It is the largest and the most important part of diencephalon. The hypothalamus constitutes the floor of the diencephalon. These ganglia are well developed in sharks and is known as geniculate lobes. The ventral part of thalamus bears geniculate nuclei or ganglia. The thalamus acts as a relay centre for transmitting olfactory and strait body impulses to thalamomedullar and thalamospinal tract. The thalamus consists of lateral walls of the diencephalon. It is also expected to be an endocrine gland with internal secretions. The secretory functions involves the external secretion related to the chemical composition of the cerebrospinal fluid or the metabolism of brain tissue. In sensory roles it acts as a baro or chemoreceptor for cerebrospinal fluid or helps in facilitating the olfactory response to sex hormones. According to the first hypothesis, the pineal body has sensory roles while the second hypothesis states its secretory function. Regarding the function of pineal body, two theories have been put forward. The epithalamus has two ganglionic masses, called the ‘habenulae’, which are of equal size and also the nerve fibres from the telencephalon which connects with the thalamus, hypothalamus and the olfactory areas of the telencephalon (Figs. The posterior part of the roof elevates to form pineal gland or epiphysis. The epithalamus contains a choroid plexus in its roof. The diencephalon is distinguished into three parts, i.e., a dorsal epithalamus, lateral thalamus and a ventral hypothalamus. The diencephalon possesses cavity inside, called third ventricle. Its small area is visible between cerebral hemispheres and optic lobes. The diencephalon is covered dorsally by the posterior bulging’s of the cerebral hemispheres. In goldfish the colour vision is suppressed as result of extirpation of forebrain. In stickleback, the spawning is suppressed and in paradise fish (Macropodus opercularis) the nest building behaviour is affected, when their forebrains are removed. The removal of forebrain from Tilapia results in neglecting their young’s, while in certain minnows (Cyprinidae) make them less cautious to their new situation. It is evident from ablation of forebrain of goldfish (Carrasius auratus) which shows less response than normal ones. The telencephalon also facilitates activities of lower brain centres and mechanism. In addition to the olfaction, the telencephalon is thought to serve other generalized function such as to regulate aggression, sexual activity and parental behaviour, maintain body equilibrium to facilitate successful breeding. The forebrain of actinopterygains has a single, median ventral ventricle in contrast to the paired ventral (basal) ventricles in lower fishes.
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