Basal View of the Brain
Like the ”lateral view of the brain”, the basal or inferior view of the brain shows the three major parts of the brain. These are the cerebrum, cerebellum and brainstem. These brain parts are marked with visible gross features like the gyri (singular: gyrus) and sulci (singular: sulcus) of the cerebrum and cerebellum. They are each also divided into subparts or regions for simplified localization of structures. For example, the brainstem is composed of the midbrain, pons and medulla oblongata, while the cerebrum is divisible into lobes. Sulci are small grooves; but there are also large grooves, and these are called fissures. Fissures divide the cerebral cortex into lobes, and also divide the cerebrum into the right and left cerebral hemispheres along the sagittal plane, thus partitioning the structure called the corpus callosum into two halves. The fissure involved in this division is called the medial longitudinal fissure.
Furthermore, the structures seen at this view are specifically referred to as the “cortex”, except for the brainstem. Thus we can see the cerebellar and cerebral cortices. The cortex is the most superficial layer of these brain parts, and make up the largest portion of the brain’s gray matter. The inferior view of the brain also shows some white matter, as well as some important structures like the olfactory bulb, tract and the optic chiasm.
The basal view of the brain specifically shows the anterior surface of the brainstem and inferior surfaces of the cerebellum and cerebrum. In the following paragraphs, an attempt is made to describe some major features of these surfaces.
The most prominent features of the anterior surface of the brainstem, which is seen at this basal view, are the pyramid and olive. The brainstem is made up of (from above downwards) the midbrain, the pons and the medulla. It is continuous above with the cerebral hemispheres, below with the spinal cord and posteriorly with the cerebellum.
The medulla is about 3 cm long, 2 cm wide, and is marked on the lateral surface by the 9th to 12th cranial nerves, as well a series of fissures or sulci that divide it into a number of regions. These fissures and regions include the anterior median fissure, anterolateral sulcus and the region between these sulci which is an elevation called the pyramid. There is also a posterolateral sulcus or fissure which defines an area between it and the anterolateral sulcus. This area is an elongated, oval swelling called olive.
The 6th to 8th cranial nerves emerge at the junction of the medulla and pons. The 5th cranial nerves (Trigeminal nerve) emerge from the anterior surface of the pons. The pons shows a convex anterior surface, marked by prominent transversely running fibres which collect laterally to form a bundle, the middle cerebellar peduncle. The 3rd and 4th cranial nerves emerge from the surface of the midbrain. However, one striking feature of the lateral view of the midbrain is the trochlear nerve (CN IV) which emerges from the velum, and then winds round the side of the midbrain to reach its ventral (anterior) aspect.
The cerebellum (or small brain) weighs about 10% of the cerebral hemispheres and it is about 150 g in the adult. It has a superficial layer of grey matter, the cerebellar cortex, and like other parts of the brain, it is marked by numerous fissures. The cerebellum lies behind the brainstem, and is separated from the cerebrum by a fold of dura mater called the tentorium cerebelli.
The cerebellum consists of a part lying near the midline called the vermis, and of two lateral hemispheres. It has two surfaces, superior and inferior. The surface of the cerebellum is marked by a series of fissures that run more or less parallel to one another. The fissures sub-divide the surface of the cerebellum into narrow leaf-like bands or folia. The long axis of the majority of folia is more or less transverse. Sections of the cerebellum cut at right angles to the axis have a characteristic tree-like appearance to which the term arbor-vitae (tree of life) is applied.
Some of the features of the surface of the cerebellum are deeper than others, and they divide the cerebellum into lobes within which smaller lobule may be recognised. The deepest fissures in the cerebellum are the primary fissure (fissure prima) which runs transversely across the superior surface, and the posterolateral fissure seen on this inferior aspect. A horizontal fissure is also seen on the inferior view of the brain and divides the cerebellum into a superior and an inferior surface.
These features divide the cerebellum into three lobes; a part anterior to the primary fissure, the anterior lobe, a part between the two fissures, the posterior lobe (sometimes called the middle lobe). The remaining part is flocculonodular lobe. The anterior and posterior lobes together form the corpus cerebelli.
The inferior view of the cerebrum is predominantly composed of the cerebral cortex which is thrown into a complicated series of tortuous folds, the gyri (singular: gyrus). Between these gyri are grooves or indentations called sulci (singular: sulcus). Also seen at this view of the cerebrum are the white matter of the brainstem and some tracts.
The highest level of cognitive functions and information processing takes place in the cerebral cortex. Some functions of the cerebral cortex include thinking and reasoning, memory, consciousness, attention, perceptual awareness and language. The cerebral cortex is made up of three large surfaces, namely:
- The superolateral surface
- The medial surface
- The inferior surface
However, only the inferior surface is seen at this basal view. These surfaces are clearly defined by three edges called borders of the cerebral hemisphere.
When the cerebrum is separated from the hindbrain by cutting across the midbrain, and is viewed from below, several structures are apparent. Posterior to the midbrain, the under surface of the splenium of the corpus callosum can be seen. Anterior to the midbrain, there is a depressed area called the interpeduncular fossa. This fossa is bounded anteriorly by the optic chiasma (optic chiasm) and on the sides by the right and left optic tracts. The optic tracts wind around the sides of the midbrain to terminate on its posterior aspect.
In this region two swellings, the medial and lateral geniculate bodies, can be seen. Anterior and medial to the crura of the midbrain are two rounded swellings called the mammillary bodies. Anterior to these bodies, there is a median elevation called the tuber cinereum, to which the infundibulum of the hypophysis cerebri is attached. The triangular interval between the mammillary bodies and the midbrain is pierced by numerous small blood vessels and is called the posterior perforated substance. A smaller area lying on each side of the optic chiasma is called the anterior perforated substance. The anterior perforated substance is bounded anterolaterally by the lateral olfactory stria and posterolaterally by the uncus. The anterior perforated substance is connected to the insula by a band of grey matter called the limen insulae which lies in the depth of the stem of the lateral sulcus.
In addition to these structures, the inferior view of the cerebrum shows markings on the orbital and tentorial parts of the cerebral cortex, as well as the feature that separate this surface into these parts. The inferior surface can easily be described by dividing the surface along the stem of lateral sulcus, to form two parts:
- Orbital part/surface: Close to the medial border of the orbital surface, there is an anteroposterior sulcus called the olfactory sulcus. This sulcus has this name because the olfactory bulb and tract lie superficial to it (hence it is not also seen on surface view). The area medial to this sulcus is called the gyrus rectus. The rest of the orbital surface is divided by an H-shaped orbital sulcus into anterior, posterior, medial and lateral orbital gyri.
- Tentorial part/surface: It is marked by two major sulci that run in an anteroposterior direction. Those are the collateral sulcus medially, and the occipito-temporal sulcus laterally. The posterior part of the collateral sulcus runs parallel to the calcarine sulcus; the area between them is the lingual gyrus. Anteriorly, the lingual gyrus becomes continuous with the parahippocampal gyrus, which is related medially to the midbrain and to the interpeduncular fossa. The anterior end of the parahippocampal gyrus is cut off from the curved temporal pole of the cerebral hemisphere by a curved rhinal sulcus. This part of the parahippocampal gyrus forms a hook-like structure called the uncus. Posteriorly, this gyrus becomes continuous with the cingulate gyrus through the isthmus. The area between the collateral sulcus and the rhinal sulcus medially, and the occipitotemporal sulcus laterally, is the medial occipitotemporal gyrus. Lateral to the occipitotemporal sulcus is an area called the lateral occipitotemporal gyrus, which is continuous (around the inferolateral margin or inferolateral border) with the inferior temporal gyrus.