Cerebellum

The cerebellum lies at the back of the skull, under the occipital and temporal lobes and behind the brainstem, from which the fourth ventricle separates it. Its name means little brain, and it resembles a small copy of the cerebrum: two hemispheres joined by a narrow midline strip called the vermis, with a tightly folded surface layer of gray matter, the cerebellar cortex, over white matter that holds deep nuclei. It accounts for roughly a tenth of the brain's mass.

The cerebellum does not start movements; it fine-tunes them. Through the pons it receives copies of the motor commands the cerebral cortex sends to the spinal cord and compares each plan with feedback from muscles, joints and the balance organs of the inner ear. When plan and result disagree, as on uneven ground, it sends corrections that keep movement smooth, accurate and well timed. It also maintains posture and is needed to learn skills such as riding a bicycle.

Damage from a stroke, a tumor or long-term heavy drinking causes ataxia (clumsy, poorly coordinated movement), an unsteady wide-based walk, a tremor that worsens as the hand nears a target, and slurred speech. Because of how its pathways cross, each half of the cerebellum controls the same side of the body.