Internal Anatomy of the Midbrain

General Regions: Tectum, Tegmentum, and Basis Pedunculi

The midbrain is divisible into three regions, which can be appreciated best in cross section. Posterior to the cerebral aqueduct is the tectum (roof) of the midbrain. The characteristic structures of this area are the superior and inferior colliculi. The periaqueductal gray (central gray) is a sleeve of neuron cell bodies that completely surrounds the cerebral aqueduct. The tegmentum of the midbrain extends from the base of the tectum to, but does not include, the substantia nigra. The anterolateral portion of the midbrain on either side is formed by the basis pedunculi, which consists of the substantia nigra and the crus cerebri. In turn, the crus cerebri is composed primarily of descending fibers. The term cerebral peduncle is sometimes used for the crus cerebri, but it actually represents the entire midbrain below the tectum (tegmentum and basis pedunculi).