Cerebral hemisphere

The cerebrum, or the largest part of the vertebrate brain, is made up of two cerebral hemispheres. The deep groove known as the longitudinal fissure divides the cerebrum into the left and right hemispheres, but the hemispheres remain united by the corpus callosum, a large bundle of nerve fibers in the middle of the brain whose primary function is to integrate sensory and motor signals between the hemispheres. In eutherian (placental) mammals, other bundles of nerve fibers like the corpus callosum exist, including the anterior commissure, the posterior commissure, and the fornix, but compared with the corpus callosum, they are much smaller in size.

Broadly, the hemispheres are made up of two types of tissues. The thin outer layer of the cerebral hemispheres is made up of gray matter, composed of neuronal cell bodies, dendrites, and synapses; this outer layer constitutes the cerebral cortex (cortex is Latin for "bark of a tree"). Below that is the larger inner layer of white matter, composed of axons and myelin.

Each hemisphere is further subdivided into a frontal, parietal, occipital, and temporal lobe. The central sulcus is a prominent fissure that separates both the frontal lobe from the parietal lobe and the primary motor cortex from the primary somatosensory cortex. Three of the four lobes also have "poles": the occipital pole, the frontal pole, and the temporal pole.

The two cerebral hemispheres are nicely macroscopic mirror images of each other, with subtle anatomical differences between them, such as the Yakovlevian torque that is sometimes seen in the human brain. Nevertheless, on a microscopic level, the functions of cells, the quantities of neurotransmitters, and the types of receptors between the hemispheres is markedly asymmetrical. While some of these hemispheric distribution differences are consistent across human beings, or even across some species, many observable distribution differences vary from individual to individual within a given species.

Each cerebral hemisphere has an outer layer of cerebral cortex which is of grey matter and in the interior of the cerebral hemispheres is an inner layer or core of white matter known as the centrum semiovale. The interior portion of the hemispheres of the cerebrum includes the lateral ventricles, the basal ganglia, and the white matter.

There are three poles of the cerebrum: the occipital pole, the frontal pole, and the temporal pole. The occipital pole is the posterior end of each occipital lobe in each hemisphere. It is more pointed than the rounder frontal pole. The frontal pole is at the frontmost part of the frontal lobe in each hemisphere, and is more rounded than the occipital pole. The temporal pole is located between the frontal and occipital poles, and sits in the anterior part of middle cranial fossa in each temporal lobe.

If the upper part of either hemisphere is removed, at a level about 1.25 cm above the corpus callosum, the central white matter will be exposed as an oval-shaped area, the centrum semiovale, surrounded by a narrow convoluted margin of gray substance, and studded with numerous minute red dots (puncta vasculosa), produced by the escape of blood from divided blood vessels.^{[citation needed]}

If the remaining portions of the hemispheres be slightly drawn apart a broad band of white substance, the corpus callosum, will be observed, connecting them at the bottom of the longitudinal fissure; the margins of the hemispheres which overlap the corpus callosum are called the labia cerebri.