Forebrain (Prosencephalon)

 Forebrain consists of three regions; namely olfactory lobes, cerebrum and diencephalon. Olfactory lobes are paired small, solid bodies present ventrally in the forebrain. They are covered by cerebral hemispheres dorsally and can be visible only from ventral side. They have olfactory bulbs and olfactory tracts. Olfactory tracts merge in the olfactory area of temporal lobe. Olfactory lobes are concerned with sense of smell. Poor sense of smell in human beings is probably due to the small size of olfactory lobes. Cerebrum is the largest and the most prominent complex part of forebrain. It makes 80-85% of the weight of the brain. Cerebrum is divided into two cerebral hemispheres by a median longitudinal fissure. Both cerebral hemispheres are interconnected by a thick band of transverse nerve fibres called corpus callosum. It is the largest commissure in human brain. A cavity filled with CSF is present in each hemisphere which is termed as lateral ventricle. The roof of lateral ventricle is pallium. The  surface of cerebral hemispheres is cerebral cortex. It is termed gray matter because it appears gray due to large collection of cytons. It is highly folded due to elevations and depressions (gyri and sulci). This folding increases the surface area. More of gray matter increases number of folds. The pattern and arrangement of these folds has relationship with intelligence. The deeper part of cerebral hemisphere is called cerebral medulla, which is mainly formed by white matter and contains axons of neurons. Masses of grey matter located in white matter are called basal nuclei. The ventrolateral walls of cerebrum (corpora striata) are thick. Three deep or wide sulci namely central sulcus, lateral sulcus and parieto-occipital sulcus, divide each cerebral hemisphere into four lobes frontal lobe, parietal lobe occipital lobe and temporal lobe. The frontal and parietal lobes are separated by central sulcus; the parietal and temporal lobes are separated by lateral sulcus while parieto-occipital sulcus separates parietal lobe from occipital lobe. General motor area which controls voluntary activities is located on the precentral gyrus of frontal lobe. The association area related with intelligence, memory, judgment, problem solving ability etc. if found on the frontal lobe. Broca's area is motor speech and controls movements of tongue, lips and vocal cards. General sensory area concerned with sensation of temperature, touch, pressure and pain, speech is located on the post central gyrus of parietal lobe. Sensary speech are is called wernicke's area. The area concerned with sense of taste, sense of hearing and sense of smell are located on the temporal lobe. Occipital lobe carries sensory visual area concerned with sense of vision and association visual area for perception, analysis and storing information obtained by sight. Diencephalon is present below the corpus callosum and above the midbrain. It is completely covered by cerebral hemispheres. It encloses a single cavity termed third ventricle (diocoel) which communicates with lateral ventricle through a narrow opening called foramen of Monro. A vascular region of anterior choroid plexus is located near the roof of third ventricle. It secretes CSF. Eptihalamus is the roof of diencephalon which gives an upward projection of pineal body. Pineal body and epithalamus function as an endocrine gland which secretes two hormones called serotonin activities while melatonin. Serotonin is involved in metabolic activities while melatonin is concerned with melanin pigment metabolism. Thalami are the lateral walls of diencephalon connected by habencular commissure. Thalami are connected to various parts of the brain by reticular activating system (RAS). All sensory impulses except olfactory impulses pass through thalamus except olfactory impulses pass through thalamus to the cerebrum. Hypothalamus is floor of diencephalon which gives a downward projection of pituitary gland. The anterior aspect of hypothalamus shows crossing of two optic nerves known as optic chiasma. The hypothalamus has many masses of grey matter called hypothalamic nuclei scattered in white matter. Diencephalon is relay centre for motor and sensory impulses between spinal cord, brain stem and various areas of cerebral cortex. It maintains homeostasis. Hypothalamus is the coordinator between nervous system and endocrine system. It secretes neurohormones (Stimulating and inhibiting factors) which influence the pituitary gland.