The diencephalon is subdivided into four layers lying on top of each other:
! The epithalamus (A – C1)
! The dorsal thalamus (A – C2)
! The subthalamus (A – C3)
! The hypothalamus (A – C4)
The simple arrangement of these layers is still clearly visible in the embryonic brain. However, it changes considerably during development owing to differences in re-gional growth. In particular, the extraordi-nary increase in mass of the dorsal thalamus and the expansion of the hypothalamus in the region of the tuber cinereum determine the structure of the diencephalon.
The epithalamus consists of the habenulae, a relay station for pathways be-tween the olfactory centers and the brain stem, and of the pineal gland (epiphysis cerebri). Owing to the increasing size of the thalamus, the dorsally situated epithalamus (B1) becomes medially transposed and ap-pears only as an appendage of the dorsal thalamus (C1).
The dorsal thalamus is the terminal station of sensory pathways (cutaneous sensibility; taste; visual, acoustic, and vesti-bular pathways). It is connected to the cere-bral cortex by efferent and afferent fiber systems.
The subthalamus is the continua-tion of the midbrain tegmentum. It contains nuclei of the extrapyramidal motor system (zona incerta, subthalamic nucleus, globus pallidus) and may be regarded as the motor zone of the diencephalon.
The globus pallidus, or pallidum (CD5), is a derivative of the diencephalon. It becomes separated from the other gray regions of the diencephalon as a result of the ingrowing fiber masses of the internal capsule (CD6) during development and finally becomes displaced into the telencephalon. Only a small medial rest of the pallidum remains within the unit of the diencephalon; this isthe entopeduncular nucleus. As a constitu-ent of the extrapyramidal system, the globus pallidus should logically be regarded as part of the subthalamus.
The hypothalamus is derived from the lowest layer and forms the floor of the diencephalon from which the neurohy-pophysis (A7) protrudes. It is the highest regulatory center of the autonomous nervous system.
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