9354 · 9 Cerebrum
Gross Anatomy and Subdivision of the Cerebrum
The cerebral longitudinal fissure (interhemispheric fissure) separates the two hemispheres down to the corpus callosum. Each hemisphere possesses lateral, medial, and basal surfaces; the transitional area between the (dorso-)lateral and medial surfaces is called the parasagittal region. Each hemisphere is also divided into four lobes, namely, the frontal, parietal, occipital, and temporal lobes (Figs. 9.49.6). The insula is sometimes counted as a fifth lobe. The massive enlargement of the mammalian neopallium (= neocortex) achieves its greatest extent in humans, enveloping the phylogenetically older cortical regions in neocortex. Thus, most of the structures derived from the paleocortex and archicortex cannot be seen on the external surface of the brain (the olfactory bulb and tract, olfactory area, paraterminal gyrus, fasciolar gyrus, indusium griseum, dentate gyrus, and hippocampal formation).
Gyri and Sulci
The massive enlargement of the neocortex causes folding of the brain surface into convolutions (gyri) separated by grooves (sulci, fissures). Only about onethird of the cerebral cortex is visible on the external surface, while two-thirds are hidden in the sulci (Figs. 9.79.9).
Only a few sulci have a relatively unchanging anatomical position. The lateral sulcus (sylvian fissure) separates the temporal lobe from the frontal and parietal lobes. Unlike other named sulci, the lateral sulcus does not merely form the border between two adjacent gyri. It extends deep under the surface of the brain, widening out into a broad, flat space containing cerebrospinal fluid, the sylvian cistern, which is not visible from the outside. The sylvian cistern is usually very narrow, almost a virtual space, except in markedly atrophic brains. Its medial wall is the insula (island of Reil; cf. Figs. 9.10 and 9.11), sometimes called the buried or central lobe of the brain. The lateral wall of the sylvian cistern is called the operculum (“lid”), because it covers the cistern like a lid; it consists of buried portions of the three lobes of the brain lying around it, which are called the temporal, frontal, and parietal opercula. A buried portion of the superior temporal gyrus contains the transverse gyri of Heschl (primary auditory cortex, Fig. 9.10).
Among the other relatively invariant sulci, the central sulcus (rolandic fissure) defines the border between the frontal and parietal lobes. The precentral gyrus, which lies in front of the central sulcus and is therefore in the frontal lobe, contains the primary motor cortex; the postcentral gyrus, which lies be-
Baehr, Duus' Topical Diagnosis in Neurology © 2005 Thieme
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Gross Anatomy and Subdivision of the Cerebrum · 357 |
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Gyrus rectus
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Lingual gyrus
Lateral occipitotemporal gyrus
Medial occipitotemporal gyrus
Fusiform gyrus
Calcarine sulcus
Fig. 9.9 Cortical gyri and sulci (basal view)
hind it and is therefore in the parietal lobe, contains the primary somatosensory cortex. On the medial surface of the hemisphere, the parieto-occipi- tal sulcus forms the border between the parietal and occipital lobes. Its inferior end joins the anterior end of the calcarine sulcus, which lies entirely in the occipital lobe and runs backward toward the occipital pole. Most of the primary visual cortex is located in the depths of this sulcus, and the remainder in the gyri on either side of it. Finally, the cingulate sulcus separates the neocortex from the mesocortex of the cingulate gyrus.
The borders of the occipital lobe are incompletely defined by the parietooccipital sulcus and the preoccipital notch (Figs. 9.7 and 9.8).
The portion of the lateral surface of the frontal lobe that lies anterior to the precentral gyrus is divided into the superior, middle, and inferior frontal gyri. For the names and locations of all gyri mentioned in this section, and a few others, as well as the names of the sulci that lie between them, see Figs. 9.79.9. The anatomy of many of the gyri and sulci varies greatly from one individual to another, and even between the two hemispheres of the same individual.
Baehr, Duus' Topical Diagnosis in Neurology © 2005 Thieme
All rights reserved. Usage subject to terms and conditions of license.