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 (Prosencephalon)
(i) Olfactory lobe – Rhinencephalon
(ii) Cerebrum – Telencephalon
(iii) Diencephalon – Diencephalon

- (2) Mid brain (Mesencephalon)
(i) Optic lobes – Mesencephalon

- (3) Hind brain (Rhambencephalon)
(i) Cerebellum – Metencephalon
(ii) Medulla oblongata – Myelencephalon
(1) Fore brain or Prosencephalon : 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 –
(a) Olfactory bulb : Anterior, swollen part, and
(b) Olfactory tract : Posterior and narrow part which ends in olfactory area of temporal lobe of cerebral hemisphere.
Function : These control the smell.
 It is normal in frog, rabbit and man.
 It is well developed in dog. So power of smell is more in dog.
 These are also well developed in dog fish and name dog fish is on the basis of well developed olfactory lobes.

# (ii) Cerebrum : (a) Structure is divided into 5 lobes (i) frontal (ii) parietal, (iii) occipital, (iv) temporal and (v) limbic.
- 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 temporal lobe e.g. Fissure lying between the frontal and parietal lobes is central fissure, that 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 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.

# (b) Histology of cerebrum : The whole brain possess grey matter outside and white matter inside around ventricle.

- (1) Grey matter : In cerebrum grey matter is very much developed, it is on an average 3.5 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 –
(i) Allocortex or paleocortex : It is the cortex of olfactory area of frontal lobe and olfactory bulbs. In lower vertebrates (cartilagenous fish) olfact lobes occupy most of the part of cerebrum. So in these animals sense of olfection is very-very much developed. Sense of olfaction is oldest sense.
(ii) Mesocortex : It is relatively not much older in development.
(iii) Neocortex or neopallium or isocortex or neencephalon : It is most recent cortex and is developed maximum only in human. It is in prefrontal cortex or prefrontal region (organ of mind), precentral and precentral gyrus etc. The neocortex is having 6 (six) layer of neurons while remaining cortex possess only 5 layers.
The cerebral cortex is having area of about 2200 cm2 while the cranial cavity is only 1450 cm3, so to accomodate cerebrum there appears foldings in the cortex. The ridges are called gyrus (or gyri) or convolution while the depression are called sulcus (sulci in plural).

- (2) White matter : It is inner part of brain. Its fibres are divide into 3 categories –
(i) Commissural fibers : These neurons connect gyri of 2 hemispheres, such as corpus callosum. habenular commissure, anterior commissure, posterior commissure.
(ii) Associate fibres : They connect gyri of same hemispheres.
(iii) Projection neuron : They are infact assending 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 columo).
(c) Associated structures of cerebrum : Cerebrum has following specific structure.
# (1) Sub cortex : Nuclei on white matter. It is cluster of grey neurons in depth of white matter, they are formed in whole brain and are named differently.

# (2) Basal granules or central nucleus : Basal ganglia is the name given to many sub cortical structure of walls of paracoel, hypothalamus and mid brain –
(i) Corpus striatum : Corpus striatum is the name given to caudate nucleus and lenticular nucleus. Caudate is tail shaped while the lenticular nucleus is lenti shaped. The lenticular nucleus is sub-divided in putamen (outer shell) and globus pallidus (ball).
(ii) Claustrum : It is the name given to grey matter present between insula and patamen.
(iii) Epistriatum or Amygdaloid body : It is structure present at the end of caudate nucleus.
(iv) Red nucleus and substantia nigra of mid brain.
The basal ganglia controls automatic movements of skeletal muscles like swinging, walking etc.

# (3) Corpus callosum : It is the band of white neurons present between both cerebral hemisphere and connect them on medial surface. It is present only mammal. It has anterior part genu, middle part trunchus and last part splenium.
Below corpus callosum there are two fused band of white neurons called fornix. There anterior part is called column and posterior part is called crura. Between column and genu a membrane is called septum lucidum or septum pellicidum. Septum lucidum encloses a space called V5 or Pseudocoel, because it is not possessing C.S.F. i.e. why it is called pseudocoel.

# (4) Limbic system : It is also called emotional brain or animal brain. Limbic system controlling emotion, animal behaviour like chewing, licking, sniffing, rage, pain, plessure, anger, sexual feelings, grooming. It has following structure
(i) Singulate gyrus : It is a region of pre central gyrus.
(ii) Hippocampal gyrus : It is a region of temporal lobe near colossomarginal sulcus. These two structure are combinely called limbic lobe.
(iii) Amygdaloid body : It is the end of caudate nucleus.
(iv) Olfactory bulb : They are on the inferior anterior surface of brain. Olfactory nerve ends in these bulb.
(v) Mammillary body : They are found in hypothalamus. Olfactory bulb and mammillary body both are centre of olfaction.
From a evolutionary point of view, the cerebral hemisphere are the highly evolved structure and this is manifested by
(a) Great increase in the number of feed back circuits between cerebral cortex and sub cortical elements.
(b) The ability of man and other primates to perform variety of complex function.
(c) The lobe of cerebrum are delineated by fissure and sulci.
(d) A corpus callosum connects the left and right cerebral hemisphere. This is a unique property of mammals as it facilitates flow of information between the 2 hemispheres.
(e) The cortical layer of cerebrum is thrown into folds (= gyri) separate by sulci. All the larger mammalian brains exhibit well developed gyri. The degree of convolutions of the cortex is a fairly reliable indicator of the evolutionary stages of development of brain. The roof of cerebrum is called pallium while the ventrolateral walls are thick and are called corpora striata.

(c) Hypothalamus : The hypothalamus is visible in the ventral view of the brain and forms the floor of diencephalon. Hypothalamus also gives a nervous process called infundibulum (forms pars nervosa) which meets a rounded non-nervous pharyngeal outgrowth called hypophysis. Both collectively form master gland called pituitary body. A stalked outgrowth of infundibulum combines with a pouch-like epithelial outgrowth (Rathke’s pouch) of the roof of embryonic mouth (= stomodaeum), forming a pituitary gland or hypophysis. Which secretes a number of hormones. In front of hypothalamus, there is cross of two optic nerves called optic chiasma. Behind the hypothalamus, there is one pair of small, rounded, nipple-like bodies called mammilary bodies or corpora mammillares. The hypothalamus consists of many masses of grey matter, called hypothalamic nuclei, scattered in the white matter.
In man and some other mammals, most fibres of optic nerves cross, but some fibres do not cross and innervate the eyes of their own respective sides. This arrangement enables man and these mammals to have a binocular vision. Rabbits simply have a monocular vision.
Pineal gland is a pine cone-shaped gland. It is located in the center of brain with which it loses all nerves connection after birth. It is innervated by sympathetic nerves. It has a photosensory role in amphibian and primitive reptiles and is called ‘Third eye’. Pinealocytes secretes melatonin. Mammalian pineal does not act as photoreceptor but it produces the hormone called melatonin which is anti FSH, and anti LH. It inhibits reproductive function. Melatonin secretions decrease after puberty.
Function of fore brain
(1) Olfactory lobe : It is centre of smell.
(2) Cerebrum : Cerebral cortex is made up of grey matter and differentiated into –
(i) Sensory area
(ii) Motor area
(iii) Associated area
Sensory and associated area confirm, recognise and evaluate for shape, colour, sound, taste and smell for sensory cells in relation with object.
Broca’s area : Known as sensory speech area or motor speech area. Translate thought into speech. Located into frontal lobe towards left side. It is associated with language area and also interpriate translation of written words into speech. Damage or injury in Broca’s area (sensory or motor speech area) may result
(i) Aphasia(Inability to speak) (ii) Word deafness, (iii) Word blindness
Cerebrum is a centre for
(i) Intelligence (ii) Emotion (iii) Will power
(iv) Memory (v) Consciousness (vi) Imagination
(vii) Experience (viii) Knowledge (ix) Reasoning
(x) Voluntary controls (xi) Weeping and laughing (xii) Micturition
(xiii) Defecation
If cerebrum is removed animal becomes simple reflax animal.
(3) Diencephalon : It is centre for
(i) Carbohydrate metabolism (ii) Fat metabolism
(iii) It relays impulses from posterior region of brain and also to posterior region of brain.
(iv) Its secretes neurohormone (v) From part of pituitary gland
(vi) Secrete cerebrospinal fluid
Hypothalamus : It is floor of diencephalon and centre for
(i) Hunger (ii) Thirst (iii) Sweating
(iv) Sleep (v) Fatigue (vi) Temperature
(vii) Anger (viii) Pleasure, love and hate (ix) Satisfaction
(x) It is also centre to release factors for endocrine glands.
(xi) It also control A.N.S (autonomic nervous system)
(xii) Centers for regulation of parasympathetic (cranio-sacral) activity. When stimulated, it causes slowing down of heart beat, contraction of the visceral muscles.

(4) Mid brain or mesencephalon : It is also completely covered by cerebral hemisphere. It is formed of two parts –
(i) Optic lobes : These are one pair, large sized lobes present on dorsal side. Each is divided transversely into upper and larger superior coliculus and lower and smaller inferior coliculus. So there are four optic lobes, so called optic quadrigemina (only in mammals). In frog these are known as bigemina. Valve of vieussens It joins the optic lobe with cerebellum.
(a) Superior optic lobe or superior celliculus : They are concerned with reflex action of eye, head and neck in response to visual stimulus.
(b) Inferior calliculus : They are concerned with movement of head and trunk in response to hearing stimulus.
(ii) Cerebral peduncle (crura cerebri) : They are the pair of thick bands of longitudinal nerve fiber present on the floor or ventral side of mid brain. The dorsal part of cerebral peduncle (white matter) is called Tagmentum while most ventral part (gray matter) is called crura cerebrae or crus of cerebrum. Dorsal thick wall of mid brain is known as optic tectum. Iter is between tegmentum and tectum. Cerebral peduncle are infect possessing assinding and desending tracts, connecting upper and lower region of brain.
In white matter of cerebral peduncle these are following sub cortical structure
(a) Red nucleus or rustrum nucleus : They are red because rich blood supply and iron containing pigment or haemoglobin.
(b) Substantia nigra : It is black because of much deposition of melanin.
(c) Occulomotor nucleus : It is origin point of 3rd cranial nerve (occulomotor) from this region 4th (Trochlear) nerve also originates.
Reticular activating system : A diffuse network of nerve cell bodies and nerve tracts extends through the brain stem. It is called reticular activating system (RAS). It screens sensory information so that only certain impulses reach the cerebrum. For example, when you are alternatively listening to a lecture in the classroom, you are unaware of rustle of papers from those around you and from the touch of your clothes to the skin. The RAS is also important in overall activation and arousal. When certain neurons in RAS are active, we are awake, when they are inhibited by other neurons, we sleep. The pons and medulla have sleep centres that cause sleep when stimulated. Midbrain has an arousal centre which causes arousal.
Function of Mid brain
(a) Pair of anterior optic lobes (which are also known as superior collici) is related with vision.
(b) Pair of posterior optic lobe (known as inferior collici) related with auditory.
(c) These act as coordination centres between hind and fore brain.

(5) Hind brain : Consists of (i) cerebellum and (ii) medulla oblongata (iii) Pons varolii.

# (i) Cerebellum (Sandwitched brain) : Cerebellum is highly convoluted and well developed in mammals. It controls the most intricate movements of the body. It coordinates sensory information received from muscles/joints, visual, auditory and equilibrium receptors as well as flow of impulses from cerebral cortex.
Cerebellum is made up of –
(a) Vermis, (b) Cerebellar lobes (= floccular lobes), (c) Lateral lobes, (d) Pons.
The pons is a thick band of transverse nerve fibers. Cerebellum is joined to parts of brain by afferent and efferent fibres. Mid brain, pons and medulla have several similar functions and they constitute the brain stem. Peripheral part is formed of grey matter and is called cerebellar cortex while the central part is formed of white matter and is called cerebellar medulla. The white matter forms a tree-like branching pattern called arbor vitae, so the cerebellum is solid internally.

# (ii) Medulla oblongata
Medulla oblongata is the hindest and posterior most part of brain. Cavity is known as IVth ventricle (metacoel). Which is continuous with central canal of spinal cord. It has a pair of lateral Foramina of Luschka and a median foramen magendic. Cerebrospinal fluid come in contact by these apertures from internal cavity of the brain to outer fluid of meninges. A arrangement on its ventral surface there are buldgings of ascending and descending tract which are called pyramids. On the ventral surface these pyramids cross each other which is called deccusatn of pyramids. On the dorsal side of medulla there are two nuclei which are called nucleus gracitis (long) and nucleus cunaeus. On floor of V4 there is groove called calamus scroptosious.
In the medulla oblongata, most of the sensory and motor fibres cross from one side to the other. Thus, the left cerebral hemisphere controls the right side of the body and vice versa. The reason for this is not known. The lower end of medulla passes into the spinal cord. There is no demarcation between the two. However, the medulla is considered to start at the level of the foramen magnum of the cranium.

# (iii) Ponus Varolii : An oval mass, called the pons varolii, lies above the medulla oblongata. It consists mainly of nerve fibres which interconnect the two cerebellar hemispheres and also join the medulla with highrt brain centres, hence its name pons means bridge. Pons possess pneumotoxic and apneustic areas or centre. From pons 5, 6, 7 and 8th cranial nerve originate.
Function of hind brain
# (1) Cerebellum –
(i) Poorly developed in frog but well developed in mammal.
(ii) It is centre for co-ordination of muscular movement.
(iii) It is primary centre for balancing, equilibrium, orientation.

# (2) Medulla oblongata contain centre for
(i) Heart beats
(ii) Respiration
(iii) Digestion
(iv) Blood pressure
(v) Gut peristalsis
(vi) Swallowing of food
(vii) Secretion of gland
(viii) Involuntory function – e.g. vomiting, coughing vasoconstrictor, vasodilater, sneezing, hiccouping.
(ix) It control urination, defecation.

The ventricles consist of four hollow fluid filled space inside the brain and same duct for connection between these ventricte.
(i) Olfactory lobe – Rhinocoel
(ii) Cerebrum – I and II ventricle or lateral ventricle or paracoel.
(iii) Foramen of monero : I and II ventricle communicating with IIIrd ventricle by foramen of monero. They are two in human and single in rabbit and frog.
(iv) Diencephalon : Third ventricle or Diocoel.
(v) Iter or cerebral aquiduct or aquiduct of sylvius : It is very narrow cavity between III and IV ventricle.
(vi) Optic lobe : Optocoel.
(vii) Cerebellum : Solid.
(viii) Medulla oblongata : 4th ventricle or metacoel.
Cavities of brain and spinal cord are modified neurocoel. They are lined by low columnar ciliated epithelium called ependyma.

The salient or mammalian features in the human brain are
(1) Relatively small, solid olfactory lobes.
(2) Very large cerebral hemispheres divided into lobes and with highly folded surface, fully cover the rest of the brain.
(3) Corpus callosum interconnecting the cerebral hemispheres.
(4) Very small pineal body.
(5) A pair of mammillary bodies joined to hypothalamus.
(6) Relatively small, solid optic lobes divided into 4 corpora quadrigemina.
(7) Large, solid cerebellum, with highly folded surface and divided into lobes.
(8) Pons varolii present anterior to the cerebellum.