Apart from certain modifications in the medulla oblongata (A1), pons (A2), and mesencephalon (A3), the brain stem has auniform structure. The phylogenetically old part of the brain stem, which is common to all three parts and contains the cranial nerve nuclei, is the tegmentum (A4). At the level of the medulla oblongata and the pons, it is overlain by the cerebellum and in the mesencephalon by the tectum (quadrigemi-nal plate) (A5). The ventral part of the brainstem mainly contains the large tracts de-scending from the telencephalon; they form the pyramids (A6) in the medulla oblongata, the pontine bulb (A7) in the pons, and the cerebral peduncles (A8) in the mesen-cephalon.
The ventricular system undergoes consider-able narrowing in the midbrain, the aque-duct of the mesencephalon (cerebral aqueduct,aqueduct of Sylvius) (A–D9). Duringdevelopment, the lumen of the neural tube becomes increasingly narrowed as the teg-mentum of the midbrain increases in volume (B), while the blueprint of the neural tube survives. The motor derivatives of the basal plate lie ventrally: the nucleus ofthe oculomotor nerve (BC10), the trochlear nucleus (eye muscles), the red nucleus (C11),and the substantia nigra (C12) (consisting of an outer reticular part and an inner compact part). The sensory derivatives of the alar plate lie dorsally: the tectum of the mesen-cephalon (quadrigeminal plate) (C13) (syn-aptic relay station for auditory and visual pathways).
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