One of the three sections of the brain of a vertebrate embryo. Unlike the forebrain and the hindbrain, the midbrain does not undergo further subdivision to form additional zones. In mammals it becomes part of the brainstem. The roof of the midbrain is called the tectum (otherwise termed the optic lobe in birds, amphibians, and fishes), while the floor is the tegmentum; between them is a narrow channel, the cerebral aqueduct. The tegmentum contains the substantia nigra (‘black matter’) and the red nucleus, both of which are involved in controlling voluntary movements. In mammals a pair of anterior prominences in the tectum (the superior colliculi) receive input from the retina of the eyes, as well as other sensory information. These integrate the information to form a visual map of the individual’s surroundings, and instigate eye movements, head turning, and other spatial movements of the body. The paired inferior colliculi in the tectum receive input from auditory fibres and project to the auditory relay nucleus of the thalamus, thus serving as a vital relay station in the circuitry responsible for hearing. Maps from different senses are held in different layers of the tectum, but are organized according to the same spatial geometry. In nonmammalian vertebrates the tectum is relatively larger than in mammals, and takes a greater role in controlling body movements.