Last's Anatomy Chapter 7: Central Nervous System PDF

# 7. Central Nervous System ## General Plan The central nervous system consists of the brain and spinal cord (spinal medulla). Developmentally the brain consists of the forebrain, midbrain and hindbrain. The forebrain is composed of the cerebrum (the two cerebral hemispheres, each with a cavity, t...

# 7. Central Nervous System ## General Plan The central nervous system consists of the brain and spinal cord (spinal medulla). Developmentally the brain consists of the forebrain, midbrain and hindbrain. The forebrain is composed of the cerebrum (the two cerebral hemispheres, each with a cavity, the lateral ventricle), and a deeper central portion, the diencephalon, whose main parts are the thalamus and hypothalamus and whose cavity is the third ventricle. Despite this academic distinction between parts, each half of the forebrain is commonly called simply the cerebral hemisphere. The midbrain is a small region whose cavity is the aqueduct and which connects the forebrain with the hindbrain, consisting of the pons, medulla oblongata and cerebellum and whose cavity is the fourth ventricle. The midbrain, pons and medulla collectively form the brainstem. All parts of the brain are contained within the cranial cavity; the medulla passes through the foramen magnum of the skull and changes its name to spinal cord where the first cervical nerve roots emerge. Cerebrospinal fluid is produced by the choroid plexuses within parts of the ventricles; its only exit is through foramina in the roof of the fourth ventricle, in the medulla. ## PART 1 ## Cerebral Hemispheres The cranial hemispheres occupy the greater part of the cranial cavity - above the floors of the anterior and middle cranial fossae, and above the tentorium cerebelli. One hemisphere, usually the left in right-handed people, is slightly larger than the other and constitutes the dominant hemisphere. The medial surface of each hemisphere is flat and lies against the falx cerebri; below the falx the two hemispheres are joined by the corpus callosum. The under surface of the hemisphere is more irregular than the medial surface; the orbital surface of the frontal lobe is slightly concave from the impression of the anterior cranial fossa, the temporal pole is boldly convex in conformity with the middle cranial fossa while the under surface of the occipital lobe slopes downwards and outwards to conform with the shape of the tentorium. The under surfaces of the two hemispheres are joined by the cerebral peduncles of the midbrain; anteriorly lie the structures of the under surface of the diencephalon (Fig. 7.22). The lateral surfaces of the hemispheres are boldly convex in conformity with the shape of the skull; the more complete term 'superolateral' is usually applied to this convex surface. All surfaces of the cerebral hemisphere are covered with a cortex of grey matter (the cells of the cerebral cortex), and internally there are further groups of cells that form such structures as the basal nuclei and thalamus. The cortex is thrown into a complicated series of tortuous folds, the gyri; the grooves between them are the sulci (Fig. 7.1). All the gyri and sulci are named but only the most important are here described. Although the patterns of no two brains are identical, there is always an underlying similarity and this general pattern common to all must be appreciated. Some of the larger sulci are used to divide the surface of the hemisphere into lobes which are named according to the cranial bones that lie adjacent when the brain is in situ: frontal, temporal, parietal and occipital lobes. Further details are given below but the essential features can be summarized by stating that the frontal lobe lies in front of the central sulcus and above the lateral sulcus; the parietal lobe is behind the central sulcus and above the lateral sulcus; the temporal lobe is below the lateral sulcus; and the occipital lobe lies below and behind the parieto-occipital sulcus. ## Surface Features ### Superolateral Surface A deep fissure that separates the frontal and temporal lobes on the under surface of the brain is continued to the lateral surface and passes backwards, above the temporal lobe. This is the lateral sulcus (fissure of Sylvius) (Fig. 7.1), although strictly speaking the part on the lateral surface is the posterior ramus of the lateral sulcus, for, at the front end of this part, there are short anterior and ascending rami that branch off from it to penetrate the inferior frontal gyrus. The areas of cortex bounding the short sulci are the orbital, triangular and opercular parts of the inferior frontal gyrus, and they have to be noted because on the left hemisphere this is the region of the motor speech area (of Broca, p. 586). The triangular part with the adjacent areas of parietal and temporal cortex form the opercula which overlie a buried part of the cortex, the insula (Fig. 7.2), composed of various long and short gyri almost completely surrounded by the circular sulcus. An oblique sulcus passes up from just behind the opercula to indent the superior border of the hemisphere just behind the midpoint. It is the only long sulcus to pass over on to the medial surface of the hemisphere. This is the central sulcus (fissure of Rolando) and it separates frontal and parietal lobes (Fig. 7.1). The precentral and postcentral gyri lie in front of and behind it; they contain the motor and sensory cortical areas. In front of the precentral gyrus the frontal lobe is divided by two horizontal sulci into three gyri, the superior, middle and inferior frontal gyri. A similar arrangement divides the temporal lobe below the lateral sulcus into superior, middle and inferior temporal gyri. Note that the central sulcus does not usually run directly into the lateral sulcus a useful point when identifying the central sulcus. The parietal lobe is divided by a transverse sulcus into superior and inferior parietal lobules. Into the latter project the lateral sulcus and the superior temporal sulcus; the posterior ends of these sulci are closed by the curved supramarginal and angular gyri respectively. An imaginary line divides the occipital lobe from the parietal and temporal lobes. It extends from the small part of the parieto-occipital sulcus visible on this lateral surface, downwards in a 45° slope to the inferior border where there is often a slight preoccipital notch (Fig. 7.2) indented in the border by a fold of dura mater over the transverse sinus. A further arbitrary line, carried backwards from the main direction of the lateral sulcus until it meets the occipital demarcation line, indicates where the parietal and temporal lobes join. ### Medial Surface The two medial surfaces are flat and lie close together; they can be inspected only when their midline connexions are divided by sagittal section (Fig. 7.3). Such a section severs the corpus callosum and the roof and floor of the third ventricle, as well as the brainstem and cerebellum if these are still attached to the cerebral hemispheres. The wall of the third ventricle thus exposed beneath the corpus callosum is described on page 593. The medial surface of the hemisphere above the corpus callosum forms the cingulate gyrus, above which is the cingulate sulcus. Above the sulcus the medial frontal gyrus extends, anteriorly, to the superior border of the hemisphere. Just behind the midpoint of the superior border the central sulcus turns on to the medial surface; it is enclosed in the paracentral lobule. At the posterior end of the hemisphere, the oblique parieto-occipital sulcus separates the parietal from the occipital lobe; it extends over the superior border to appear, as previously noted, on the superolateral surface. The medial surface of the occipital lobe is wedge-shaped and is named the cuneus. Between the parieto-occipital sulcus and the paracentral lobule is the precuneus. The cuneus is limited inferiorly by the calcarine sulcus which runs forward from the occipital pole to the medial surface of the temporal lobe. The sulcus is of great importance because of the associated visual area of the cortex (p. 586). The parieto-occipital sulcus runs into it. In old terminology the part of the calcarine sulcus below the cuneus was called the posterior calcarine sulcus. Note that the parieto-occipital and calcarine sulci form an easily identifiable pattern like the letter Y on its side; the common stem of the Y is the anterior part of the calcarine sulcus, and the two limbs are the parieto-occipital sulcus and the posterior part of the calcarine sulcus. The lingual gyrus lies below the posterior part of the calcarine sulcus and is limited at the border between the medial and inferior surfaces of the occipital lobe by the collateral sulcus (see below). ## Internal Structure The interior of the cerebrum is characterized by the presence within the white matter of large masses of grey matter and also by cavities which contain the cerebrospinal fluid. The largest mass of cells in each hemisphere is the thalamus. It belongs to the diencephalon, the central part of the forebrain, and is described on page 595. Other cell groups belong to the lateral part of the forebrain and some of them constitute the basal nuclei (still commonly called by their old name, basal ganglia). They are usually classified anatomically as consisting of the caudate nucleus, lentiform nucleus (which has an outer part, the putamen, and an inner part, the globus pallidus), the amygdaloid body and the claustrum. Unfortunately there is no agreement on what should be included among the basal nuclei; the amygdaloid body is often excluded and placed among the components of the limbic system because of its association with memory and behaviour, while the substantia nigra and subthalamic nucleus (although mainly in the midbrain rather than the cerebrum) are often included because of their profuse connexions with the lentiform nucleus. However, among these uncertainties there are two highly important and universally recognized facts: (1) the caudate nucleus and the putamen part of the lentiform nucleus are joined by many interconnecting fibres to form what is collectively known as the corpus striatum (from the striated naked-eye appearance), and (2) fibres from the globus pallidus part of the lentiform nucleus form the main efferent pathway from the corpus striatum. The caudate nucleus has the shape of a highly curved comma (Figs 7.5 and 7.6) with a head, body and tail. The bulbous head tapers back to the body which, curving back round the lateral part of the thalamus, bends sharply forwards into the long thin tail that joins the amygdaloid body. The caudate nucleus is curled snugly round the internal capsule like a hand holding a bunch of flowers. The whole length of its convexity projects into the lateral ventricle. The lentiform nucleus is the shape of a biconvex lens, completely buried in the hemisphere (Figs 7.6 and 7.7). It is oval in outline and has two parts: the large lateral putamen, curved and roughly quadrilateral in shape, and the small medial globus pallidus, bluntly conical. The putamen is joined to the head of the caudate nucleus by bundles of myelinated and unmyelinated fibres which, by passing through the anterior part of the internal capsule, give the area a striated appearance. As noted above the corpus striatum is the collective name for the caudate nucleus and the putamen part of the lentiform nucleus.. The amygdaloid body (often called 'the amygdala' by clinicians) consists of several nuclear masses and is connected with the tip of the tail of the caudate nucleus in the roof of the inferior horn of the lateral ventricle. It is functionally part of the limbic system (p. 588). The claustrum is a thin lamina, circular in outline and curved into a saucer-shape. It lies lateral to the putamen, and although easy to identify in horizontal or coronal sections (e.g. Fig. 7.7) its significance is unknown. Other functional components of the basal nuclei are considered elsewhere: the subthalamic nucleus on page 594, and the substantia nigra on page 606. Functionally the basal nuclei exert a supraspinal control over skeletal muscle movements by influencing their rate, range and co-ordination. The corpus striatum, often called simply 'the striatum' by neurologists, can be regarded as the input side of the basal nuclei, receiving fibres mainly from the cerebral cortex, thalamus and substantia nigra. The globus pallidus ('the pallidum') is the output side, sending fibres to the thalamus, and also to the subthalamic nucleus, substantia nigra and the reticular formation. The fibres to the thalamus run in two large bundles, the ansa lenticularis and the ansa fascicularis, which pass ventral and dorsal to the subthalamic nucleus respectively. The ansa fascicularis passes through the fibres of the internal capsule, as does another bundle, the subthalamic fasciculus, intercon-necting the globus pallidus and the subthalamic nucleus. Thus there are, for example, cortico-striato-pallido-thalamo-cortical pathways by which these subcortical cell groups can exert their influence on movement. Different pathways involve different transmitters which include acetyl choline, dopamine, glutamate, serotonin and GABA. The commonest disease of the basal nuclei is parkinsonism, characterized by tremor, rigidity and akinesia; there is a decrease of dopamine in the nigro-striatal pathway. The white matter of the cerebral hemisphere is made up of fibres belonging to three main groups. * **Commissural fibres** join the cortices of the two hemispheres. Most of them are gathered together in the corpus callosum; a few lie in the anterior, posterior and habenular commissures. They radiate widely and symmetrically through the white matter of the hemispheres. * **Association (arcuate) fibres** are confined to their own hemisphere, in which they connect different parts of the cortex. * **Projection fibres** are those which join the grey matter of the hemisphere with subcortical nuclei in the hemispheres and with nuclei in the brainstem and spinal cord. The verb 'project' is often used to indicate a connexion between one structure and another, e.g. the caudate nucleus projects to the putamen, meaning that the axons of the cell bodies in the caudate nucleus pass to make synaptic connexion with cell bodies in the putamen. In the base of the hemisphere a major collection of projection fibres lies lateral to the thalamus and the head of the caudate nucleus, forming the internal capsule. The lentiform nucleus lies lateral to the internal capsule and the tail of the caudate nucleus curls around, also lateral (Fig. 7.5). From the internal capsule the fibres radiate upwards and outwards in the shape of a curved fan to reach the cortex and similarly pass from the cortex down to the capsule; this fan-shaped arrangement is the corona radiata. Fibres of the corpus callosum intersect it. ### Internal Capsule The internal capsule consists of afferent fibres passing up to the cortex from cell bodies in the thalamus, and of efferent fibres passing down from cell bodies in the cortex to the cerebral peduncle of the midbrain. It lies within the concavity of the C-shaped caudate nucleus, which separates it from the C-shaped concavity of the lateral ventricle (Figs 7.5 and 7.7). The internal capsule is seen in a typical horizontal section through the hemisphere (e.g. at a level through the interventricular foramen and the pineal body) as a band of white matter that is not a straight line but bent into a lateral concavity by the convex medial border of the lentiform nucleus (i.e. by the globus pallidus). Because of this almost L-shape the internal capsule is described as having an anterior limb, genu and posterior limb, and there are also two other portions posteriorly: the sublentiform and retrolentiform parts. The anterior limb lies between the head of the caudate nucleus medially and the lentiform nucleus laterally. It contains frontopontine fibres from cell bodies in the frontal cortex. They pass down below the thalamus into the cerebral peduncle, where they occupy the medial third of the base of the peduncle. They arborize round the pontine nuclei. The anterior limb also probably contains fibres running from the frontal eye field to the oculomotor nucleus, concerned with the accommodation-convergence reflex (p. 518). The genu is the region of the bend in the capsule (as seen in horizontal section), at the apex of the globus pallidus. Its principal constituents are the corticonuclear fibres which pass from the cerebral cortex to the motor nuclei of cranial nerves in the brainstem (p. 612). The posterior limb lies between the thalamus medially and the lentiform nucleus laterally. Occupying the anterior two-thirds of the posterior limb (right behind the corticonuclear fibres in the genu) are the corticospinal fibres. From cell bodies in the cortex the fibres pass down through this part of the capsule, then through the brainstem to the lower medulla where most of them decussate to form the lateral corticospinal tract and eventually arborize with the anterior horn cells that innervate skeletal muscle. Thus passing through a small part of the internal capsule - genu and anterior two-thirds of the posterior limb are the motor fibres that control all the skeletal muscle in the body. The head (corticonuclear) fibres lie most anteriorly and immediately behind them are corticospinal fibres for the arm, hand, trunk, leg and perineum in that order from front to back. (In the cerebral peduncle of the midbrain the head fibres lie medially and the fibres for the perineum laterally, in the same order). It is in this part of the internal capsule that haemorrhage or thrombosis of a striate artery commonly occurs. The muscles of the opposite side of the body are thus paralysed; they become spastic with increased stretch reflexes, the signs of an upper motor neuron lesion (p. 625). Fibres from the speech (Broca's) area are interrupted in lesions of the left internal capsule; thus loss of speech accompanies hemiplegia of the right side of the body. By reading this paragraph you have read about some of the most important facts in the whole of human anatomy. Beside and behind the corticospinal fibres in the posterior limb of the capsule there are thalamocortical fibres passing from cell bodies in the thalamus to the cerebral cortex. These include sensory fibres mediating impulses derived from the opposite side of the body which run upwards through the corona radiata to the sensory cortex. There are also large numbers of corticopontine fibres. In the retrolentiform part of the capsule, at the posterior end of the lentiform nucleus, are parieto-, occipito- and temporopontine (corticopontine) fibres which will occupy the lateral third of the base of the cerebral peduncle. But much more importantly, this part of the capsule contains visual fibres passing from cell bodies in the lateral geniculate body to the visual area of the cortex as the optic radiation (p. 587). A further group of fibres runs from the medial geniculate body below the posterior end of the lentiform nucleus, so forming the sublentiform part of the capsule. These are the fibres of the auditory radiation which reach the auditory area of the cortex in the superior temporal gyrus (p. 586). Although the corticopontine fibres (about 20 million) form the largest group of internal capsule components, the corticonuclear, corticospinal and thalamocortical fibres and those of the optic radiation are of greater clinical importance, though much smaller in number. The corpus callosum (Fig. 7.3) consists of a mass of 100 million commissural fibres, each of which extends from cortex to cortex between symmetrical parts of the two hemispheres. It commences at the anterior commissure, at the upper end of the lamina terminalis of the diencephalon and, traced from here to its termination, it becomes increasingly thicker. It is described as having four parts: the rostrum, genu, body and splenium. From the anterior commissure the mass passes upwards and forwards as the rostrum. It now takes a sharp bend backwards as the genu. From here it is gently convex upwards (the body of the corpus callosum) and it ends posteriorly as a thick rounded free border, the splenium. The corpus callosum can be seen by separating the two hemispheres, and its cut surface is exposed in a midline sagittal section through the brain (Fig. 7.11). The fibres of the corpus callosum extend to all parts of the cerebral cortex. In a horizontal section the fibres of the genu are seen arching forwards on each side to the frontal cortex; this appearance gives them the name forceps minor. Similarly, the fibres of the massive splenium curve backwards symmetrically to the occipital cortex, forming the forceps major (Fig. 7.7). Between forceps minor and forceps major the fibres of the corpus callosum spread out to the cortex on the lateral surface of the hemisphere. They pass across the anterior horn and body of the lateral ventricle, for each of which they form the roof. As they turn down into the temporal lobe they form the lateral wall of the inferior and posterior horns of the lateral ventricle, where they are known as the tapetum. ## Cortical Areas Certain areas of the cerebral cortex have long been identified with specific functions. Although these areas are still clinically relevant, modern investigations are modifying traditional concepts as far as the separation of motor and sensory functions is concerned. Many motor fibres, for example, have their origin outside the traditional motor cortex and some arise from what were previously regarded as purely sensory areas. A new terminology has emerged, and it is now customary to refer to a combined 'sensorimotor cortex', subdividing it into four areas designated by the letters Ms and Sm the capital M or S indicating whether the association is predominantly with motor or sensory functions. Thus the area MsI (first or primary motosensory area) includes the old 'motor and premotor' regions of the precentral and other gyri of the frontal lobe (corres-ponding to areas 4 and 6 as described by Brodmann in his now classical study of cortical histology). The area MsII (the supplementary motor area) is on part of the medial surface of the frontal lobe (part of areas 6 and 8). Similarly SmI (first sensorimotor area) includes most of the postcentral gyrus (areas 3, 1 and 2) and its extension on to the medial surface of the parietal lobe, and SmII is the lowest part of the postcentral gyrus (areas 40 and 43). These four main motor and sensory areas have many interconnexions, both within their own and with the opposite hemisphere. The area MsI is where movements of the various parts of the body are initiated, and it receives its main inputs from the cerebellum and thalamus. Some of the cortical cells send their axons down as the corticonuclear and corticospinal (pyramidal) tracts (p. 612). MsII receives many fibres from the basal nuclei and is concerned with postural mechanisms, but this area is not yet clearly understood. In the precentral gyrus of MsI the body is represented upside down along this cortex, although the face itself is represented the right way up. The face lies lowest, then the hand (a very large area), then arm, trunk and leg. The leg and perineum areas overlap the superior border and extend down on the medial surface of the hemisphere into the paracentral lobule. The motor (anterior) speech area (of Broca, areas 44 and 45) is usually situated in the inferior frontal gyrus on the left side (in right-handed and in most left-handed people), below and in front of the face area and centred on the pars triangularis between the anterior and ascending rami of the lateral fissure. Damage to it produces motor aphasia difficulty in finding the right words, but not paralysis of laryngeal musculature. The posterior speech area (of Wernicke) is in the posterior parts of the superior and middle temporal gyri and extends into the lower part of the parietal lobe. Its integrity is necessary for the understanding of speech. The frontal eye field, involved in voluntary eye movements and the accommodation pathway (p. 519), is in the centre of the middle frontal gyrus (parts of areas 6, 8 and 9). The areas SmI and SmII receive a large thalamic input. SmI is for the appreciation of touch, kinaesthetic and vibration sense, and the parts of the body are represented in roughly the same way as in MsI. SmII appears to be associated with pain and temperature sensations. Although the conscious appreciation of pain may occur at the thalamic level, the cortex is necessary for its localization. The gustatory area for the conscious appreciation of taste lies in the inferior part of the postcentral gyrus (frontoparietal operculum), near the tongue area of SmI. The auditory area (areas 41 and 42) is mostly hidden in the lateral sulcus, in the anterior transverse temporal gyrus. It extends into the superior temporal gyrus below the sulcus, and is here surrounded by the auditory association area (area 22). These regions receive fibres from the medial geniculate body via the auditory radiation. The cochleae are bilaterally represented, so a lesion of one cortex does not cause deafness. The olfactory area is in the uncus at the front of the parahippocampal gyrus (Fig. 7.3A) and adjacent parts of the cortex. The visual area (area 17) is mainly on the medial surface of the occipital lobe in the depths of the calcarine sulcus; more precisely, it lies along the lower lip of the anterior part of the sulcus and along both upper and lower lips of the posterior part, and it extends for a short distance (1 cm or so) on to the lateral surface of the occipital lobe as far as the lunate sulcus. The true visual area is characterized by a white line (stria of Gennari) which bisects the grey matter of the cortex; in cortical sections it is easily seen with the naked eye, hence the name 'striate cortex' often given to this area. The cortex adjacent to the striate part on the medial and lateral surfaces of the hemisphere (and which does not have the stria) forms the visual association area (areas 18 and 19). Each visual area receives from its own half of each retina, i.e. it registers the opposite visual field. In each cortex the upper half receives from the upper half of each half-retina, the lower half from the lower half of each half-retina, i.e. the upper and lower visual fields are crossed. The macula registers at the posterior end of the visual area and more peripheral parts of the retina progressively more anteriorly. When thinking about visual anatomy, do not confuse parts of the visual fields with parts of the retina. The temporal (lateral) half of the visual field of one eye conveys its impressions to the nasal (medial) half of the retina of that eye; similarly the temporal half of the retina receives its impressions from the nasal half of the visual field. ### Visual Pathways The peripheral nerves of ordinary sensation, with their cell bodies in posterior root ganglia, are represented in the visual pathway by the bipolar cells of the retina (Fig. 7.8A). These cells receive impulses from the retinal rods and cones. The bipolar cells synapse with ganglion cells in the inner part of the retina (next to the vitreous body, 1 cell for each cone, 1 cell for 80 rods). These are homologous with the second neuron cell bodies in the central nervous system in the other sensory pathways. Their axons run on the surface of the retina and enter the optic disc and so pass to the optic nerve. The optic nerve is not a nerve in the sense of the other cranial and spinal nerves; it is an elongated tract of white matter stretched out from the brain and enclosed in the meninges thereof as far forward as its attachment to the sclera. Histologically it is identical with white matter of the central nervous system, and there is no effective regeneration when divided. In the orbit it is surrounded by a tube of dura mater and arachnoid, with cerebrospinal fluid in the subarachnoid space. At the optic foramen the dura and arachnoid leave it and the nerve, still sheathed in pia mater, passes up to meet its fellow at the optic chiasma, which is attached to the anterior part of the floor of the third ventricle. In the chiasma the nasal fibres of each optic nerve decussate and pass into the optic tract of the opposite side; the temporal fibres from each retina pass on to their own side (Fig. 7.8A). Thus the right optic tract contains fibres from the right half of each retina, i.e. it carries impressions from the nasal field of the right eye and the temporal field of the left eye. Likewise, the left optic tract contains fibres from the left half of each retina and since there is no further decussation this holds true right back to the visual cortex. Cortical pathways for common sensation consist of three neurons. They reach the opposite hemisphere by a complete decussation of the second order neurons. The visual pathway by the half decussation of its second order neurons at the chiasma achieves the same object. There is complete crossing of the visual fields. One hemisphere registers common sensation from the opposite half of the body and also from the opposite half of the visible environment. The optic tract passes from the chiasma around the cerebral peduncle, high up against the temporal lobe and, reaching the side of the thalamus, divides into two branches. The larger of these enters the lateral geniculate body, in which the fibres synapse. These are visual fibres. The smaller branch (superior brachium see below) passes down medially, between the lateral and medial geniculate bodies, and synapses in the superior colliculus and the pretectal nuclei; these are fibres mediating light reflexes (p. 518). **Blood supply.** The optic tract is supplied chiefly by the anterior choroidal and posterior communicating arteries, the chiasma and intracranial part of the optic nerve by the anterior cerebral. In the orbit the nerve is supplied by the ophthalmic artery and, distally, by the central artery on its way to the retina. The lateral geniculate body, which is a part of the thalamus, is a small rounded elevation below the pulvinar on the posterior surface of the thalamus; identify it by following the optic tract backwards into it (Fig. 7.4). From the ganglion cells of the second neuron (in the retina) axons pass in the optic nerve, chiasma and optic tract to synapse with cells in six layers of the geniculate body. The fibres from the half-retina of the same side (i.e. temporal fibres) synapse at layers 2, 3 and 5, those from the half-retina of the opposite side (i.e. nasal fibres) synapse at layers 1, 4 and 6. From these layers the cell bodies send their axons through the optic radiation to the occipital cortex (Fig. 7.7). Because these fibres are proceeding from the lateral geniculate body to the visual cortex which largely borders the calcarine sulcus, the optic radiation is sometimes called the geniculocalcarine tract. The superior brachium is the name given to the small (medial) branch of the optic tract. It passes down on the thalamus to the midbrain, where it ends in the tectum. The fibres in the superior brachium arborize around cells in the superior colliculus. The cell bodies in the superior colliculus send their fibres, by tectobulbar and tectospinal tracts, to motor nuclei in the brainstem and spinal cord for the mediation of general light reflexes (e.g. reflex blinking and jumping or turning away from a flash of bright light). The superior colliculi are united by the posterior commissure (at the entrance to the aqueduct, Fig. 7.11), and thus general body reflexes to light are usually bilateral. The special fibres concerned in the pupillary light reflex (p. 518) do not synapse in the colliculus, but pass bilaterally to each pretectal nucleus. The pretectal nucleus is a small group of cells lying in the tegmentum under the upper and lateral margin of the superior colliculus. It passes light impulses to each Edinger-Westphal nucleus and so to the sphincter pupillae. A lesion here produces the Argyll Robertson pupil; contraction to light is lost, but the pupil still contracts to accommodation and convergence. (The pathway of these reflexes is explained on p. 518.) ## Limbic System and Olfactory Pathways Surrounding the corpus callosum and diencephalon are a number of features that have come to be known collectively as the limbic system. Because the olfactory tracts and its associated structures were originally included in this descriptive concept, much of its function was thought to be concerned with olfaction. However, this view in no longer tenable and it is now known to play a role in such abstract functions as behaviour, mood and memory; thus lesions of one of its major constituents, the hippocampus, result in loss of memory for recent events, although the memory of distant events is retained. Much remains to be discovered, but for the present purpose it is sufficient to note the component parts and to comment on selected items that have not been mentioned previously. ### Limbic System Apart from the olfactory nerves, bulb and tract (see below), the following are among the major components of the limbic system: 1. The septal and piriform areas of cerebral cortex, near the lamina terminalis (anterior boundary of the third ventricle, p. 592) 2. The uncus (p. 581), the insula (p. 578), and the cingulate and parahippocampal gyri (pp. 579 and 581) 3. The amygdaloid body (p. 582) 4. The hippocampus, fimbria, fornix and mamillary body (see below). Some authorities would also include the hypothalamus and the anterior part of the thalamus in view of their functional connexions with limbic structures. There is as yet no universal agreement on what should be included. ### Olfactory Pathways The olfactory tract, which is an elongated extension of the white matter of the brain (like the optic nerve), lies in the olfactory sulcus beside the gyrus rectus on the inferior surface of the frontal lobe (Fig. 7.4). Its anterior end is expanded as the olfactory bulb, containing the mitral cells with which the olfactory nerve filaments synapse after passing through the cribriform plate of the ethmoid (p. 473). The axons of the mitral cells run back in the tract to the anterior perforated substance, through which some of the fibres reach the region of the uncus (at the front of the parahippocampal gyrus, Fig. 7.4) and adjacent parts of the cortex. Other fibres make complex connexions with parts of the limbic system. Note that from the olfactory receptors in the nasal mucosa to the cortex there are two groups of neurons, and that the second neuron has reached the cortex without relay in the thalamus a unique occurrence Further synapses connect the olfactory bulb with the hypothalamus and brainstem, as is the case with other sensory pathways (light, sound, taste, touch) for visceral and somatic effects, distinct from conscious appreciation. ### Hippocampus Just above the anterior part of the parahippocampal gyrus (here known as the subiculum) lies the hippocampal sulcus, which is projected into the floor of the inferior horn of the lateral ventricle as the hippocampus. Viewed from above the anterior part of it (the pes hippocampi) has the appearance of the knuckles of a clenched fist (Fig. 7.7). On its ventricular surface is a thin film of white matter, the alveus; its cell bodies are in the hippocampus and subiculum. The fibres of the alveus thicken medially to form the fimbria. This breaks free from the hippocampus as the crus (posterior pillar) of the fornix. The dentate gyrus is a small part of the hippocampus which, as seen from the medial side, lies between the fimbria and the parahippocampal gyrus. ### Fornix The fornix is the great efferent pathway from the hippocampus. As a flat band continuous with the fimbria, it curves up behind the thalamus to join its fellow in a partial decussation across the midline, the commissure of the fornix. It is really a chiasma, and is an association tract rather than a true commissure. The conjoined mass of white matter, lying beneath the corpus callosum, is the body of the fornix. From it the conjoined anterior columns arch down in front of the anterior poles of the thalami, forming the anterior margins of the interventricular foramina (Fig. 7.11). The columns of the fornix pass both anterior and posterior to the anterior commissure. The anterior fibres pass mainly to the septal nuclei near the lamina terminalis (not in the septum pellucidum). The posterior fibres pass directly to the thalamus or into the mamillary body. From the mamillary body fibres pass in the lateral wall of the third ventricle as the mamillothalamic tract to the anterior pole of the thalamus. Here they relay and the thalamic neurons send their fibres through the internal capsule to the cingulate gyrus. On the upper surface of the corpus callosum is a thin film of grey matter, the induseum griseum, beneath which lie the medial and lateral longitudinal striae. These appear to be aberrant fibres of the fornix. ## Ventricles of the Brain The central nervous system is hollow; it develops from a neural tube whose cavity persists. The cavity is lines throughout with ependyma, a single epithelial-like layer of cells. The brain formation requires cerebrospinal fluid, whatever the reasons may be, and this fluid is produced within the cavity. The places where the cerebrospinal fluid is produced are the ventricles. In each ventricle the cavity comes to the surface without opening thereon; that is to say, the lining ependyma comes into contact with the surface pia mater, with no grey or white matter between. This is to allow the invagination of a mass of blood capillaries which thus becomes covered by pia and ependyma. This combination of capillaries, pia and ependyma constitutes the choroid plexus which secretes the cerebrospinal fluid. These vascular fringes invaginate the whole length of each surface encroachment of the ventricle. Each cerebral hemisphere possesses its cavity, the lateral ventricle, and this comes to the surface at a curved slit, the choroid fissure (p. 59