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par Opercula Insulæ.-The overlapping portions of the cerebral substance which cover over the insula are termed the insular opercula, and they form, by the apposialtion of their margins, the three rami of the lateral fissure. The rami of the ned fissure extend from the exposed surface of the hemisphere to the submerged surface nis of the insula, and, in this manner, separate the opercula from each other.

rds. The temporal operculum (pars temporalis) extends upwards over the insula from dis the temporal region, and its superior margin forms the inferior lip of the posterior noramus of the lateral fissure.

or The superior operculum is carried downwards from the parietal (pars parietalis) Sand frontal (pars frontalis) regions over the insula, and its inferior margin, meeting en the temporal operculum, forms the superior lip of the posterior ramus of the lateral fissure.

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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.

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Superior operculum

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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

any one sense, but as the place of meeting (and the physical counterpart of the blending in consciousness) of the impressions of different senses. In the human brain the neopallium becomes mapped out into a large series (more than forty) of areas, which differ one from the other in structure and in their connexions, and presumably therefore in their functions; and many of these areas may be further subdivided into a series of less obtrusively differentiated territories (Figs. 553 and 581).

The gray matter of the neopallium is spread over the surface of the white matter as a thin film (cortex cerebri), which is nowhere more than 4 millimetres, and may be only 1.25 millimetres thick. In different regions it presents every gradation of thickness between these two extremes. As the cortex increases in volume it does so not by any addition to its depth, but solely by an expansion of its superficial area. Thus it happens that in all larger mammalian brains, as the cerebral hemisphere expands and there is an increasing disproportion between the bulk of the hemisphere and the area of its surface, the cortex must become folded to accommodate itself to the limited area of surface upon which it has to be packed. But this process of folding does not take place in any haphazard or purely mechanical way. The situations of the furrows or sulci which make their appearance are determined, for the most part, by the arrangement and the relative rates of expansion of the various areas into which the neopallium becomes differentiated.

The great majority of the furrows belong to a group, which we may call (1) sulci terminales, i.e. they make their appearance along the boundary lines between areas of different structure. The fissura rhinalis and sulcus centralis are examples of this group. Another group, which may be called (2) sulci axiales, develop by the folding of areas of uniform structure, i.e. along the axis of certain territories. The retro-calcarine sulcus and the sulcus occipitalis lateralis belong to this group. There is a third group of (3) sulci operculati, where the edge of one area becomes pushed over an adjoining territory, so that a trough is formed (Fig. 575, C), which is neither a limiting nor an axial sulcus. The sulcus lunatus is an example. And finally there is a fourth group, in which some more definitely mechanical factor comes into play to complicate the operation of these other factors, or even to determine the development of a furrow. The sulcus parieto-occipitalis and the fissura lateralis are examples of the fourth group.

[It is the custom to call certain furrows sulci and others fissures, and to call some of them complete, because they indent the whole thickness of the wall of the ventricle, and to call the rest incomplete. There is no justification whatever for any such distinctions.

It is usual also to subdivide the surface of the hemisphere in a purely arbitrary manner into "lobes" and to speak of interlobar fissures, but this is an artificial and misleading terminology which we shall avoid as far as possible.]

Fissura Longitudinalis Cerebri.-The longitudinal fissure is not a fissure of the cortex but is the great cleft between the two cerebral hemispheres. In front and behind it separates the cerebral hemispheres completely the one from the other. In its middle part, however, the fissure is interrupted and floored by the corpus callosum, a white commissural band, which passes between the hemispheres and connects them together. The superior surface of the corpus callosum is displayed when the contiguous medial surfaces of the cerebral hemispheres are drawn asunder. The longitudinal fissure is occupied by a median fold of dura mater, termed the falx cerebri, which partially subdivides the part of the cranial cavity allotted to the cerebrum into a right and left chamber.

External Configuration of each Cerebral Hemisphere.-Each cerebral hemisphere presents a lateral, a medial, and an inferior surface. The lateral surface is convex and is adapted accurately to the internal surface of the cranial vault. The medial surface is flat and perpendicular, and bounds the longitudinal fissure. In great part it is separated from the corresponding surface of the opposite hemisphere by the falx cerebri. The inferior surface is irregular and is adapted to the anterior and middle cranial fosse of the cranial floor and, behind these, to the superior surface of the tentorium cerebelli. Traversing this surface in a transverse

direction, nearer the anterior end of the hemisphere than the posterior end, is the stem of the lateral fissure. This deep cleft divides the inferior surface into an anterior or orbital area, which rests on the orbital part of the frontal bone and is consequently concave from side to side, and a more extensive posterior or tentorial area, which lies on the floor of the lateral part of the middle cranial fossa and upon the superior surface of the tentorium cerebelli. This surface is arched from before backwards, and looks medially as well as downwards. In its posterior two-thirds it lies above the cerebellum, from which it is separated by the tentorium cerebelli.

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The borders which intervene between these surfaces are the supero-medial, the superciliary, the infero-lateral, the medial occipital and medial orbital. The superomedial border, convex from before backwards, intervenes between the convex lateral surface and the flat medial surface of the hemisphere. The superciliary border is highly arched and separates the orbital surface from the lateral surface. The infero-lateral border marks off the tentorial surface from the lateral surface. The medial occipital border can be seen only in cases where the brain has been hardened in situ and faithfully retains the natural form. It extends from the posterior end of the hemisphere towards the posterior extremity of the corpus callosum, and intervenes between the medial and tentorial surfaces. It is the border which lies along the straight blood sinus, and it therefore occupies the angle which is formed by the attachment of the posterior part of the falx cerebri to the superior surface of the tentorium cerebelli. The medial orbital border separates the medial surface from the orbital surface.

The most projecting part of the anterior end of the cerebral hemisphere is called

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FIG. 575.-DIAGRAMS TO EXPLAIN THREE TYPES OF CEREBRAL FURROWS.

the frontal pole, whilst the most projecting part of the posterior end is termed the occipital pole. On the inferior surface of the hemisphere the prominent point of cerebral substance which extends forwards below the lateral fissure receives the name of the temporal pole. In a well-hardened brain a broad groove is usually present on the medial and inferior aspect of the occipital pole of the right hemisphere. This corresponds to the commencement of the right transverse venous sinus. A less distinct groove on the occipital pole of the left hemisphere frequently indicates the commencement of the left transverse sinus. On the tentorial surface, a short distance behind the temporal pole, a well-marked depression, impressio petrosa, is always visible. This corresponds to the elevation on the anterior surface of the petrous portion of the temporal bone over the superior semicircular canal.

THE WHITE MATTER OF THE CEREBRAL HEMISPHERES.

According to the connexions which they establish the fibres forming the white medullary matter of the hemispheres may be classified into three distinct groups, viz., (1) commissural fibres; (2) association fibres; and (3) projection fibres.

Commissural Fibres.-These are fibres which link together portions of the gray cortex of opposite cerebral hemispheres. They are arranged in three groups forming three definite structures, viz., the corpus callosum, the anterior commissure, and the hippocampal commissure.

The corpus callosum has in a great measure been already studied (p. 628). As it enters each hemisphere, its fibres spread out in an extensive radiation (the radia

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

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