The olfactory bulb and medial hemisphere wall of the rat-fish, Chimaera

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 hemispheres 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. Each olfactory bulb is divided into a dorsal and ventral part and each has a dorsal and a ventral accessory olfactory bulb. A dorsal and a ventral olfactory nerve unite the respective parts of the bulb with a nasal sac. The cup-shaped 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. Septal nucleu recognizable in this form include: a medial and a lateral septal 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.