Diencephalon [interbrain]

The diencephalon is a portion of the encephalon. Together with the telencephalon (the cerebral hemispheres plus the basal nuclei) it constitutes the brain. Median and symmetrical, with a truncated pyramid shape, it is intercalated between the lower axial centers (spinal cord, brainstem) and the upper axial centers (telencephalon). Within it is found the cavity of the third ventricle.

The diencephalon is located rostrally with respect to the midbrain, with which it shares some nuclear formations, and is completely lined by the telencephalon (with the exception of its ventral surface, which freely emerges at the encephalic surface). Consequently, it is visible only after extensive demolition of the telencephalic hemispheres and the white matter surrounding it.

It includes the 3rd ventricle, covered by the respective chorioid canvas. There is a dorsal part, epithalamus, a lateral one, thalamus, and a ventral one, hypothalamus. In the epithalamus there are two symmetrical lateral formations called abenular ganglia present in all vertebrates but relatively more developed in lower forms, having mainly value of olfactory centers, and two ependymal extroversions, the epiphysis or pineal organ and the parietal organ.

In the Petromyzonts both these vesicles form rudimentary eyes capable of distinguishing light from darkness. In Reptiles, especially Rhynchocephals, the parietal organ forms a rudimentary eye, while in Birds and Mammals it is barely sketched. In all Vertebrates, except Mammals, posterior to the epiphysis is formed the so-called subcommissural organ, consisting of cells secreting a viscous substance, whose function is still uncertain, which flows through the ventricles in a caudal direction to the height of the spinal cord, where it fluidizes.

In Mammals this organ is transformed into a group of ependymal rosettes with endocrine function. The lateral parts of the diencephalon, the thalami, constitute an important center of integration of all impulses arriving to the brain from the periphery and of those leaving the encephalon. In general, the dorsal part of the thalamus contains sensory nuclei, the ventral part contains motor nuclei, which increase in number, development and specialization as we climb the Vertebrate scale. The lower part of the diencephalon, the hypothalamus, includes four regions recognizable in all Vertebrates: preoptic, chiasmatic, infundibular, and posterior recessus.

In the chiasmatic region the optic nerves cross (decussation of the optic fibers is complete in all Vertebrates except Mammals). The infundibular region includes the infundibulum, at the end of which there is the neurohypophysis with two lateral lobes, developed in Fishes, more reduced in Amphibians and Cyclostomes, absent in other Vertebrates. In Fishes, in relation with the bottom of the infundibulum, the vascular sac develops, provided with neuroepithelium with function of receptor of hydrostatic pressure and of regulator of the amount of fluid in the ventricles.

In Mammals the infundibular region is represented by the tuber cinereum ending with the infundibulum in a narrow sense. The region of the posterior recess is also called mamillary because in Mammals and also in Chondrichthyes two protuberances called mamillary bodies develop there. In all classes of Vertebrates the hypothalamus is a superior center of integration of the visceral activities of the organism. It also exerts considerable control over the activity of the pituitary gland. The hypothalamus presides over water turnover, homeothermic thermal regulation, and generally the regulation of the central nervous system.

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