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THE CEREBRAL HEMISPHERES

 
 The two cerebral hemispheres enclosed within the cranium and meninges have an inferior surface or base, a medial surface where the two hemispheres oppose one another and a lateral and superolateral surface.  There are four LOBES, the FRONTAL, TEMPORAL, OCCIPITAL and PARIETAL, each underlying the skull bone of the same name. The most anterior point of the frontal lobe is the frontal pole, the temporal pole fits into the middle cranial fossa and the occipital pole protrudes posteriorly.
 The surface features of the hemispheres are dominated by the presence of numerous GYRI and SULCI. The gyri consist of a central core of white matter covered by  a layer of grey matter called the cortex. The sulci vary in depth forming grooves and fissures. The infolding into gyri and sulci is a mechanism to increase the total cortical area, 2/3rds of it is deep in the  walls of the sulci. The layer of cortex is a continuous sheet of grey matter (neurons) covering the entire hemisphere. Its structure will be dealt with separately.

The major sulci are


(a) the CENTRAL SULCUS: separating the frontal and parietal lobes, it begins on the superomedial surface of the hemisphere, crosses the superomedial border and extends down the superolateral surface of the hemisphere anteroinferiorly to reach the posterior ramus of the lateral sulcus. Apart from separating the frontal and parietal lobes it is separating the precentral gyrus from the  postcentral gyrus. This precentral gyrus or motor  area is the region where fibers of the corticospinal and corticonuclear tracts arise.  The postcentral gyrus or sensory area is the terminus for sensory impulses from the thalamus and originally from the medial lemniscus and spinothalamic tracts which relayed in the thalamus. The control in both the motor and sensory cortices is contralateral with the right hemisphere controlling the left half of the body and vice versa.

(b) the LATERAL SULCUS: On the lateral surface of the hemisphere the folds of cortex converge and bury an area of cortex deep in the hemisphere, this is termed the INSULA. Overlying the insula the folds of cortex form the lateral sulcus. The stem of the sulcus begins in the vallecula and runs laterally between the temporal and the orbital part of the frontal lobe. On the superolateral surface the stem divides into a posterior, anterior and ascending ramus. If the edges of the sulcus are pulled apart the middle cerebral artery  and veins can be seen coursing out over the superolateral surface of the hemisphere.

(c) the CALCARINE SULCUS: On the medial aspect of the posterior part of the hemisphere a Y shaped structure is formed by the meeting of two sulci-
the PARIETO-OCCIPITAL sulcus and the calcarine sulcus. The parieto-occipital sulcus separates the parietal lobe from the occipital lobe. The calcarine sulcus is of importance as the region where nerve fibers conveying visual impulses  end in the cortex of its walls. This cortex can be recognised by its linear appearance and is termed the STRIATE CORTEX.

FRONTAL LOBE
 This lobe extends from the anterior tip of the brain to the central sulcus and inferiorly it is limited by the lateral sulcus. On the medial surface of the hemisphere it extends to an imaginary line from the central sulcus to the corpus callosum. The lateral surface of this lobe is made up of four gyri.
 The PRECENTRAL GYRUS is parallel to and anterior to the central sulcus and extends to the precentral sulcus. The other gyri are the SUPERIOR, MIDDLE AND INFERIOR FRONTAL GYRI, which are oriented in a horizontal plane, perpendicular to the precentral gyrus. The superior frontal gyrus extends on the medial surface of the hemisphere to the cingulum. The inferior frontal gyrus is divided into three parts, the anterior orbital part, the posterio opercular part and wedged between these two parts is the triangular part.
 The frontal lobe contains the following functional  areas:
(a) The PRIMARY MOTOR CORTEX, made of of most of the precentral gyrus. It is involved in the initiation of WILLED VOLUNTARY MOVEMENT and contains many of the cells of origin of the descending motor pathways.
(b) The PREMOTOR AREA  forms the remainder of the precentral gyrus and parts of the superior middle and inferior frontal gyri and is  also involved in the initiation of willed voluntary movement.
(c) BROCA'S AREA is in the opercular and triangular parts of the inferior frontal gyrus of the dominant hemisphere (usually the left) and is necessary for the production of SPOKEN and WRITTEN LANGUAGE.
(d) The FRONTAL EYE FIELD area is in the MIDDLE FRONTAL GYRUS and it controls CONJUGATE EYE MOVEMENTS.
(e) The PREFRONTAL CORTEX comprises the remainder of the frontal lobe and is involved in PERSONALITY, INSIGHT and FORESIGHT.

 PARTIETAL LOBE
 This lobe extends from the central sulcus to an imaginary line from the top of the parietooccipital sulcus to the preoccipital notch. Inferiorly, it is bounded by the lateral sulcus and a further imaginary line extending from this sulcus to the posterior boundary of the parietal lobe. On the medial surface of the hemisphere it is limited inferiorly by the corpus callosum and the calcarine sulcus, posteriorly by the parieto-occipital sulcus and anteriorly by the frontal lobe.
 The gyri of the lateral surface of the parietal lobe are the postcentral gyrus, and the superior and inferior parietal lobules. The POSTCENTRAL GYRUS is posterior to the central sulcus and lies parallel to it extending posteriorly to the postcentral sulcus. The SUPERIOR PARIETAL LOBULE and INFERIOR PARIETAL LOBULE are separated by the INTRAPARIETAL SULCUS extending from the postcentral sulcus towards the occipital lobe. This inferior parietal lobule is composed of the ANGULAR GYRUS and the SUPRAMARGINAL GYRUS. The medial surface of the parietal lobe has the medial extension of the postcentral gyrus and part of the cingulate gyrus. It extends to the parietooccipital sulcus and and is completed by the PRECUNEUS.
 
The parietal lobe contains the following functional areas:
(a) The postcentral gyrus corresponds to the PRIMARY SOMATOSENSORY CORTEX, involved in the initial processing at the cortical level of TACTILE and PROPRIOCEPTIVE information.
(b) Along with part of the temporal lobe, most of the interior of the parietal lobe is involved in the comprehension (usually in the left hemisphere).
(c) the rest of the parietal lobe is involved with the complex orientation of a person in time and place.

TEMPORAL LOBE
 This lobe extends superiorly to the lateral sulcus and the imaginary line limiting the inferior part of the parietal lobe.Posteriorly it extends to the line joining the parietooccipital sulcus to the preoccipital notch. On the medial surface of the hemisphere it is limited posteriorly by an imaginary line from the preoccipital notch to the splenium of the corpus callosum. The superior, middle, and inferior temporal gyri form the lateral surface of the temporal lobe.
 The SUPERIOR TEMPORAL GYRUS forms the inferior margin of the lateral sulcus, the temporal operculum. The INFERIOR TEMOPORAL GYRUS continues onto the inferior surface of the hemishpere. The remainder of the inferior surface of the temporal lobe forms the OCCIPITOTEMPORAL and PARAHIPPOCAMPAL gyri  separated from one another by the COLLATERAL SULCUS.  The parahippocampal gyrus is continuous with the cingulate gyrus at the back of the splenium of the corpus callosum. The anterior end of the parahippocampal gyrus forms a medially directed mass, the UNCUS. The superior border of the parahippocampal gyrus is the HIPPOCAMPAL SULCUS. An area of cortex, folded into the TEMPORAL LOBE at this sulcus is the HIPPOCAMPUS.
 The temporal lobe contains the following functionAL areas;
(a)  the PRIMARY AUDITORY CORTEX for hearing  is that part of the superior temporal gyrus which lies in the lateral sulcus.
(b) The parahippocampal gyrus and the hippocampus are involved in EMOTIONAL & VISCERAL RESPONSES.
(c) The temporal lobe is also involved in the complex activities of LEARNING and  MEMORY.

 OCCIPITAL LOBE
 The parietal and temporal lobes bound the anterior end of the occipital lobe on both the lateral  and medial sides of the hemisphere. The gyri on the lateral side of the occipital lobe, termed LATERAL OCCIPITAL GYRI  are very variable in position and orientation. The wedge shaped area on the medial surface, between the parietooccipital sulcus and the CALCARINE sulcus is the CUNEUS. Inferior to the calcarine sulcus is the LINGUAL gyrus. This is separated from the occipitotemporal gyrus  by the COLLATERAL SULCUS.
  The occipital lobe contains the following functional area.
The primary visual cortex  is in the walls of the calcarine sulcus and the immediate surrounding cortex. The rest of the occipital lobe is termed the occipital association cortex involved with the processing of visual information.
 

    WHITE MATTER
 
 The white matter of the cerebral hemispheres lies deep to the cortex and extends in varying ways through the hemispheres, between the hemispheres and connects with the lower parts of the brain. It can basically be divided into three types of fiber systems:
(a)Commissure  fibers which join like areas in both cerebral hemispheres.
(b) Association  fibers which join areas in each hemisphere.
(c) Projection    fibers which radiate from the cortex down to lower centers.

(a) COMMISSURES

(1) The CORPUS CALLOSUM is largest of the commissural systems linking both cerebral hemispheres.  This is found in  the longitudinal fissure which, when separated exposes the mass of white matter connecting both hemispheres. It is best seen on a saggital section of the hemisphere. Its posterior part, the SPLENIUM is evident as a rounded mass above the pineal body. Closely related between the splenium and pineal at this point is the GREAT CEREBRAL VEIN. Extending anteriorly from the splenium is the BODY of the corpus callosum lying below the cingulate gyrus and above the fornix and septum pellucidum. The body leads into the GENU and the ROSTRUM of the corpus callosum is attached to the lamina terminalis. The corpus callosum links both medial surfaces of the hemispheres for nearly half their entire length.
 As stated the upper surface of the corpus callosum forms the floor of the LONGITUDINAL FISSURE and embedded in its pia mater lie the anterior cerebral vessels and the Induseum Griseum. When viewed from above,the corpus callosum shows an arching of fibers laterally and then medially into the frontal lobes termed the FORCEPS MINOR and an arch of fibers extending backwards from the splenium termed the FORCEPS MAJOR. Some of the fibers termed the TAPETUM pass through the splenium and turn down to reach the temporal and inferior part of the occipital lobe. This tapetum is closely related to the posterior and inferior horns of the lateral ventricles.  The inferior surface of the body of the corpus callosum overlies the third ventricle and is separated from it by the tela choroidea of the third ventricle.

(2) THE ANTERIOR COMMISSURE: Lying in the superior part of the lamina terminalis, just anterior to the column of the fornix this small commissure connects the temporal lobes. There are also some fibers which connect both olfactory bulbs.

(3)POSTERIOR COMMISSURE:Dorsal to the upper part of the cerebral aqueduct this slender commissure contains fibers which arise and interconnect the superior colliculi  and the medial longitudinal fasiculus.

(4)HABENULAR COMMISSURE: Interconnects the habenulae. Separating the posterior commissure from the habeular commissure is a small pineal recess and superior to the habenular commissure is the thin roof of the third ventricle.

(b) ASSOCIATION FIBERS:
The white matter connecting various parts of the cortex of the same hemisphere  are of varying lengths.


(1)SHORT ASSOCIATION FIBERS pass from one part of a gyrus to another part of the same gyrus or they may loop around a sulcus to an adjacent gyrus.

(2)LONG ASSOCIATION FIBERS can extend between poles of a hemisphere and they form recognisable bundles.

(a) the CINGULUM (girdle) embedded in the cingulate gyrus surrounds the corpus callosum and it links the temporal lobe with the medial surface ofthe frontal lobe.

(b) The SUPERIOR LONGITUDINAL BUNDLE connects the frontal lobe with the occipital and

(c) the INFERIOR LONGITUDINAL BUNDLE connects the occipital and the temporal lobes.

(d) the FASCICULUS UNCINATUS is a bundle which connects the frontal with the temporal lobe.

(c) PROJECTION FIBERS:
These are the fibers which connect the cerebral cortex with  deeper nuclear masses in the hemispheres, with the thalamus and with the brain stem nuclei and spinal cord. They form the characteristic CORONA RADIATA of fibers radiating from all parts of the cortex and narrowing through to form the internal capsule as the fibers squeeze through to the brainstem areas.