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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
Intermediate part of inferior frontal gyrus Gyrus frontalis superior (anterior part) Anterior ascending ramus of lateral fissure
Anterior horizontal ramus of lateral cerebral fissure
Sulcus temporalis superior
Pars intermedia (superior temporal gyrus)
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
Area piriformis | Limen insulæ
Area acustica extending on to
Line of obliterated rhinal fissure the superior temporal gyrus FIG. 582.-PART OF A LEFT CEREBRAL HEMISPHERE WITH THE OPERCULA OF THE INSULA WIDELY SEPARATED TO EXPOSE THE INSULA AND THE SUPERIOR SURFACE OF THE TEMPORAL OPERCULUM. The area acustica is coloured a uniform blue, the area intermedia with large blue spots and the area circumambiens with fine blue dots.
covers over a small part of the anterior portion of the insula, and is sometimes termed the pars triangularis.
The orbital operculum is, for the most part, on the inferior surface of the hemisphere. It lies below and to the medial side of the horizontal anterior ramus of the lateral fissure, and proceeds backwards from the orbital aspect of the frontal lobe over the anterior part of the insula.
Development of the Lateral Fissure and of the Insular District of the Cerebral Hemisphere. It is only during the latter half of the intra-uterine period of development that the opercula take shape and grow over the insula, so as to shut it out from the surface. In its early condition the insula presents the form of a depressed area on the side of the cerebral hemisphere, surrounded by a distinct boundary wall formed by the surrounding more elevated surface of the hemisphere (Fig. 583, A). After a time this depressed area, which is called the fossa lateralis, assumes a triangular outline, and then the bounding wall is observed to be composed of three distinct parts, viz., a superior or fronto-parietal, an inferior or temporal, and an anterior or orbital part (Fig. 583, B). The angle formed by the meeting of the superior and anterior portions of the boundary may become flattened, and a short oblique part of the limiting wall develop into a small triangular frontal operculum (Fig. 583, F). Each of these portions of the bounding wall of the fossa becomes a line of growth, from which an operculum
tion of the corpus callosum). It thus comes about that every part of the cerebral cortex, with the exception of the bulbi olfactorii, the olfactory parts of the hemisphere, and the inferior and anterior part of the temporal lobe, is reached by the callosal fibres. But it should be clearly understood that all the regions of the cortex do not receive an equal proportion of fibres; in other words, some cortical areas would appear to be more plentifully supplied than others. Another point of some importance consists in the fact that the callosal fibres do not, as a rule, connect together symmetrical portions of the gray cortex. As the fibres cross the median plane they become greatly scattered, so that dissimilar parts of the cortex of opposite hemispheres come to be associated with each other.
The commissura anterior is a structure supplemental to the corpus callosum, although originally it was the principal cerebral commissure long before the corpus callosum was evolved. It connects together the two olfactory bulbs, and also portions of the opposite temporal lobes. It presents a cord-like appearance and in median section appears as a small oval bundle in the lamina terminalis (Fig. 544, p. 617). The middle free portion is placed immediately in front of the columns of the fornix as they curve downwards, and also in intimate relation to the anterior end of the third ventricle. Posteriorly, the small portion of the anterior commissure which appears in the ventricle between the two columns of the fornix is clothed with the ventricular ependyma; anteriorly, the commissure is connected with the lamina terminalis as it stretches from the optic chiasma upwards towards the inferior (anterior) end of the hippocampal commissure.
The lateral part of the anterior commissure penetrates the cerebral hemisphere, and, gaining the inferior part of the anterior end of the internal capsule, divides into two portions, viz., a small inferior olfactory part and a much larger temporal part.
The olfactory portion of the anterior commissure is an exceedingly small fasciculus. It passes downwards and forwards, and finally enters the olfactory tract. It is composed (1) of true commissural fibres, which bind one olfactory bulb to the other; and (2) of other fibres, which connect the olfactory bulb of one side with the piriform area of the other side.
The temporal portion is formed of almost the whole of the fibres of the commissure. It is carried laterally under the lentiform nucleus, until it gains the interval between the globus pallidus and the putamen. At this point it changes its direction and sweeps backwards. In frontal sections through the brain, posterior to this bend, the temporal portion of the anterior commissure appears as an oval bundle of fibres cut transversely and placed in close contact with the inferior surface of the lentiform nucleus (Fig. 576). Finally, it turns sharply downwards on the lateral aspect of the amygdaloid nucleus, and its fibres are lost in the white medullary centre of the temporal lobe. When the lateral part of the anterior commissure is displayed by dissection, it is seen to be twisted like a rope.
The hippocampal commissure is composed of fibres which connect the hippocampus of one side with the corresponding structure of the opposite side. It is described on p. 629.
Association Fibres.-The association fibres bind together different portions of the cortex of the same hemisphere. They are grouped into long and short association bundles.
The greater number of the short association fibres pass between adjacent gyri. They curve round the bottoms of the sulci in U-shaped loops. Some of these occupy the deepest part of the gray cortex itself, and are termed intracortical association fibres (Figs. 577 and 578); others lie immediately subjacent to the gray matter-between it and the general mass of the white matter and receive the name of subcortical fibres. Many groups of short association fibres, instead of linking together contiguous gyri, pass between gyri more or less remote. It is only after birth, when intellectual effort and education have stimulated different portions of the cortex to act in harmony and in conjunction with each other, that these association fibres assume their sheaths of myelin and become functional.
The long association fibres are arranged in bundles which run for considerable distances within the white medullary centre of the cerebral hemisphere, and unite
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
CULUS LONGITUDINALIS SUP
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
Fasciculus occipito frontalis [inferior] Temporal
FASC. LONGITUDINALIS INF
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
Fasciculus longitudinalis superior passing over the lateral side of the
Fasciculus occipito-frontalis infer
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.