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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
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
relationship with cells of the ventro-lateral nucleus. The fibres arising from this nucleus proceed to the gyrus centralis posterior, and convey impulses to it, which may excite a consciousness of touch, pressure, pain, heat, or cold. Some of these spino-thalamic fibres enter the medial lemniscus in the medulla oblongata, but others remain separate from it (Fig. 580) until they reach the level of the pons, where they become added to the lateral margin of the bulbo-thalamic tract.
[In Fig. 580 the line from the label "lemniscus medialis" points to the place of junction of the spino- and bulbo-thalamic tracts.]
Other groups of fibres, serially homologous to both the spino-thalamic and the bulbo-thalamic tracts, come from the various sensory cerebral nerves-trigeminal,
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
the parietal tuberosity of the cranial wall by turning upwards into the parietal region in the form of an ascending terminal piece.
The anterior horizontal ramus extends horizontally forwards in the frontal region for a distance of not more, as a rule, than three-quarters of an inch, immediately above and parallel to the posterior part of the superciliary margin of the hemisphere.
The anterior ascending ramus proceeds upwards and slightly forwards, into the inferior part of the lateral surface of the frontal region for a variable distance (an inch or less). In many cases the two anterior limbs spring from a common stem of greater or less length, and not infrequently there is only a single anterior limb.
Sulcus Circularis.-If the lips of the posterior ramus of the lateral fissure are pulled widely asunder from each other, the insula (island of Reil) will be seen at the
FIG. 581.- -A DIAGRAM OF THE LATERAL ASPECT OF THE LEFT CEREBRAL HEMISPHERE.
The inferior frontal sulcus (the superior boundary of the inferior frontal gyrus), the middle frontal sulcus (separating the anterior and middle frontal areas), and the superior frontal sulcus (bounding the superior frontal gyrus) are not labelled.
bottom. The insular district of the cortex is completely hidden from view, when the lateral fissure is closed, by overlapping portions of the cerebral hemisphere, and, when brought into view in the manner indicated, it is observed to present a triangular outline and to be surrounded by a limiting sulcus, of which three parts may be recognised, viz., a superior part, bounding it above and separating it from the parietal and frontal regions; an inferior part, marking it off below from the temporal region; and an anterior part, separating it in front from the frontal region.
The insula consists of three areas of different structure. At the antero-inferior corner (where the sulcus circularis is deficient) the knee-like bend of the area piriformis (see Figs. 582 and 584) appears at the limen insulæ. The rest is subdivided by an oblique furrow (sulcus centralis insula) into a posterior part divided into gyri longi and an anterior part divided into gyri breves.
Opercula Insulæ.-The overlapping portions of the cerebral substance which cover over the insula are termed the insular opercula, and they form, by the apposition of their margins, the three rami of the lateral fissure. The rami of the fissure extend from the exposed surface of the hemisphere to the submerged surface of the insula, and, in this manner, separate the opercula from each other.
The temporal operculum (pars temporalis) extends upwards over the insula from the temporal region, and its superior margin forms the inferior lip of the posterior ramus of the lateral fissure.
The superior operculum is carried downwards from the parietal (pars parietalis) and frontal (pars frontalis) regions over the insula, and its inferior margin, meeting the temporal operculum, forms the superior lip of the posterior ramus of the lateral fissure.
The small triangular piece of cerebral substance which sometimes intervenes between the ascending and horizontal anterior rami of the lateral fissure is formed by the bending downwards of the front part of the upper operculum. It
Gyrus temporalis transversus anterior
FIG. 582.-PART OF A LEFT CEREBRAL HEMISPHERE WITH THE OPERCULA OF THE INSULA WIDELY
Area piriformis | Limen insulæ
Area acustica extending on to
for the most part, on the inferior surface of the hemio the medial side of the horizontal anterior ramus of the backwards from the orbital aspect of the frontal lobe insula.
Fissure and of the Insular District of the Cerebral Hemiatter half of the intra-uterine period of development that the ver the insula, so as to shut it out from the surface. In its nts the form of a depressed area on the side of the cer tinct boundary wall formed by the surrounding more 583, A). After a time this depressed area, which is c lar outline, and then the bounding wall is observed t 4., a superior or fronto-parietal, an inferior or temp 583, B). The angle formed by the meeting of the ndary may become flattened, and a short oblique par all triangular frontal operculum (Fig. 583, F). Each o of the fossa becomes a line of growth, from which an oper