The Chimaerae are phylogenetically old cartilagenous fish which are included in the class Chondrichthyes with the sharks and rays. The telencephalon of these fish is connected to the diencephalon by a unique fibrous stalk. The telencephalic heniispheres are united only ventrally at midhemisphere levels in the region of the hippocampal and anterior commissures. The lateral ventricles join the midline ventricle through the interventricular foramen at this level.Ea.ch olfactory bulb is divided into a dorsal and ventral part and each has a dorsal and a ventral accessory olfactory bulb. A dorsaI and a ventral olfactory nerve unite the respective parts of the bulb with a nasal sac. The cupshaped mitral cell laminae share a common granular cell layer where they are in apposition. The granular lamina is replaced caudally by the anterior olfactory nucleus.The medially situated primordial hippocampal formation consists of the anterior continuation, the primordial subiculum, the cornu ammonis and the dentate gyrus.Septa1 nuclei recognizable in this form include: a medial and a lateral septa1 nucleus, a septohippocampal nucleus, a nucleus triangularis, an accumbens nucleus, and a nucleus of the diagonal band. The medial zone of the olfactory tubercle occupies the ventromedial hemisphere wall. Major fiber connections of the medial limbic lobe include: the medial olfactory tract, the fornix, the medial forebrain bundle and the medial corticohabenular tract.
The lateral telencephalon of Chimaera possesses several unique features but also has nuclei and fiber systems homologous with those of other sub-mammalian vertebrates. Ventricular ridges, similar to those of reptiles, are quite evident. ACcessory olfactory bulbs are associated with the dorsal and ventral parts of each olfactory bulb. These contribute to the lateral olfactory tract.The internal granular layer caudal to the olfactory and the accessory bulbs blends with the anterior olfactory nucleus. Caudal to this nuclear area, the nuclei of the rostral telencephalon are well differentiated.Nuclear areas distinguishable in the lateral hemisphere include: the primordial dorsal pallium, the primordial piriform cortex, the primordial striatal and amygdaloid nuclei, and the lateral zone of the oHactory tubercle. These areas replace dorsal, dorsolateral, ventrolateral and ventral parts of the anterior olfactory nucleus, respectively. The primordial striatum is subdivided into hyperstriatum, neostriatum, paleostriatum augmentatum and paleostriatum primitivum. The amygdaloid area has anterior, corticomedial and basolateral nuclear groups. The basolateral area is best differentiated. The hyperstriatum forms a rostral ventricular eminence; the basolateral amygdaloid nucleus is present in a larger caudal ventricular ridge.Fiber tracts of the lateral wall include the lateral olfactory tract, the lateral corticohabenular tract, the lateral forebrain bundle and the stria terminalis. Nuclei of medial and lateral walls are interrelated through the hippocampal and the anterior commissures.
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