Cerebral hemisphere

Two cerebral hemispheres form the cerebrum, or the largest part of the vertebrate brain. A deep groove known as the longitudinal fissure divides the cerebrum into left and right hemispheres. The inner sides of the hemispheres, however, remain united by the corpus callosum, a large bundle of nerve fibers in the middle of the brain whose primary function is to integrate and transfer sensory and motor signals from both hemispheres. In eutherian (placental) mammals, other bundles of nerve fibers that unite the two hemispheres also exist, including the anterior commissure, the posterior commissure, and the fornix, but compared with the corpus callosum, they are significantly smaller in size.

Two types of tissue make up the hemispheres. The outer layer of the cerebral hemispheres is made up of grey 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 inner layer of white matter, composed of axons and myelin.

Each hemisphere further subdivides into a frontal, parietal, occipital, and temporal lobe. The central sulcus is a prominent fissure that separates the parietal lobe from the frontal lobe and the primary motor cortex from the primary somatosensory cortex. In addition, three of the four lobes are associated with "poles": the occipital pole, the frontal pole, and the temporal pole.

The hemispheres are macroscopically 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 cytoarchitecture of the cerebral cortex shows that the functions of cells, the quantities of neurotransmitters, and the types of receptors between the hemispheres is markedly asymmetrical.^[1][2] 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.