Structure of human brain(Encephalon):
It is soft,whitish,large sized and slightly flattened structure present inside cranial cavity of cranium of the skull.In man ,it is about 1200-1400 gm in weight and has about 10,000 million neurons.Brain is made up of 3 parts:
1.Fore brain or Prosencephalon:
Prosencephalon divides into an anterior telencephalon and posterior diencephalon; the mesencephalon remain unchanged. The rhombencephalon divides into an anterior metencephalon and posterior myelencephalon; Ultimately ,telencephalon develops into cerebral hemisphere and basal ganglia and houses lateral ventricle. Diencephalon develops into thalamus,hypothalamus ,and pineal gland and houses cerebral aqueduct.Mesencephalon develops into pons and cerebellum; and myelencephalon develops into medulla oblongata ,houses 4th ventricle. The area of neural tube inferior to myelencephalon gives rise to spinal cord.
It forms anterior two-third of brain and is formed of three parts:
These are one pair,small sized,club-shaped,solid,completely covered by cerebral hemisphere dorsally. Each is differentiated into two parts:
Posterior and narrow part which ends in olfactory area of temporal lobe of cerebral hemisphere.
These control the smell
(a).It is normal in frog,rabbit and man.
(b).It is well developed in dog,So power of smell is more in dog.
(c).These are also well developed in dog fish and name dog fish is on the basis of well developed olfactory lobes.
What is the cerebrum:
Cerebrum is divided into 5 lobes:
A lobe called insula is hidden as it lies deep in the sylvian fissure.The cerebral hemisphere are separated from olfactory lobes by rhinal fissure. The median fissure divides the cerebrum into a right and a left cerebral hemisphere.
A few sulci are well developed and form three deep and wide fissures which divide each cerebral hemisphere into four lobes: Anterior frontal lobe,middle parietal lobe,posterior occipital lobe and lateral frontal lobe e.g.Fissure sulcus lying between the frontal and parietal lobes is central fissure or sulcus,that is lying between the parietal and occipital lobes is parieto-occipital fissure and that demarcating frontal and parietal lobes from the temporal lobe is lateral or Sylvian fissure.Each cerebral hemisphere is with a fluid-filled cavity called lateral ventricle or paracoel.
Two cerebral hemispheres are interconnected by thick band of transverse nerve fibres of white matter called corpus callosum.The peripheral portion of each cerebral hemisphere is formed of grey matter and is called cerebral cortex,while deeper part is formed of white matter and is called cerebral medulla. Cerebral cortex is the highest centre for many sensations and activities and is with a number of sensory areas.Cerebral cortex 2-4 mm thick.
Histology of cerebrum:
The whole brain possess grey matter outside and white matter inside around ventricle.
In cerebrum grey matter is very much developed,it is on an average 2-4 mm.thick but at poles its thickness is 1.3 mm. It is thickest at pre central gyrus (4.5 mm thick).Grey matter of cerebrum is called cortex or pallium.Phyllogenetically or evolutionarily cortex is divided into 3 parts:-
(a)Allocortex or Paleocortex:
It is the cortex of olfactory area of frontal lobe and olfactory bulbs.In lower vertebrates(cartilagenous fish) olfactory lobes occupy most of the part of cerebrum.So in these animals sense of olfaction is very-very much developed.Sense of olfaction is oldest sense.
It is relatively not much older in development.
(c).Neocortex or neopallium or isocortex or neencephalon:
It is most recent cortex and is developed maximum only in human.It is prefrontal cortex or prefrontal region(organ of mind),precentral and postcentral gyrus etc.The neocortex is having 6 layer of neurons while remaining cortex possess only 5 layers. The cerebral cortex is having area of about 2200cm2while the cranial cavity is only 1450 cm3,so to accomodate cerebrum there appears folding in the cortex.The ridges are called gyrus(or gyri) or convolution while the depression are called sulcus(sulci in plural).
It is inner part of brain.White matter is aggregation of myelinated and unmyelinated axons of many neurons.Its fibres are divided into 3 categories:
These neurons connect gyri of 2 hemispheres,such as corpus callosum.habencular commissure,anterior commissure,posterior commissure.
They connect gyri of same hemisphere.
They are infact ascending and descending nerve tract,they connect one part of brain to another part of brain or to spinal cord.(In spinal cord they were called as column).
Associated structures of cerebrum:
C1erebrum has following specific structure.
Nuclei on white matter matter.It is cluster of grey neurons in depth of white matter,they are formed in whole brain and are named differently.