thalamocortical-corticothalamic fibers (as part of the central thalamic radiations) that interconnect nuclei of the dorsal thalamus with the overlying cortex. Studies in humans have revealed that corticospinal fibers are somatotopically arranged in about the caudal half of the posterior limb. Geniculotemporal radiations (auditory radiations) convey auditory information from the medial geniculate nucleus to the transverse temporal gyri through the sublenticular limb. Visual input from the lateral geniculate body to the occipital cortex is conveyed via geniculocalcarine radiations (optic radiations) through the retrolenticular limb (Fig. 16-14). Optic radiations form a distinct lamina of fibers immediately lateral to the tapetum as they course caudally into the occipital lobe. Fibers of the internal capsule flare out into the hemisphere as they pass distal to the caudate and putamen. This abrupt divergence of internal capsule fibers forms the corona radiata (“radiating crown”), which contains converging corticofugal fibers as well as diverging corticopetal fibers (see Fig. 16-20).