The midbrain, or mesencephalon, is a fairly short portion of the brainstem that better preserves the basic organization of the neural tube than do other parts of the brainstem.
The midbrain is represented on the ventral surface of the intact brain by the crus cerebri (cerebral peduncles Fig. 8.19/12), the interpeduncular fossa, and the superficial origin of the oculomotor nerves (III).
The midbrain is concealed dorsally by the overhanging cerebral hemispheres and cerebellum. Its lumen, the mesencephalic aqueduct, is a simple canal joining the much larger cavities of the third and fourth ventricles. The mesencephalon has a stratified structure, comprising from dorsal to ventral: tectum, tegmentum, and cerebral peduncle (Fig. 8.29).The tectum lies dorsal to the aqueduct. Its major features are four rounded surface swellings (see Fig. 8.22). The paired caudal swellings, the caudal colliculi, are widely spaced and are joined by a substantial commissure. They serve as important integration centers for auditory pathways (p. 287). The brachium, an axonal tract from of the caudal colliculus to the ipsilateral medial geniculate body in the thalamus, is visible as a distinct ridge. The rostral colliculi are closer together and are joined to the lateral geniculate bodies of the thalamus by similar but less obtrusive brachia. The rostral colliculi, an important integration center for visual pathways, are involved in somatic reflexes initiated by visual input, such as head movements and startle responses.
The tegmentum constitutes the core of the midbrain and is directly continuous with the corresponding region of the pons. As such, much of the midbrain tegmentum is formed by the reticular formation. The principal mesencephalic nuclei include the nuclei associated with cranial nerves—the mesencephalic nuclei of the trigeminal nerves (V), the trochlear nuclei (IV), the principal and parasympathetic oculomotor nuclei (III)—the red nuclei (named for their pronounced vascularity), and the periaqueductal gray, a core of gray substance surrounding the aqueduct. The substantia nigra is a prominent lamina in the tegmentum that can be identified in transverse sections by its darker color, which is due to the gradual accumulation of melanin pigment within the constituent neurons. Like the red nucleus, it is associated with the basal nuclei (p. 277) in the control of voluntary movement.
The crura cerebri are visible on the ventral surface of the brain. They comprise fiber tracts that travel from the telencephalon to the caudal brainstem. These fibers converge as they emerge from the telencephalon, although they are separated by the interpeduncular fossa (see Fig. 8.19/13). The oculomotor nerves (III) also emerge in this region, directly rostral to the pons.