Topography of the Cerebral Hemispheres
The cerebrum is the largest part of the brain, spanning all three cranial fossae. The composite parts can be classified based on their embryological origin, functional roles or their topography. This article will be looking at the organization of the brain with respect to the latter. Additionally, the article will look at the cerebrum from different points of view, as some structures are only visible from a particular angle.
Firstly, the cerebrum is divided into two hemispheres – a left and a right – by the falx cerebri (inferior projection of the dura mater containing the superior and inferior sagittal sinuses) along the longitudinal cerebral fissure. Each hemisphere can then be subdivided into lobes that are named according to the cranial bones under which they reside. There is one exception to this rule that will be discussed later. The forebrain undergoes a significant amount of folding during development. This resulted in the surface of the cerebral cortices being arranged in a vast array of hills and valleys. The hills are known as gyri (lobules) and the valleys are the sulci (or fissures). It should be noted that the pia mater (inner most meningeal layer) is closely associated with the brain tissue and follows it into the sulci. Conversely, the arachnoid mater (middle meningeal layer) covers the gyri and covers over the sulci (without going into the depression).
Knowledge of the sulci helps with the identification of the different lobes. The largest sulcus is the central sulcus of Rolando. It’s a midline structure that separates the frontal lobe (anteriorly located) from the parietal lobe. The lateral sulcus of Sylvius (or the Sylvian fissure) is an inferolateral sulcus that separates the temporal lobe from the frontal and parietal lobes (both of which are superior to the sulcus). The preoccipital notch is a posteroinferior point that demarcates the transition from the temporal lobe to the occipital lobe. An imaginary line from the preoccipital notch to the parieto-occipital sulcus also helps to define the boundary between the parietal and the occipital lobes. A horizontal imaginary line extending from the upper part of the Sylvian fissure to intersect the previously described line will also help in marking the transition from the parietal lobe to the temporal lobe.
The frontal lobe occupies the anterior cranial fossa. It is made up of four gyri – superior, middle, inferior and precentral – that are separated from each other by a superior frontal, an inferior frontal and a precentral sulcus. The superior frontal gyrus is cranial to the superior frontal sulcus. Below that sulcus is the middle frontal gyrus; caudal to that is the inferior frontal sulcus and gyrus. All three gyri are anterior to the vertical precentral sulcus and gyrus (the gyrus is just anterior to the central sulcus of Rolando). The inferior frontal gyrus is further subdivided into three parts by the anterior and ascending rami of the lateral sulcus of Sylvius. Anterior to the anterior ramus is the pars orbitalis of the inferior frontal gyrus. Between the anterior ramus and the ascending ramus is the pars triangularis. Finally, posterior to the ascending ramus is the pars operculum, which is also bounded posteriorly by the Sylvian fissure’s posterior ramus.
The temporal lobe can be found in the middle cranial fossa. It has three gyri and two sulci. The superior temporal gyrus is just caudal to the Sylvian fissure, but cranial to the superior temporal sulcus. The middle temporal gyrus rests between the superior and inferior temporal sulci; while the inferior temporal gyrus is inferior to the latter sulcus.
The parietal lobe also has three gyri and two sulci. The central sulcus of Rolando is its anterior boundary of the postcentral gyrus. Posterior to the postcentral gyrus is the postcentral sulcus. An intraparietal sulcus (roughly perpendicular to the postcentral sulcus) travels toward the occipital lobe with the superior parietal lobule above and the inferior parietal lobule below. The latter is further subdivided into a supramarginal gyrus anteriorly and an angular gyrus posteriorly.
If the upper and lower margins of the Sylvian fissure are retracted superiorly and inferiorly, respectively, a fifth lobe can be observed. The insula (or island of Reil) is a deep cerebral lobe that has two sulci and two gyri. There is a short gyrus that is located anteriorly and a long gyrus posteriorly. Between both gyri is the central sulcus of the insula. A circular sulcus courses along the medial parts of the opercula of the frontal, parietal, orbital and temporal lobes.
The occipital lobe is the most posterior of the five lobes. It occupies the posterior cranial fossa and has several sulci in its substance. There is a small lateral projection of the calcarine sulcus, a transverse occipital sulcus and an inconstant lunate sulcus. The gyri of the occipital lobe are better appreciated when the cerebrum is viewed medially.
A sagittal section along the longitudinal cerebral fissure reveals the medial surface of the cerebrum. Additional gyri and sulci of the previously mentioned lobes can be better appreciated in this view. Inferior to the genu of the corpus callosum and anterior to the lamina terminalis of the hypothalamus is the subcallosal gyrus. The cingulate gyrus commences just anterior to the parolfactory area (inferior to the genu of the corpus callosum) and follows the contour of the commissural body and the callosal sulcus. The cingulate sulcus is superior to the cingulate gyrus and separates it from the medial frontal gyrus. The medial frontal gyrus is limited posteriorly by the paracentral sulcus. Posterior to the paracentral lobule (which is behind the paracentral sulcus and contains a small medial projection of the central sulcus of Rolando), the cingulate sulcus courses superiorly and becomes the marginal sulcus. The lobule just posterior to the marginal sulcus is the precuneus. It is separated posteriorly from the cuneus by the parieto-occipital sulcus. The calcarine sulcus travels horizontally from the occipital pole to meet the parieto-occipital sulcus just posterior to the venous junction of the great vein of Galen and the straight sinus. Inferior to the calcarine sulcus is the lingual gyrus of the occipital lobe, which rests on the cerebral surface of the tentorium cerebelli (roughly horizontal projection of the dura mater).
The medial view also reveals diencephalic structures of the brain. The corpus callosum can be observed arching anteroposteriorly, deep within the cerebrum. Beneath it is the septum pellucidum, which connects the latter to the fornix anteriorly. The medial wall of the thalamus and its interthalamic adhesion is caudal to the fornix and the choroid plexus of the 3rd ventricle. The hypothalamic sulcus – marking the border between the thalamus and the hypothalamus – can also be seen taking an anteroposterior course from the anterior commissure to the opening of the cerebral aqueduct. Inferior to the hypothalamus is the optic chiasm and the hypophysis cerebri. Finally, posterosuperior to the thalamus is the epithalamus (Habenular nucleus and the pineal gland).
The inferior view of the brain reveals more details of the frontal, temporal and occipital lobes. On the frontal lobe (from medial to lateral), there is a rectus gyrus (straight gyrus) between the longitudinal cerebral fissure and an olfactory sulcus. The olfactory sulcus, which is more lateral than the gyrus rectus, provides a pathway for the olfactory tract to course towards its destination. There are also two orbital sulci and two orbital gyri found on the inferior aspect of the frontal lobe.
The uncus of the temporal lobe is lateral to the hypophysis cerebri, tuber cinereum, mammillary bodies and posterior perforated substance (in the interpeduncular fossa). It is also posterior to the anterior perforated substance and anterior to the crus cerebri and lateral geniculate body of the thalamus. It is adjacent to the parahippocampal gyrus. The parahippocampal gyrus is laterally limited by the rhinal sulcus anteriorly and the collateral sulcus posterior. It also continues posteriorly as the medial occipitotemporal gyrus. The collateral sulcus serves as a medial border for the lateral occipitotemporal gyrus, which is limited laterally by the occipitotemporal sulcus.