Each cerebral hemisphere includes a large volume of white matter, sometimes called the medullary center, that accommodates vast numbers of axons running to and from all parts of the cortex. Axons that establish connections between the cortex and subcortical gray matter continue into the internal capsule. The lateral ventricles, one in each hemisphere, are the largest of the four ventricles of the brain and are important in the dynamics of the cerebrospinal fluid (CSF) system.
Cerebral White Matter
Three types of axons are present in the cerebral white matter (Fig. 16-1). Association fibers are confined to a hemisphere and connect one cortical area with another. Many of these fibers accumulate in named longitudinally running bundles that can be displayed by dissection. Commissural fibers connect the cortices of the two hemispheres; most are located in the corpus callosum, and the remainder are located in the anterior commissure. Projection fibers establish connections between the cortex and such subcortical structures as the corpus striatum, thalamus, brain stem, and spinal cord. They are afferent (corticopetal) or efferent (corticofugal) with respect to the cortex. Most corticopetal projection fibers originate in the thalamus; some ascend from nuclei in the hypothalamus and brain stem (see Chapters 9 and 11).
Association fibers are the most numerous of the three types of fiber noted. Operative procedures, vascular accidents, and lesions that transect the fasciculi may lead to dysfunction
by disconnecting functionally related regions of the cerebral cortex.
FIGURE 16-1 Coronal section through a cerebral hemisphere, indicating the major bodies of gray matter (yellow) and positions of the larger bundles of association, commissural, and projection fibers (blue). The choroid plexus of the lateral and third ventricles is outlined in red.
The cingulum, which is most easily displayed by dissection in the cingulate gyrus (Figs. 16-2 and 16-3), is an association fasciculus of the limbic lobe. The axons in this longitudinal bundle run in both directions and interconnect the cingulate gyrus, parahippocampal gyrus of the temporal lobe, and septal area below the genu of the corpus callosum.
The superior longitudinal fasciculus (see Figs. 16-2 and 16-3), also known as the arcuate fasciculus, runs in an anteroposterior direction above the insula, and many of the fibers turn downward into the temporal lobe. This, similar to the other large association bundles, consists of axons of various lengths that enter or leave the fasciculus at any point along its course. The superior longitudinal fasciculus provides important communications between cortices of the parietal, temporal, and occipital lobes and the cortex of the frontal lobe. These provide a pathway whereby interpreted sensory signals (especially visual and proprioceptive) from the parietal cortex influence the formulation in the frontal lobe of neuronal programs for appropriate movements. The arcuate fasciculus also includes fibers that connect the receptive (sensory) and expressive (motor) language areas (see Chapter 15). Aninferior longitudinal fasciculus, beneath the lateral and ventral surfaces of the occipital and temporal lobes, is difficult to demonstrate by dissection.
The inferior occipitofrontal fasciculus and uncinate fasciculus are components of a single association system (Figs. 16-4 and 16-5). The fibers are compressed into a well-defined bundle below the insula and lentiform nucleus. The longer part of the fiber system, extending the length of the hemisphere, is the inferior occipitofrontal fasciculus. The uncinate fasciculus is the part that hooks around the stem of the lateral sulcus to connect the frontal lobe, especially
cortex on its orbital surface, with cortex in the region of the temporal pole.
FIGURE 16-2 Dissection of the right cerebral hemisphere: dorsal view with frontal pole at the right.
FIGURE 16-3 Dissection of the right cerebral hemisphere: dorsal view with frontal and temporal poles at the right.
FIGURE 16-4 White matter of the right cerebral hemisphere after removal of the superior longitudinal fasciculus, insula, and underlying structures down to the external capsule.
The superior occipitofrontal fasciculus, also called the subcallosal bundle, is located deep in the hemisphere (see Fig. 16-1). Its fibers spread out to cortex of the frontal lobe and to cortex in the posterior part of the hemisphere.
Large numbers of arcuate fibers connect adjacent gyri. These short subcortical association fibers are oriented at right angles to the gyri and bend sharply under the intervening sulci. Spread of activity along a gyrus or sulcus is provided by other subcortical association fibers and by axons within the cortex.
Most of the neocortical commissural fibers constitute the corpus callosum; the remainder are included in the anterior commissure, along with fibers of other than neocortical origin. The corpus callosum varies considerably in size and shape. The sectional area of the corpus callosum in the midline may be, on average, slightly larger in right- than in left-handed people, although this observation has been disputed. In laboratory animals, commissural fibers from an area of cortex in one hemisphere have been shown to terminate in the corresponding area and in cortex closely related functionally with that area, in the other hemisphere. The hand areas of the primary somatosensory cortices and large parts of the primary visual areas are notable in that they are not directly connected by commissural fibers. They communicate functionally, however, through callosal fibers that connect the adjacent association areas. Much of the cortex of the temporal lobe makes its commissural connections by way of the anterior commissure rather than the corpus callosum.
The trunk of the corpus callosum is the compact part of the commissure in and near the midline (see Fig. 16-2). As they pass laterally,
the callosal fibers intersect association bundles and projection fibers. The trunk of the corpus callosum is considerably shorter than the hemispheres; this accounts for the enlargements of the ends, which are the splenium posteriorly and the genu anteriorly (see Fig. 13-2). The splenium and the radiations that connect the occipital lobes constitute theforceps occipitalis (forceps major) (Fig. 16-6), and the genu and the radiations that connect the frontal lobes form the forceps frontalis (forceps minor). The genu tapers into therostrum of the corpus callosum, which is continuous with the lamina terminalis forming the anterior wall of the third ventricle. Callosal fibers that form a thin sheet over the temporal horn of the lateral ventricle constitute the tapetum (see Fig. 16-6), which provides some of the communication between the cortices of the temporal lobes.
FIGURE 16-5 The dissection shown in Figure 16-4 has been continued by removal of the external capsule to expose the lentiform nucleus.
The ventral surface of the corpus callosum forms the roof of the lateral ventricles and has relations with the fornix and septum pellucidum in the midline. The fornix, consisting of symmetrical halves, is a robust fiber system that connects the hippocampal formation of each temporal lobe with the hypothalamus (see Fig. 18-2) and the septal area of the forebrain. The crura of the fornix begin at the posterior end of each hippocampus; they curve forward and merge to form the body of the fornix, which is in contact with the undersurface of the trunk of the corpus callosum. The body of the fornix divides into two columns that turn ventrally away from the corpus callosum; they form the anterior boundaries of the interventricular foramina and continue to the hypothalamus. The resulting interval between the fornix and corpus callosum is bridged by the septum pellucidum(see Fig. 11-2), a thin sheet of neuroglial tissue that contains scattered groups of neurons at its anterior end and is covered on each side by ependyma. The septum pellucidum separates the frontal horns of the lateral ventricles; it is a double membrane containing a slit-like cavity, the cavum septi pellucidi, which does not communicate with the ventricular system or with the subarachnoid space.
FIGURE 16-6 Dissection of parts of the corpus callosum in the right hemisphere. The posterior half of the cingulum has been removed, and the longitudinal striae are visible on the upper surface of the exposed corpus callosum.
The anterior commissure is a bundle of axons that crosses the midline in the lamina terminalis; it traverses the anterior parts of the corpora striata and provides for additional communication between the temporal lobes (Fig. 16-7). The anterior commissure includes fibers that connect the middle and inferior temporal gyri of the two sides; this is a neocortical component similar to the corpus callosum. Other fibers run between the olfactory cortex of the temporal lobes (the lateral olfactory areas), for which the uncus is a landmark. Also present are axons that interconnect the olfactory bulbs, but these are a minor component of the human anterior commissure.
A large hole in the septum pellucidum is often present in the brains of professional boxers. No functional disability is known to result from this perforation, but boxers commonly have numerous other small lesions that transect axons in cerebral white matter. The resulting generalized reduction in the number of cortical connections leads to the condition known as chronic traumatic encephalopathy or dementia pugilistica, popularly called “punch-drunkenness.” Whereas deterioration of the personality, impairment of memory, and possibly some features of parkinsonism are attributable to functional disconnections in the cerebrum, dysarthria and ataxia may be caused by similar multiple interruptions of cerebellar connections.
FUNCTIONS OF THE CEREBRAL COMMISSURES
The interhemispheric connections provided by the corpus callosum and anterior commissure contribute to the bilaterality of memory traces. All knowledge that arrives from the senses is
collected by both cerebral hemispheres. In some people with severe epilepsy, the corpus callosum has been transected to confine the epileptic discharge to one hemisphere and the seizures to one side of the body. This operation leads to no significant changes in intellect, behavior, or emotional responses that can be attributed to commissurotomy. A task that has been newly learned with one hand, however, is no longer transferable to the other hand.
FIGURE 16-7 Dissection exposing the anterior commissure, photographed by a camera anterior to the left frontal pole of the dissected brain.
A particularly significant result of commissurotomy is related to language. In most people, the linguistic faculties reside in the left hemisphere. After recovering from the operation, the patient is unable to describe an object held in the left hand (with the eyes closed) or seen only in the left visual field, although the nature of the object is understood. There is no such difficulty when the sensory data reach the left hemisphere. After commissurotomy, the right hemisphere is rendered mute and agraphic because it has no access to memory for language in the left hemisphere. The hemisphere that is subordinate with respect to language is superior in certain other activities, however. These include copying drawings that include perspective and arranging blocks in a prescribed manner. The nonlinguistic hemisphere is therefore the more proficient side of the brain in functions that require special competence in three-dimensional perspective. Interhemispheric differences are discussed in more detail in Chapter 15.
INTERNAL CAPSULE AND PROJECTION FIBERS
The projection fibers are concentrated in the internal capsule and fan out as the corona radiata in the cerebral white matter (see Fig. 16-5). The internal capsule consists of ananterior limb, a genu, a posterior limb, a retrolentiform
part, and a sublentiform part, all of which have topographic relations with adjacent gray masses. The anterior limb is bounded by the lentiform nucleus and by the head of the caudate nucleus. The genu is located medially to the apex of the lentiform nucleus, and the posterior limb intervenes between the lentiform nucleus and the thalamus. The retrolentiform part of the internal capsule occupies the region behind the lentiform nucleus, and the sublentiform part consists of fibers that pass beneath the posterior part of the lentiform nucleus. The anatomical relations of the internal capsule are best appreciated in a horizontal section at the level of the insula (Fig. 16-8).
FIGURE 16-8 Horizontal section of the cerebrum at the level of the insula, stained to distinguish gray matter (dark) from white matter (light). The genu and limbs of the internal capsule are labeled. The sublentiform part of the internal capsule is ventral to the plane of this section below the posterior part of the lentiform nucleus. For other structures seen in a section at this level, see Figure 12-2.
Many of the projection fibers establish reciprocal connections between the thalamus and the cerebral cortex. The anterior thalamic radiation, located in the anterior limb of the internal capsule, consists mainly of fibers connecting the mediodorsal thalamic nucleus and prefrontal
cortex. The middle thalamic radiation is a component of the posterior limb of the internal capsule. This radiation includes the somatosensory projection from the ventral posterior thalamic nucleus to the somesthetic area in the parietal lobe; these fibers run in the posterior part of the posterior limb, where they are partly intermingled with motor projection fibers. Other fibers of the middle thalamic radiation establish reciprocal connections between the thalamus and the association cortex of the parietal lobe. Fibers from the ventral anterior and ventral lateral nucleus of the thalamus reach the motor, premotor, supplementary motor, and cingulate motor areas of the frontal lobe by traversing the genu and adjacent region of the posterior limb of the internal capsule.
The posterior thalamic radiation establishes connections between the thalamus and cortex of the occipital lobe. The geniculocalcarine tract that ends in the visual cortex is a particularly important component of this radiation. Originating in the lateral geniculate body, the geniculocalcarine tract first traverses the sublentiform and retrolentiform parts of the internal capsule. The constituent fibers then spread out into a broad band bordering the lateral ventricle and turn backward into the occipital lobe. Some of the fibers, constituting Meyer's loop, proceed forward for a considerable distance into the temporal lobe above the temporal horn of the lateral ventricle before turning back into the occipital lobe (see Fig. 20-7). The posterior thalamic radiation also contains fibers that establish reciprocal connections between the pulvinar of the thalamus and the cortex of the occipital lobe. The inferior thalamic radiation consists of fibers directed horizontally in the sublentiform part of the internal capsule that connect thalamic nuclei with cortex of the temporal lobe. Most of the fibers are included in the auditory radiation, which originates in the medial geniculate body and terminates in the primary auditory area, on the superior surface of the superior temporal gyrus.
MOTOR PROJECTION FIBERS
The remaining projection fibers are corticofugal, and many of them have motor functions. The corticobulbar (corticonuclear) and corticospinal tracts, which together constitute the pyramidal motor system, originate in the motor, premotor, supplementary motor, and cingulate motor areas in the frontal lobe and in the rostral (anterior) parts of the parietal lobe. These axons are probably accompanied by motor corticoreticular fibers (see below). The descending axons converge as they traverse the corona radiata and enter the anterior half of the posterior limb. In their passage caudally through the internal capsule, the motor fibers are shifted into the posterior half of the posterior limb by frontopontine fibers that have already traversed the anterior limb. Corticobulbar fibers are most anterior, followed in sequence by corticospinal fibers related to the upper limb, trunk, and lower limb. There is considerable overlap of the territories occupied by fibers for the major regions of the body, so a small destructive lesion in the internal capsule has serious effects.
Corticopontine fibers originate in all four lobes of the cerebral cortex but in greatest numbers in the frontal and parietal lobes. They terminate in the pontine nuclei (nuclei pontis) in the basal part of the pons. Fibers of the frontopontine tract traverse the anterior limb of the internal capsule and the anterior part of the posterior limb. Most of the fibers of theparietotemporopontine tract originate in the parietal lobe and traverse the retrolentiform part of the internal capsule.
Corticostriate fibers originate in all parts of the neocortex and end in the striatum. The caudate nucleus and putamen receive these fibers from the internal capsule; the putamen receives some from the external capsule as well.
Other projection fibers pass caudally to nuclei in the brain stem. Corticorubral fibers arise from the motor areas of the frontal lobe and end in the red nucleus. The corticoreticular fibers begin in the motor cortex and in the cortex of the parietal lobe, especially the primary somesthetic area. They terminate mainly in the central group of reticular nuclei.Cortico-olivary fibers, also mostly from the motor areas, go to the inferior olivary complex of nuclei. These descending pathways accompany the axons of the pyramidal system through the internal capsule and basis pedunculi into the pons and medulla. Along with the corticospinal and corticobulbar tracts, they are severed by destructive lesions in the internal capsule.
Such lesions also involve the thalamocortical fibers from the ventral lateral and ventral anterior thalamic nuclei to the motor areas of the cortex.
Internal Capsule Lesions
An infarction in the posterior part of the internal capsule results in serious neurological deficits. These include the effects of an “upper motor neuron lesion” (see Chapter 23) caused mainly by interruption of pyramidal and corticoreticular fibers. Hemiparesis is weakness of all the muscles of the opposite side of the body, and hemiplegia is complete paralysis of the affected side. A lesion in the internal capsule may also cause general sensory deficits by involvement of the thalamocortical projection to the somesthetic area and a visual field defect by interruption of geniculocalcarine fibers.
The composition of the external capsule is incompletely understood, but it is known that this thin layer of white matter between the putamen and claustrum consists mainly of projection fibers. These include some of the corticostriate fibers that end in the putamen and some of the corticoreticular fibers.
The lateral ventricles, one in each cerebral hemisphere, are roughly C-shaped cavities lined by ependyma and filled with CSF. Each lateral ventricle consists of a central part in the region of the parietal lobe from which horns extend into the frontal, occipital, and temporal lobes. The principal features of the ventricular walls are shown in Figures 16-9 and 16-10. The configuration of the entire ventricular system of the brain is shown in Figure 16-11.
Within the temporal lobe, the temporal horn is normally too small to show on a CT scan. It becomes visible if the ventricle is enlarged. Dilatation of the lateral ventricle may be caused by obstructed flow of CSF or by atrophy of the surrounding brain tissue.
The floor of the temporal horn includes an important structure, the hippocampus (see Fig. 16-10). The hippocampus may be visualized as an extension of the parahippocampal gyrus on the external surface that has been “rolled into” the floor of the temporal horn. The slightly enlarged anterior end of the hippocampus is known as the pes hippocampi because it resembles an animal's paw. Efferent fibers from the hippocampus form a ridge, the fimbria, along its medial border. The fimbria continues as the crus of the fornix at the posterior end of the hippocampus beneath the splenium of the corpus callosum. The choroid plexus of the central part of the ventricle continues into the temporal horn, where it is attached to the margins of the choroid fissure above the fimbria of the hippocampus.
The central part of the lateral ventricle has a flat roof formed by the corpus callosum. The floor includes part of the dorsal surface of the thalamus, of which the anterior tubercle is a boundary of the interventricular foramen (foramen of Monro) that leads to the third ventricle. The tail of the caudate nucleus forms a ridge along the lateral border of the floor. The stria terminalis, a slender bundle of fibers originating in the amygdaloid body in the temporal lobe, lies in the groove between the tail of the caudate nucleus and the thalamus along with the thalamostriate vein (vena terminalis). The fornix completes the floor medially, and the choroid plexus is attached to the margins of the choroid fissure, which intervenes between the fornix and thalamus. The stria terminalis and fornix are association fasciculi of the limbic system.
FIGURE 16-9 Dissection of the right cerebral hemisphere: dorsolateral view. The roof of the lateral ventricle has been removed.
The frontal horn of the ventricle extends forward from the region of the interventricular foramen. The corpus callosum continues as the roof, and the genu of the corpus callosum limits the frontal horn in front. The septum pellucidum bridges the interval between the fornix and corpus callosum in the midline, separating the frontal horns of the two lateral ventricles. The occipital horn, which is of variable length, is surrounded by cerebral white matter (Fig. 16-10). Two elevations on the medial wall of the occipital horn are the bulb of the occipital horn, raised by the forceps occipitalis, and the calcar avis, which corresponds to the calcarine sulcus.
The slender temporal horn extends to within about 3 cm of the temporal pole. A triangular area, called the collateral trigone, is found in the floor of the ventricle where the occipital and temporal horns diverge from the central part of the ventricle. A substantial part of the choroid plexus of the lateral ventricle rests on the trigone and can be seen in computed tomography scans of the brain because it contains small amounts of calcified material. The collateral sulcus on the external surface of the hemisphere is located immediately below the trigone and may produce a collateral eminence there. The tail of the caudate nucleus, now considerably attenuated, extends forward in the roof of the temporal horn as
far as the amygdaloid body. This latter is a group of nuclei above the anterior end of the temporal horn, close to the uncus on the external surface. The stria terminalis and thalamostriate vein run along the medial side of the tail of the caudate nucleus.
FIGURE 16-10 Dissection of the right cerebral hemisphere: lateral view showing the occipital and temporal horns of the lateral ventricle.
FIGURE 16-11 A cast of the ventricular system of the brain. (A) Left lateral ventricle. (B) Interventricular foramen. (C) Third ventricle. (D) Cerebral aqueduct. (E) Fourth ventricle. (Prepared by Dr. D. G. Montemurro.)
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