The brain develops from three enlargements of the rostral end of the embryonic neural tube (Table 18-1). The enlargements become the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon). Subsequently the forebrain and hindbrain differentiate further, producing five primary divisions of the brain: telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon (Fig. 18-1). TABLE 18-1 7. Commissure of caudal colliculi 9. Decussation of trochlear nerves in rostral medullary velum 10. Middle cerebellar peduncle 11. Caudal cerebellar peduncle 12. Rostral cerebellar peduncle 14. Median sulcus in fourth ventricle 15. Nucleus cuneatus lateralis 18. Spinal tract of trigeminal nerve 19. Superficial arcuate fibers 20. Cochlear nuclei (dorsal and ventral) 21. Brachium of caudal colliculus 23. Brachium of rostral colliculus 24. Cut surface of internal capsule The nucleus ambiguus is a column of sparse neurons located ventral to the nucleus of the spinal tract of the trigeminal nerve (Fig. 18-9). Except for some visceral efferent neurons that innervate the heart (Fig. 18-7), the nucleus ambiguus contains somatic efferent neurons. Via vagus and glossopharyngeal nerves, the neurons send axons to striated muscles of the pharynx, larynx, and esophagus. The motor nucleus of the facial nerve (nucleus motorius n. facialis) is located ventrally in the medulla oblongata (Fig. 18-11). The nucleus contains somatic efferent neurons that innervate muscles of facial expression. Neurons are topographically arranged within the nucleus: rostral to caudal positioned neurons innervate rostral to caudal muscles, dorsal neurons innervate ventral muscles, and vice versa (Berman, 1968). White matter at the lateral edge of the medulla oblongata constitutes the caudal cerebellar peduncle (pedunculus cerebellaris caudalis) (formerly restiform and juxtarestiform bodies). The axons of the peduncle pass deep to the acoustic stria and turn abruptly dorsad to join the cerebellum (Fig. 18-10). The lateral recess of the fourth ventricle is located immediately caudal to the abrupt turn of the peduncle (Fig. 18-11). 8. Rostral cerebellar peduncle 12. Spinal tract of trigeminal nerve 13. Superficial arcuate fibers 14. Trapezoid body (transected) 15. Location of pontine sensory nucleus of trigeminal nerve
The Brain
EMBRYONIC BRAIN DIVISION
DERIVED BRAIN STRUCTURES
DEFINITIVE BRAIN CAVITIES
Forebrain
Telencephalon
Cerebral hemispheres
Lateral ventricles
Diencephalon
Thalamus, hypothalamus, epithalamus, subthalamus, metathalamus
Third ventricle
Midbrain
Mesencephalon
Cerebral peduncles and tectum
Mesencephalic aqueduct
Hindbrain
Metencephalon
Pons and cerebellum
Fourth ventricle
Myelencephalon
Medulla oblongata
Fourth ventricle
The Brainstem
The Medulla Oblongata
Caudal Half of the Medulla Oblongata
Level of the Facial Nucleus
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The Brain
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