Approximately 2 cm in length, the midbrain connects the forebrain to the pons caudally and the cerebellum posteriorly (eFig. 9.32A). Along the midline of the anterior surface there is a deep depression, the interpeduncular fossa, which has several perforations on its surface where small vessels perforate through the floor of the fossa. On either side of the interpeduncular fossa are the crus cerebri (eFig. 9.32B). Projecting medially from the crus is cranial nerve III, the oculomotor nerve (eFig. 9.33A). The most prominent features on the posterior surface are the superior and inferior colliculi (eFig. 9.32A). Between the inferior colliculi, cranial nerve IV, the trochlear nerve, emerges, crosses the midline, and wraps around the lateral aspect of the midbrain (eFig. 9.32A). Laterally, the superior brachium and inferior brachium can be seen as they project in an anterolateral direction from the superior and inferior colliculi.