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Hemispheres of the large brain
Last reviewed: 20.11.2021
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The terminal brain (telencephalon) consists of two hemispheres of the large brain, divided by a longitudinal slit and connected to each other in the depth of this slit with the help of a corpus callosum, anterior and posterior adhesions, and also adhesions of the arch. The cavity of the terminal brain forms the right and left lateral ventricles, each of which is in the corresponding hemisphere. The hemisphere of the large brain consists of the outer covers - the cerebral cortex (cloak) lying deeper than the white matter and the accumulations of gray matter in it - the basal nuclei. The boundary between the final and the next intermediate brain passes in the place where the inner capsule lies to the lateral side of the thalamus.
[The cerebral hemisphere
The cerebral hemisphere (hemispherium cerebralis) is covered from the outside with a thin plate of gray matter - the cerebral cortex. Each hemisphere has three surfaces: the most convex upper-lateral (facies superolateral, hemispherii), flat, facing the neighboring hemisphere medial (facies medialis hemispherii) and lower (facies inferior hiispherii). The latter has a complex relief, corresponding to the inner base of the skull. Surfaces of the cerebral hemispheres are separated from each other by margins: margo superior, margo inferior and margo medialis. The most projecting anterior and posterior regions of the hemisphere were called poles: the frontal pole (polus frontalis), the occipital pole (polus occipitalis) and the temporal pole (polus tiporalis). The relief of the surfaces of the hemispheres is very complicated due to the presence of more or less deep grooves of the large brain and the cylindrical elevations located between them - gyri. The depth, the extent of the furrows and convex convolutions, their shape and direction are very variable.
Upper-lateral surface of the hemisphere
In the anterior part of each hemisphere of the large brain is the frontal lobe (lobus frontalis). It ends at the front with a frontal pole and is bounded from below by a lateral furrow (sulcus lateralis; sylvian furrow), and behind it by a deep central furrow. The central furrow (sulcus centralis; Roland furrow) is located in the frontal plane. It starts at the top of the medial surface of the cerebral hemisphere, cuts across its upper edge, descends, not interrupted, along the upperlateral surface of the hemisphere down and ends slightly before reaching the lateral furrow.
Behind the central sulcus is the parietal lobe (lobus parietalis). The posterior border of this lobe is the parieto-occipital furrow (sulcus parietooccipitalis). This furrow is located on the medial surface of the cerebral hemisphere, deeply dissecting the upper edge of the hemisphere and passes to its upper-lateral surface.
The occipital lobe (lobus occipitalis) is located behind the parieto-occipital furrow and its conditional continuation on the upperlateral surface of the hemisphere. In comparison with other shares, it has small dimensions. The occipital lobe ends with the occipital pole (polus occipitalis). Furrows and convolutions on the upperlateral surface of the occipital lobe are very variable.
The temporal lobe (lobus temporalis) occupies the lower lobe segments of the hemisphere and is separated from the frontal and parietal lobes by a deep lateral groove. The margin of the temporal lobe, covering the islet portion, was called the temporal cover (operculum temporale). The anterior part of the temporal lobe forms the temporal pole (polus temporalis). On the lateral surface of the temporal lobe, two furrows are seen - the upper and lower temporal (sulci temporales superior and inferior), almost parallel to the lateral sulcus. The convolutions of the temporal lobe are oriented along the furrows.
The islet portion, the islet (lobus insularis, S. Insula) lies in the depth of the lateral sulcus. This proportion can be seen if the portions of the frontal, parietal and temporal lobes that have been given the name of the tire cover are moved apart or removed. A deep circular furrow of the islet (sulcus circularis insulae) separates the islet from the surrounding parts of the brain. On the surface of the island there are meander islands, long and short (gyri insulae, longus et breves). Between the long gyrus, located at the rear of the islet and oriented from top to bottom and forward, and short gyruses that occupy the upper part of the islet, is the central furrow of the islet (sulcuscentralis insulae). The lower part of the islet is devoid of furrows and has a small thickening - the threshold of the island (limen insulae).
The medial surface of the hemisphere
All parts of the hemisphere, with the exception of the islet, take part in the formation of its medial surface. Over the corpus callosum, separating it from the rest of the hemisphere, is the furrow of the corpus callosum (sulcus corporis callosi). Bending behind the plenum (splenium) of the corpus callosum, this furrow goes downwards and continues into the furrow of the hippocampus, or the hippocampal furrow (sulcus hippocampi, s. Hippocampalis). Above the furrow of the corpus callosum is the sulcus cinguli. This furrow starts anterior and downward from the beak of the corpus callosum, rises, then turns back and follows the furrow of the corpus callosum. The furrow ends higher and posteriorly from the corpus callosum as an subtermic sulcus (sulcus subparietalis). At the level of the corpus callosum, the marginal part (pars marginalis, BNA) branching upward and posterior to the upper edge of the hemisphere of the large brain branches off from the waist furrow. Between the furrow of the corpus callosum and the waist furrow is the gyrus cinguli, which surrounds the corpus callosum in front, from above and from behind. Behind and downward from the corpuscle of the corpus callosum girdle narrows, forming an isthmus gyri cinguli isthmus. Further down and forward the isthmus passes into the wider gyrus of the hippocampus, or the para-hippocampal gyrus (gyrus parahippocampalis), bounded from above by the hippocampal groove. The cingulate gyrus, isthmus and para-hippocampal gyrus are known as the arched gyrus (gyrus fornicatus - BNA). In the depth of the furrow of the hippocampus is a rather thin strip of gray color, separated by small transverse grooves, - a dentate gyrus (gyrus dentatus). The region of the medial surface of the hemisphere, located between the waist furrow and the upper edge of the hemisphere, refers to the frontal and parietal lobes.
Anterior to the upper edge of the central groove is the medial surface of the upper frontal gyrus, and a paracentral lobe (lobulus paracentralis) is bound directly to the indicated section of the central sulcus, bounded posteriorly by the marginal part of the sulcus furrow. Between the marginal part from the front and the parietal-occipital furrow from the rear, precuneus is found - a part of the cerebral hemisphere belonging to the parietal lobe.
On the medial surface of the occipital lobe there are two deep furrows merging with each other at an acute angle, open to the back, parietal-occipital, separating the parietal lobe from the occipital, and the sulcus calcaneus. The latter begins on the medial surface of the occipital pole and is directed forward to the isthmus of the cingulate gyrus. The portion of the occipital lobe lying between the parieto-occipital and the furrowed grooves and having the shape of a triangle facing the point of confluence of these furrows is called a wedge (cuneus). The spur groove, which is clearly visible on the medial surface of the hemisphere, restricts from above the lingual gyrus (gyrus hingualis), extending from the occipital pole to the bottom of the neck of the cingulate gyrus. A collateral furrow (sulcus collateralis), which already belongs to the lower surface of the hemisphere, is located at the bottom of the lingual gyrus.
Lower hemisphere surface
The relief of the lower surface of the hemisphere is very complicated. The anterior parts of this surface are formed by the frontal lobe of the hemisphere, behind which the temporal pole protrudes, and also the lower surfaces of the temporal and occipital lobes, passing one into the other without noticeable boundaries.
On the lower surface of the frontal lobe, somewhat lateral and parallel to the longitudinal slit of the large brain, the olfactory furrow passes (sulcus olfactorius). From the bottom, an olfactory bulb and the olfactory tract, which goes from behind to the olfactory triangle, adhere to it. In the region of this triangle, the medial and lateral olfactory striae are visible (striae olfactoriae medialis et lateralis). The portion of the frontal lobe between the longitudinal slit of the large brain and the olfactory groove was called the direct gyrus (gyrus rectus). The surface of the frontal lobe, lying lateral to the olfactory sulcus, is divided by shallow orbital furrows (sulci orbitales) into several gyrus-shaped convolutions (gyri orbitales) that are variable in shape, location and size.
In the posterior part of the lower surface of the hemisphere, the collateral groove, located downwards and laterally from the lingual gyrus on the inferior surface of the occipital and temporal lobes, lateral to the parahypocampal gyrus, is clearly discernible. A little anterior to the anterior end of the collateral groove is the nasal furrow (sulcus rhinalis). It restricts, from the lateral side, the curved end of the parigypocampal gyrus - the hook (lincus). The medial occipital-temporal gyrus (gyrus occipitotemporalis medialis) lies lateral to the collateral groove. Between the gyrus and the lateral occipital-temporal gyrus (gyrus occipitotemporalis lateralis) located outside it, is the occipital-temporal groove (sulcus occipitotemporalis). The boundary between the lateral occipital-temporal and the lower temporal gyrus is not the sulcus, but the lower-lateral edge of the cerebral hemisphere.
A number of brain regions located mainly on the medial surface of the hemisphere and being a substratum for the formation of such general conditions as wakefulness, sleep, emotions, motivation of behavior, etc., are identified under the name limbic system. These reactions were formed in connection with the primary functions of smell (in phylogeny), so their morphological basis is the brain regions that develop from the lower-lateral parts of the cerebral bladder and belong to the so-called olfactory brain (rhinencephalon). The limbic system consists of the olfactory bulb, the olfactory tract, the olfactory triangle, the anterior perforated substance located on the lower surface of the frontal lobe (peripheral region of the olfactory brain), as well as the girdle and para-hippocampal (along with the hook) gyrus, dentate gyrus, hippocampus (central part of the olfactory brain ) and some other structures. Inclusion of these parts of the brain in the limbic system was possible in connection with the general features of their structure (and origin), the presence of mutual connections and the similarity of functional reactions.