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districts of gray cortex which may be far removed from each other. The better known of these fasciculi are the following: (1) the uncinate bundle; (2) the cingulum; (3) the superior longitudinal bundle; (4) the inferior longitudinal; and (5) the occipito-frontal.
The fasciculus uncinatus is composed of fibres which arch over the stem of the
Cavum septi pellucidi
FIG. 576.-TWO FRONTAL SECTIONS THROUGH THE CEREBRAL HEMISPHERES OF AN ORANG,
A, Section through the left hemisphere in a plane a short distance behind B, which is a section through the right hemisphere. The positions of the great longitudinal association tracts are indicated in red.
Fasciculus longitudinalis superior
lateral cerebral fissure and connect the frontal pole, and the orbital gyri of the frontal lobe, with the anterior portion of the temporal lobe.
The cingulum is a very well-marked and distinct band, which is closely associated with the medial edge of the neopallium. Beginning in front, in the region of the anterior perforated substance, it arches round the genu of the corpus callosum and
FIG. 577.-DIAGRAMS OF THE LEADING ASSOCIATION BUNDLES OF THE CEREBRAL HEMISPHERE.
B, Medial aspect of hemisphere.
is carried backwards on the superior surface of this structure at the place where its fibres pass into the callosal radiation. The cingulum, therefore, lies under cover of the gyrus cinguli and stands in intimate relation to the white centre of this gyrus (Fig. 559, p. 631). At the posterior end of the corpus callosum the cingulum turns round the splenium and is carried forwards, in relation to
the hippocampal gyrus, to the uncus and the temporal pole. The cingulum is composed of several systems of fibres which run only for short distances within it.
The fasciculus longitudinalis superior is an arcuate bundle which is placed on the lateral aspect of the foot or basal part of the corona radiata and connects the frontal, occipital, and temporal regions of the hemisphere. It lies in the base of the superior operculum and sweeps backwards over the insular region to the posterior end of the lateral cerebral fissure. Here it bends downwards round the posterior end of the putamen and proceeds forwards in the temporal lobe, to reach its anterior extremity. As it turns downwards to reach the temporal lobe numerous fibres radiate from it into the occipital lobe.
The fasciculus longitudinalis inferior is a very conspicuous bundle which extends along the whole length of the occipital and temporal regions (Fig. 577, B). Curran has recently demonstrated that the fasciculus uncinatus and the inferior longi
Optic radiation seen in a gap cut out of
FIG. 578.-DISSECTION TO DISPLAY SOME OF THE PRINCIPAL ASSOCIATION BUNDLES OF THE CEREBRAL
The occipito-temporal extremity of the superior longitudinal bundle has been cut away in order to expose the subjacent inferior occipito-frontal bundle, parts of which in turn have been removed to expose the origin and termination of the still deeper optic radiation (coloured blue); (acoustic fibres, yellow).
tudinal bundle are merely the shorter inferior fibres of a much bigger and longer tract (Fig. 578), to which he has applied the name occipito-frontalis inferior. The arrangement of these longitudinal tracts may be put concisely by saying that fibre connexions of differing lengths link together the various cortical areas in the longitudinal direction, the deeper fibres (i.e. those furthest removed from the cortex, medial, lateral, superior or inferior) being progressively longer than the superficial. The deepest fibres extend the whole length of the hemisphere and are pushed aside by the insula (Fig. 578) and collected into two great bundles, a superior longitudinal and an inferior occipito-frontal bundle. In the occipital lobe the inferior occipito-frontal bundle is placed on the lateral aspect of the optic radiation, which takes a similar direction and from which it is distinguished by the greater coarseness of its fibres (Figs. 576, p. 649; 578; 559, p. 631). It is not present in the macaque monkey (Ferrier and Turner), but is well developed in the orang and the chimpanzee.
The fasciculus occipito-frontalis superior is a bundle of fibres which runs in a sagittal direction in intimate relation to the lateral ventricle (Fig. 576, p. 649). It may be regarded as the medial edge of the superior longitudinal bundle. It has been pointed out (Forel, Onufrowicz, and others) that, in cases where the corpus callosum fails to develop, the tapetum remains apparently unaffected, and Déjerine has endeavoured to prove that the fibres of this layer really belong to the fasciculus occipito-frontalis. The fasciculus occipito-frontalis lies on the medial aspect of the corona radiata in intimate relation to the caudate nucleus, and posteriorly it spreads out over the superior and lateral aspect of the lateral ventricle, immediately outside the ependyma, where, according to Déjerine, it constitutes the tapetum (see p. 632).
Projection Fibres.-We have already seen that every part of the cerebral cortex is linked to other cortical areas, not only in its own neighbourhood (short association fibres) (Fig. 578), but also in the most distant parts of the hemisphere (long association fibres), as well as to the cortex of the other hemisphere (commissural fibres). In addition there are two large series of fibres: (i.) an ascending group which conveys to the cerebral cortex impulses coming from the thalamus and metathalamus, the corpora quadrigemina and the red nucleus, and the various other sensory nuclei scattered throughout the brain stem and spinal medulla; and (ii.) a descending group connecting the cerebral hemisphere with the corpus striatum, various parts of the diencephalon, mesencephalon and cerebellum, as well as with all the motor nuclei scattered throughout the central nervous system. These two groups of tracts, respectively passing to and from the cerebral cortex, are known collectively as its projection fibres.
While examining the general arrangement of these projection fibres of the cerebral hemisphere it is convenient to refer incidentally to certain other fibre-tracts which do not fall strictly within this group.
Post. fun. of
Post. roots of
Thalamic radiation -Bulbothalamic tract
FIG. 579.-DIAGRAM OF THE SENSORY TRACTS FROM THE
The Sensory Tracts.-A certain proportion of the fibres that enter the spinal medulla by its posterior root, which are supposed to be the sensory nerves of muscles, tendons, and joints, pass upwards without interruption in the posterior funiculi throughout the whole length of the spinal medulla until they reach the medulla oblongata, where they end in the nucleus gracilis and nucleus cuneatus. From these nuclei, arcuate fibres (fasciculus bulbothalamicus) arise and, after crossing the median plane, proceed upwards in the medial lemniscus of the other side to end in the ventro-lateral nucleus of the thalamus, from which a third group of neurones arises and proceeds upwards through the internal capsule to the cerebral cortex, where the impulses conveyed by it excite a consciousness of position and movement. But other sensory fibres end in the spinal medulla near their place of entry into it, and from the cells related to the endings of these fibres a new tract (fasciculus spinothalamicus) arises, crosses the median plane to reach the antero-lateral funiculus of the opposite side, in which it proceeds upwards throughout the whole length of the spinal medulla (that lies above its origin), the rhombencephalon and mesencephalon to the thalamus, where it ends alongside the bulbo-thalamic tract in
cells of Betz in the posterior part of the precentral cortex (p. 663) in the district immediately in front of the sulcus centralis. The fibres descend through the corona radiata into the posterior limb of the internal capsule. From this point the further course of the pyramidal tract has been traced, viz., through the central part of the basal region of the cerebral peduncle and pons, and the pyramid of the medulla oblongata. At the level of the foramen magnum it decussates in the manner already described, and enters the spinal medulla as the lateral cerebrospinal and anterior cerebro-spinal tracts. The fibres composing these end in connexion with the ventral or motor column of cells, from which the fibres of the anterior roots of the spinal nerves arise.
Similar fibres arise from the inferior part of the precentral area and proceed through the internal capsule and cerebral peduncle to all the motor nuclei upon the opposite side of the brain stem (fasciculi cerebronucleares). Hence the cerebral cortex of one hemisphere can control all the muscles of the opposite side of the body.
The fronto-pontine strand is composed of fibres which arise as the axons of the cells in the cortex which covers the frontal region that lies in front of the precentral furrows. It descends in the anterior limb of the internal capsule, enters the medial part of the base of the cerebral peduncle, through which it gains the basilar part of the pons. In this its fibres end amongst the cells of the nuclei pontis, from which axons arise and establish relations with the cortex of the opposite cerebellar hemisphere.
The temporo-pontine tract consists of fibres which spring from the cells of that part of the cortex which covers the middle portions of the lower two temporal gyri. The temporo-pontine tract passes medially under the nucleus lentiformis, enters the retrolenticular part of the posterior limb of the internal capsule, and thus gains the lateral part of the cerebral peduncle. From this it descends into the basilar part of the pons, in which it ends in the nuclei pontis.
Cortico-striate and other Descending Fibres.- From the fibres of the internal capsule numerous collateral branches are given off to the nucleus caudatus and nucleus lentiformis, and from these basal ganglia fibres arise which enter the cerebral peduncle as constituent elements of the great cerebro-spinal tract.
Some of the fibres from the corpus striatum, especially the nucleus lentiformis, as well as others descending from the frontal cortex, pass into the red nucleus (Fig. 571), which also receives afferent tracts from the tectum mesencephali and from the cerebellum: it emits an important efferent tract (fasciculus rubrospinalis), which crosses the median plane and descends in the brain stem and spinal medulla to the various motor nuclei (see Figs. 454 and 475).
THE SULCI AND GYRI OF THE CEREBRAL HEMISPHERES. Fissura Cerebri Lateralis (O.T. Fissure of Sylvius). This is the most conspicuous furrow on the surface of the cerebral hemisphere. In reality it is formed, not as a furrow upon the surface of the hemisphere, but as a great fossa, the margins of which develop into large lip-like folds that bulge over the fossa and meet to form the superficial pattern of the lateral fissure. It is composed of a short main stem, from the lateral extremity of which two or three branches or limbs radiate. The stem of the lateral fissure is placed on the inferior surface of the hemisphere. It begins at the substantia perforata anterior and passes laterally, forming a deep cleft between the temporal pole and the orbital surface of the frontal region. Appearing on the lateral surface of the hemisphere, the fissure immediately divides into two or three radiating rami. These are: (1) the ramus posterior; (2) the ramus anterior horizontalis; (3) the ramus anterior ascendens, of which the last is inconstant.
The posterior ramus is the longest and most constant of the three limbs. It extends backwards, with a slight inclination upwards, on the lateral surface of the hemisphere for a distance which may vary from about two to three inches. It intervenes between the frontal and parietal regions, which lie above it, and the temporal region which lies below it; and it finally ends in the region subjacent to