General Structure of the Hemispheres
The cerebrum (cerebrum) is a derivative of the telencephalon. The right and left hemispheres (hemispheria cerebri dextrum et sinistrum) are separated by the deep longitudinal cerebral fissure — fissura longitudinalis cerebri. From the underlying cerebellum, the hemispheres are separated by the transverse cerebral fissure (fissura transversa cerebri).
The structure of each hemisphere includes three major systems:
- Cerebral cortex (cortex cerebri or pallium);
- Basal nuclei (nuclei basales);
- Rhinencephalon (rhinencephalon), which occupies the most ventral position.
Collections of gray matter from these three systems frame the internal white matter of the hemispheres. Inside the telencephalon lie cavities — the two lateral ventricles (ventriculi laterales), with the left conventionally considered the first.
Surfaces, Margins, and Poles
Each hemisphere has a complex three-dimensional shape anatomically featuring three surfaces connected by margins, and projecting poles.
Surfaces:
- Superolateral surface (facies superolateralis hemispherii) — convex, facing the internal surface of the cranial vault bones.
- Inferior surface (facies inferior hemispherii) — its anterior and middle parts rest on the cranial base, while the posterior parts lie on the tentorium cerebelli.
- Medial surface (facies medialis hemispherii) — flat, facing the longitudinal cerebral fissure.
These surfaces form three margins: superior (margo superior), inferolateral (margo inferior), and inferomedial (margo medialis). The latter is additionally subdivided into medial occipital and medial orbital margins.
The most projecting parts of the hemispheres are called poles. These include the frontal pole (polus frontalis), occipital pole (polus occipitalis), and temporal pole (polus temporalis).
Lobes of the Brain
Each hemisphere is traditionally divided into five lobes:
- Frontal lobe (lobus frontalis);
- Parietal lobe (lobus parietalis) — located posterior to the central sulcus. It is separated from the temporal lobe by the lateral sulcus and partially separated from the occipital lobe by the parieto-occipital sulcus. Running through its substance is the intraparietal sulcus, dividing the lobe into superior and inferior parietal lobules.
- Temporal lobe (lobus temporalis) — has the most distinct boundaries. On its superolateral surface run the superior and inferior temporal sulci, which form the superior, middle, and inferior temporal gyri.
- Occipital lobe (lobus occipitalis) — on its convex surface, it lacks distinct boundaries with the parietal and temporal lobes. The pattern of its sulci and gyri is highly individualized and variable.
- Insular lobe or insula (lobus insularis / insula) — situated deep within the lateral fossa of the cerebrum (fossa lateralis cerebri) and physically separates the frontal lobe from the temporal lobe.
Major Sulci, Gyri, and Their Functions
The surface of the hemispheres is furrowed by sulci (sulci), between which lie gyri (gyri). Each cortical area is responsible for specific functions.
| Surface | Sulcus | Gyrus or Structure | Main Function |
|---|---|---|---|
| Superolateral | Sulcus centralis | Gyrus precentralis (frontal lobe) | Primary motor cortex (voluntary movement) |
| Superolateral | Sulcus centralis | Gyrus postcentralis (parietal lobe) | Primary somatosensory cortex |
| Superolateral | Sulcus temporalis superior | Gyrus temporalis superior | Hearing, sensory speech |
| Medial | Sulcus cinguli | Gyrus cinguli | Limbic system (emotions, motivation) |
| Medial | Sulcus calcarinus | Cuneus, gyrus lingualis | Primary visual cortex |
| Inferior | Sulcus olfactorius | Gyrus rectus | Olfaction |
| Inferior | Sulcus hippocampi | Gyrus dentatus | Memory and learning |
Insular Lobe (Insula)
The insula lies at the floor of the lateral cerebral fossa. It is shaped like a three-sided pyramid, with its apex directed superiorly and laterally — directly toward the lateral sulcus.
Peripherally, the insula is surrounded by the frontal, parietal, and temporal lobes, which form the walls of the lateral sulcus. Its base is bounded by the circular sulcus of the insula (sulcus circularis insulae), which gradually flattens and disappears on the inferior surface.
Running across the surface of the insula is the central sulcus of the insula (sulcus centralis insulae). It divides the lobe into two parts:
- Anterior (larger) part — contains several short gyri (gyri breves insulae).
- Posterior (smaller) part — formed by a single long gyrus (gyrus longus insulae).