Cerebrum
The cerebrum is the largest part of the human brain that fills most of the cranial cavity. Its large size is the result of a progressive (telencephalization) centralization of the various higher sensory and motor centres of the brain during evolution.
The cerebrum is a heavily, convoluted bilobed structure (Fig. 12.1). The two lateral halves are called cerebral hemispheres. When the two cerebral, hemispheres are viewed together from above, they assume the shape of an ovoid mass, which is broader behind than in front. The widest transverse diameter corresponds with a line connecting the two parietal tuberosities.
A deep median cleft, the longitudinal cerebral fissure, incompletely separates the two cerebral hemispheres. Both in front and behind, the cleft is complete, but in the central part the cleft extends downwards up to the corpus callosum which is a large mass of white fibres joining the two cerebral hemispheres across the median plane.
The longitudinal cerebral fissure is occupied by the following structures:
1. Falx cerebri (a sickle-shaped fold of dura mater).
2. Fold of arachnoid that follows the surfaces of the falx cerebri.
3. Pia mater covering the medial surface of the falx cerebri.
4. Anterior cerebral arteries and veins (which lie in the subarachnoid space between the arachnoid and the pia).
Each cerebral hemisphere consists of: (a) an outer layer of grey matter called cerebral cortex, (b) an inner mass of white matter, (c) large masses of grey matter embedded in the basal part of the white matter called basal ganglia/basal nuclei, and (d) a cavity within it called lateral ventricle (Fig. 12.2).
Fig. 12.2 Coronal section of the cerebral hemisphere showing its structure. Also note the borders and surfaces of the cerebral hemisphere. (CN = caudate nucleus, T = thalamus, P = putamen, C = claustrum, A = amygdaloid body.)
External Features of the Cerebral Hemisphere
The external features of the cerebral hemisphere include poles, surfaces, borders, sulci, and gyri.
Poles (Figs 12.2 and 12.3)
• The frontal pole at the anterior end of the hemisphere is more rounded than the occipital pole. It lies opposite the medial part of the superciliary arch.
• The occipital pole at the posterior end of the hemisphere is more pointed than the frontal pole. It lies at a short distance superolateral to the external occipital protuberance.
• The temporal pole between frontal and temporal poles points forwards. It fits into the anterior part of the middle cranial fossa and is overhung by the lesser wing of the sphenoid.
Surfaces
Each cerebral hemisphere has three surfaces – superolat-eral, medial, and inferior (Fig. 12.2).
1. The superolateral surface is most convex and most extensive. It faces upwards and laterally and conforms to the corresponding half of the cranial vault.
2. The medial surface is flat and vertical. It presents a thick C-shaped cut surface of the corpus callosum.
3. The inferior surface is irregular to adopt the floors of anterior and middle cranial fossae. It is divided into two parts by a deep horizontal groove or sulcus, the stem of lateral sulcus, viz. (a) a small anterior part, the orbital surface, and (b) a large posterior part, the tento-rial surface.
Borders
Each cerebral hemisphere presents six borders (Figs 12.2 and 12.3), viz. superomedial, superciliary, inferolateral, medial orbital, medial occipital and inferomedial.
1. The superomedial border separates the superolateral surface from the medial surface.
2. The superciliary border is at the junction of superolateral and orbital surfaces. It lies just behind the superciliary arch hence its name strictly speaking, it is the orbital part of the inferolateral border.
3. The inferolateral border separates the superolateral surface from the tentorial surface. Posteriorly this border exhibits a notch, the preoccipital notch about 3 cm in front of the occipital pole. This notch is used as a useful surface landmark.
4. The medial orbital border (Fig. 12.3) separates the medial surface from the orbital surface.
5. The inferomedial/hippocampal border (Fig. 12.3) surrounds the cerebral peduncle. It is formed by the medial aspect of the uncus and parahippocampal gyrus.
6. The medial occipital border (Fig. 12.3) separates the medial surface from the tentorial surface.
Sulci and Gyri
The cerebral cortex (the surface layer of grey matter) is highly extensive in man. To accommodate it in the limited space available within the rigid cranial cavity, the surface of cerebral hemisphere becomes folded, producing numerous convolutions separated by fissures. These convolutions and fissures are termed gyri and sulci respectively. In human brain the total surface area of cerebral hemisphere is about 2000 cm but approximately two-third of this is hidden from the surface view within the walls of the sulci.
A brain with convoluted cerebral cortex is termed gyrencephalic while the one with smooth cortex, lissen-cephalic (Gk. lissos = smooth).
In general, man and other higher mammals have gyren-cephalic brain while the reptiles, birds and lower mammals have lissencephalic brain.
The sulci vary in depth from slight grooves to deep fissures and some of them are sufficiently deep to indent the wall of the lateral ventricle in the depth of the hemisphere.
The gyri consist of a central core of white matter (nerve fibres running to and from the overlying cortex) covered by a layer of grey matter, the cerebral cortex.
There is a great deal of individual variations in the details of sulci and gyri. Therefore, the following account deals with only some important sulci and gyri.
Main cerebral sulci
Main cerebral sulci are fairly constant in position and shape and include lateral, central, parieto-occipital and calcarine sulci.
Lateral sulcus (of Sylvius) (Figs 12.4, 12.6)
The anterior horizontal ramus is about 2.5 cm long and passes forwards into the inferior frontal gyrus. The anterior ascending ramus runs upwards for about 2.5 cm in the same gyrus. The posterior ramus (the main part of the sulcus) is about 7.5 cm long and runs posteriorly and slightly upwards across the lateral surface and ends in the inferior parietal lobule by an upturned posterior end.
Calcarine sulcus
Calcarine sulcus is present on the medial surface of the cerebral hemisphere. It begins as a deep fissure, a little below the posterior end of the corpus callosum, the sple-nium and follows an arched course with a convexity upwards to the occipital pole and may extend slightly on to the superolateral surface.
Parieto-occipital sulcus
Parieto-occipital sulcus is present on the medial surface of the hemisphere. It begins at the midpoint of the calcarine sulcus and courses upwards and slightly backwards to cut the superomedial border of the hemisphere about 5 cm in front of the occipital pole, and may extend slightly on to the superolateral surface.
Lobes of Cerebral Hemisphere (Fig. 12.4)
To discuss further about sulci and gyri and other aspects of the cerebral hemisphere, the superolateral surface of the hemisphere is arbitrarily divided into four lobes – frontal, parietal, temporal and occipital with the help of: (a) three main sulci, central, lateral and parieto-occipital, and (b) two imaginary lines. The first imaginary line is a vertical line joining the parieto-occipital sulcus to the preoccipital notch, and the second line is a backward continuation of the horizontal part of the posterior ramus of the lateral sulcus till it joins the first line (Fig. 12.4).
The frontal lobe lies anterior to the central sulcus, and above the posterior ramus of the lateral sulcus.
The parietal lobe lies behind the central sulcus and in front of the upper part of the first imaginary line. Below it is bounded by the posterior ramus of lateral sulcus and the second imaginary line.
The temporal lobe lies below the posterior ramus of lateral sulcus and second imaginary line. It is separated from the occipital lobe by the lower part of the first imaginary line.
The occipital lobe lies behind the vertical line joining the parieto-occipital sulcus and preoccipital notch.
Insula/island of Reil (also called central lobe)
It is customary to consider the insula separately from the four main lobes (vide supra) of the cerebral hemisphere.
The insula is the submerged (hidden) portion of the cerebral cortex in the floor of the lateral sulcus (Fig. 12.5). It has been submerged from the surface during development of brain due to the overgrowth of the surrounding cortical areas and can be seen only when the lips of the lateral sulcus are widely pulled apart. It is triangular in shape and surrounded all around by a sulcus, the circular sulcus except anteroinferiorly at its apex called limen insulae which is continuous with the anterior perforated substance.
Fig. 12.5 The insula (island of Reil) exposed by removing the opercula. Note: Insula is also called ‘central lobe.’
Fig. 12.6 Superolateral surface of the left cerebral hemisphere showing lobes, sulci and gyri. (SFG = superior frontal gyrus, MFG = middle frontal gyrus, IFG = inferior frontal gyrus (a. pars orbitalis, b. pars triangularis, c. pars opercularis), STG = superior temporal gyrus, MTG = middle temporal gyrus, ITG = inferior temporal gyrus, IPL = inferior parietal lobule.)
The insula is divided into two regions – anterior and posterior by a central sulcus. The anterior region presents 3 or 4 short gyri called gyri brevia and the posterior region presents 1 or 2 long gyri called gyri longa.
The insula is hidden from the surface view by the overgrown cortical areas of frontal, parietal and temporal lobes. These areas are termed frontal, frontoparietal and temporal opercula (operculum = lid). The superior surface of the temporal operculum presents anterior and posterior transverse temporal gyri.
The middle cerebral artery and deep middle cerebral vein lie on the surface of the insula.
Sulci and Gyri on the Superolateral Surface of the Cerebral Hemisphere (Fig. 12.6)
In the frontal lobe
• The prefrontal sulcus often broken into two or three parts, runs downwards and forwards parallel and little anterior to the central sulcus. The area between the central and precentral sulci is called precentral gyrus.
• Anterior to the precentral sulcus there are two sulci called superior and inferior frontal sulci which run horizontally. These sulci divide the region of frontal lobe in front of precentral sulcus into superior, middle, and inferior frontal gyri.
• The anterior and ascending rami of lateral sulcus divide the inferior frontal gyrus into three parts. The part below the anterior ramus is called pars orbitalis, the part between the anterior and ascending rami the pars trian-gularis and the part posterior to the ascending ramus, the pars opercularis.
In the parietal lobe
• The postcentral sulcus runs downwards and forwards, a little behind and parallel to the central sulcus. The area between these two sulci is called the postcentral gyrus.
• The rest of the parietal lobe is divided into a superior and inferior parietal lobules by an intraparietal sulcus which runs horizontally backwards from the postcentral sulcus.
• The upturned posterior end of the posterior ramus of lateral sulcus, and the posterior ends of superior and inferior temporal sulci extends into the inferior parietal lobule to divide it into three parts: (a) the part that surrounds the posterior ramus of lateral sulcus is called supra marginal gyrus, (b) the part surrounding the superior temporal sulcus, the angular gyrus, and (c) the part surrounding the inferior temporal sulcus, the arcus temporo-occipitalis.
In the temporal lobe
There are two sulci in this lobe that run parallel to the posterior ramus of the lateral sulcus. These are termed superior and inferior temporal sulci, and divide the temporal lobe into superior, middle and inferior temporal gyri.
The superior surface of superior temporal gyrus presents two transverse temporal gyri. The anterior transverse temporal gyrus also called Heschl’s gyrus forms the primary auditory area of the cortex.
In the occipital lobe
The occipital lobe possesses rather three short sulci, lateral and transverse occipital sulci and lunate sulcus.
1. Lateral occipital sulcus runs horizontally and divides this lobe into superior and inferior occipital gyri.
2. Lunate sulcus is C-shaped sulcus with forward convexity just in front of the occipital pole.
3. Transverse occipital sulcus runs downwards into the uppermost part of the occipital lobe from the supero-medial border of hemisphere, a little behind the parieto-occipital sulcus.
Sulci and Gyri on the Medial Surface of the Cerebral Hemisphere (Fig. 12.7)
The presence of corpus callosum is the most conspicuous feature seen on the medial surface of the cerebral hemisphere. It is C-shaped thick bundle of commissural fibres. It consists of a central part, the trunk, a thick posterior end, the splenium and curved anterior end, the genu.
Fig. 12.7 Medial surface of the left cerebral hemisphere showing lobes, sulci and gyri. (Note: In this figure tentorial surface of hemisphere is also seen.) The interrupted lines indicate the approximate boundaries of the lobes. (PCL = paracentral lobule.)
The sulci and gyri on the medial surface are located above, in front and behind the corpus callosum.
• Cingulate sulcus: It is the most prominent sulcus which follows a curved course about 1 cm above and parallel to the upper convex margin of the corpus callosum. Anteriorly it ends below the genu of corpus callosum, posteriorly it turns upwards to reach the superomedial border of the hemisphere a little behind the upper end of the central sulcus.
The area between the cingulate sulcus and the corpus callosum is termed cingulate gyrus.
• Just in front of lamina terminalis, there are paraterminal and parolfactory gyri and anterior and posterior parol-factory sulci.
• Callosal sulcus. It separates the cingulate gyrus from the corpus callosum.
• The part of medial surface between the cingulate sulcus and the superomedial border of hemisphere is divided by a short offshoot sulcus ascending from the cingulate sulcus above the middle of the trunk of corpus callosum into two parts:
The boundaries of paracentral lobule needs to be elaborated, as it is the cortical (highest) centre of micturition and defecation. The paracentral lobule is bounded above, by the superomedial border of the hemisphere, below by the cingulate sulcus, and posteriorly by the upturned posterior end of the cingulate sulcus. The paracentral lobule is invaded by the downturned upper end of the central sulcus.
• The posterior part of medial surface behind the para-central lobule has two main sulci: the calcarine sulcus, and the parieto-occipital sulcus.
(a) Calcarine sulcus. It is already described on page 142. A small region between the splenium and calcarine sulcus is termed isthmus.
(b) Parieto-occipital sulcus. It is already described on page 142.
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