The midbrain, the upper portion of which evolved from the optic lobes, is the main centre of sensory integration in fish and amphibians. It also is involved with integration in reptiles and birds. In mammals the midbrain is greatly reduced, serving primarily as a connecting link between the hindbrain and the forebrain. Connected to the medulla, pons, and midbrain by large bundles of fibres is the cerebellum. Relatively large in humans, this “little brain” controls balance and coordination by producing smooth, coordinated movements of muscle groups.