- 29 May 2017
It is the main part of the nervous system. The brain lies in a protective bony box, called cranium. The brain is the anterior part of centrai nevous system, lies in the head. It is derived from embryonic ectoderm. The structural and funcitonal units of the brian are neurons.
These are the connective tissue coverings around the brain. The meninges are three. These are duramater, arachnoid membrane and pia mater.
The duramater is outer and lies closely with the inner surface of cranial cavity. The space between the duramater and arachnoid membrane is called sub dural sinus.
The arachnoid membrane is middle and non vascular. The space between the arachnoid membrane and pia mater is called sub arachnoid sinus. These sinusus are filled with a lymph, called cerebrospinal fluid. The fluid and membranes protect the brain from external shocks, prevent dessication of brain and nourish the brain. The piamater is inner and closely associated with the outer surface of brain. It is vascular
B. PARTS OF BRAIN
The brain is mainly divided into three parts. These are fore brain, mid brain and hind brain.
The fore brain is also called prosencephalon, it is divided into two parts. These are telencephalon and diencephalon. Telencephalon contains two parts. These are olfactory lobes and cerebral hemispheres.
i. Olfactory lobes
These are a pair, clubshaped, lie side by side anteriorly. The olfactory lobes are longitudinally seperated by groove, called longitudinal fissure. These are hollow. The cavities of olfactory lobes are called olfactory ventricles or rhinocoels. Each olfactory lobe develops a nerve at its anterior end. This first cranial nerve is called olfactory nerve.
ii. CEREBRAL HEMISPHERES
These two together called large brain. The cerebral hemispheres are a pair, lie side by side behind the olfactory lobes. Each is half globe shaped. Each cerebral hemisphere is broader at the posterior end and narrower at the anterior end. These are seperated by a longitudinal groove called cerebral fissure. The surface is smooth without sulci and gyrae.The thin dorsal wall of cerebral hemisphere is called pallium. These are also hollow. The cavities are called cerebrocoels or lateral ventricles or 1 and '2' ventricles or paracoels. The cerebral hemispheres contain thick ventro lateral walls. These walls are called corpora striata. The thick ventrolateral walls contain many centres, nerve ganglia and nerve tracts. The corpora striata are connected by transverse fibres, called anterior commissure. These fibres lie on floor of paracoels. The roof of the paracoels contain fibres, called corpus callosum. It is the characteristic feature of mammals brain. Each cerebral hemisphere contains a central groove in the lateral surface. This groove is called Sylvian fissure. The part of cerebral hemisphere anterior to the sylvian fissure is called frontal lobe and posterior part is called temporal lobe.
Paracoels anteriorly open into olfactory ventricles and posteriorly into the third ventricle through a common aperture, called foramen of Monro.
It is an unpaired part of fore brain. The dience phalon is diamond shaped. It is anteriorly covered by posterior ends of cerebral hemispheres. The dorsal wall contains a pineal stalk and anterior chorid phlexus. The pineal stalk contains pineal gland. The anterior choroid phlexus is net like. It is non nervous blood capillary net like structure. The lateral walls of diencephalon are thick. These thick walls are called optic thalami. Hence diencephalon is also called thalamiencephalon. The ventral wallof diencephalon is called hypothalamus. It contains thirsty and hunger centres. The ventral wall contains another stalk, called infundibulum. This infundibulum contains an endocrinal gland, called pituitary. The diencephalon is also hollow. The cavity is called third ventricle or diocoel.
iv. Mid brian
It is also called mesencephalon. The mid brain contains four optic lobes. These four optic lobes together called corpora quadrigemina. It is the characteristic feature of mammals brain. The optic lobes are hollow. The cavities are called optocoels. These open into iter or aquiductus sylvius. The ventral walls of optic lobes are thick, called crura cerebri. The crura cerebri are connected by transverse nerve fibres called posterior commissure.
The optic nerves or second pair of cranial nerves are developed from cruracerebri. They cross one another and form an 'X' shaped optic chiasma on the ventral surface of diencephalon.
v. Hind brain
It is also called rhombencephalon. The hind brain contains two parts. These are cerebellum and medulla oblongata. These are unpaired.
It is the unpaired part. It lies only dorsally. The cerebellum is commonly called small brain. It contains three parts. The central part is called vermes. The two lateral lobes are called floculi. The cerebellum is solid and lies transversely.
It is the posterior part of brain. Medulla oblongata is the most important part of the brain. It is hollow. The cavity is called fourth ventricle or myelocoel or medullary coel. The third and fourth ventricles are connected by a tube called iter or aquiduct of Sylvius. The fourth ventricle posteriorly opens into the central canal of spinal cord. The medulla posteriorly continuous as the spinal cord. The dorsal wall of medulla contains nonnervous blood capillary net called posterior choroid phelxus.
FUNCTIONS OF BRAIN
Brain is the centre for control and coordination of all vital activities. It receives impulses from different parts of body through sensory nerves and sends orders through motor nerves to different parts of the body for necessary action.
The functions of various parts of the brain are given beiow:
- The olfactory lobes are the centres of sense of smell.
- Cerebral hemispheres are the centres of consciousness, intelligence, memory and voluntary actions.
- Diencephalon of the brain controls metabolism of fats and carbohydrates. It also regulates genital functions and sleep. It plays a role in perceiving the sense of vision. Removal of this part of brain causes blindness.
- Optic lobes are concerned with sense of sight. They also control the movement of eye muscles.
- The cerebellum serves for coordinating muscular actions and maintaining balance during movement.
- Medulla oblongata is an important part of the brain. It controls involuntary functions such as heart beat, respiration, excretion, taste, hearing and sound production. Removal of medulla oblongata causes the death of the animal.