The cerebrum is divided into two symmetric cerebral hemispheres; left and right hemispheres. The cerebrum lies on top of the brainstem and is the largest and most well-developed of the five major divisions of the brain. It is the newest structure in the phylogenetic sense, with mammals having the largest and most well-developed among all species. The cerebrum is composed of the cerebral cortex, basal ganglia, and the olfactory bulb. In human beings, the cerebral cortex is folded into many gyri and sulci, which has allowed the cortex to expand in surface area without taking up much greater volume.
The cerebrum surrounds older parts of the brain. Motor areas of the cerebral cortex as well as limbic and olfactory systems project fibers from the cerebrum to the brainstem and spinal cord. Fibers connect the cognitive and volitive systems of the cerebrum to the thalamus and other regions of the midbrain. The cerebrum's networks of neurons make possible complex behavior such as social interactions, speech and language, learning, memory, and planning.
The cerebrum is also the center of consciousness and volition (will), located in the frontal lobe. Upper motor neurons in the primary motor cortex send their axons to the brainstem and spinal cord to synapse on the lower motor neurons that contract the muscles. Damage to motor areas of cortex can lead to certain types of motor neuron disease such as paralysis.
Language is attributed to certain parts of the cerebral cortex. Motor portions of language are attributed to Broca's area within the frontal lobe; this area is responsible for the dextrous and precise movement of the tongue. The Wernicke's area, at the temporal-parietal lobe junction, is responsible for speech comprehension. The arcuate fasciculus, a large white matter tract, interconnects these two regions. Damage to the Broca's area can result in expressive aphasia, while damage to Wernicke's area results in receptive aphasia.