Basal view of the brain

Overview

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.

Brainstem

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.

Cerebellum

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.

Cerebrum