It is formed of nervous tissue and contains a cavity which is continuous with the III ventricle or diocoel of diencephalon. Just behind the optic chiasma is a flattened bilobed infundibular lobe or infundibulum extending posteriorly and divided by a median longitudinal groove. On the ventral side of diencephalon is the optic chiasma or crossing of the optic nerves which go the eyes. The pineal stalk or epiphysis, which originally was continuous with the brow spot, becomes constricted off from it in early larval life. Its roof is thin and lined with a vascular membrane, the anterior choroid plexus.īehind it arises a hollow, thin-walled stalk, called the pineal stalk which terminates dorsally at the brow spot. Its lateral walls are thick called optic thalami (singular thalamus) and its thick floor is called the hypothalamus. (c) Diencephalon or thalamencephalon is a short unpaired structure of the forebrain situated behind the cerebral hemispheres.
They have a thick roof called the pallium in which more nerve cells have moved to the periphery. The corpora striata of two hemispheres are joined by a transversely running tract of fibres called anterior commissure and above which is another commissure (upper line) partly representing the hippocampal commissure of the brain of reptiles and mammals. On ventro-lateral side of each of the cerebral hemispheres there is a thick fibrous tract called the corpus striatum containing a network of white medullated nerve fibres and nerve cells. Fibres of the olfactory, tactile and optic impulse reach the cerebral hemisphere which may act as correlating centres but the hemispheres are largely olfactory in function. The nerve cell bodies form masses around the lateral ventricles and lie in layers. Posteriorly the lateral ventricles communicate with each other and with the ventricle of diencephalon called diocoel by an opening, the interventricular foramen or foramen of Monro. Each cerebral hemisphere has a large lateral ventricle or paracoel which is continuous anteriorly with the rhinocoel. They are separated from each other by a deep median longitudinal fissure. They are wider behind and narrower in front.
The two cerebral hemispheres are long, oval structures separated from olfactory lobes by a slight constriction. Each lobe gives off an olfactory nerve and possesses a small cavity rhinocoel or olfactory autricle. The olfactory lobes are anterior small, spherical structures which are fused together in the median plane. It is the largest part of the brain consisting of a pair of anteriorly directed olfactory lobes, a pair of cerebral hemispheres, and a diencephalon.
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