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The biology of the Brain

In such simple animals as worms and insects, the brain consists of small groups of nerve cells. All animals with a backbone have a complicated brain made up of many parts. Animals that have an exceptionally well-developed brain include apes, dolphins, and whales. Human beings have the most highly developed brain of all. It consists of billions of interconnected cells and enables people to use language, solve difficult problems, and create works of art. 


The human brain is a greyish-pink, jellylike ball with many ridges and grooves on its surface. A newborn baby's brain weighs less than 0.5 kilogram. By the time a person is 6 years old, the brain has reached its full weight of about 1.4 kilograms. Most of the brain's nerve cells are present at birth. The increase in weight comes from growth of nerve cells, development and growth of supporting cells, and development of connections among cells. During this six-year period, a person learns and acquires new behaviour patterns at the fastest rate in life. 

A network of blood vessels supplies the brain with the vast quantities of oxygen and food that it requires. The human brain makes up only about 2 per cent of the total body weight, but it uses about 20 per cent of the oxygen used by the entire body when at rest. The brain can go without oxygen for only three to five minutes before serious damage results. 

The brain is at the upper end of the spinal cord. The spinal cord is a cable of nerve cells that extends from the neck about two-thirds of the way down the backbone. The spinal cord carries messages between the brain and other parts of the body. In addition, 12 pairs of nerves connect the brain directly with certain parts of the body. For more information about the nervous system and the brain's place in it.. 

The brain works somewhat like both a computer and a chemical factory. Brain cells produce electrical signals and send them from cell to cell along pathways called circuits. As in a computer, these circuits receive, process, store, and retrieve information. Unlike a computer, however, the brain creates its electrical signals by chemical means. The proper functioning of the brain depends on many complicated chemical substances produced by brain cells. 

Scientists in various fields work together to study the structure, function, and chemical composition of the brain. This field of study, called neuroscience or neurobiology, is rapidly increasing our understanding of the brain. However, much still remains to be learned. Scientists do not yet know how the physical and chemical processes in the brain produce much of the brain's activity. 

This article deals chiefly with the human brain. The last section of the article discusses the brain in various kinds of animals. 

 

The parts of the brain 

The brain has three main divisions: (1) the cerebrum, (2) the cerebellum, and (3) the brain stem. Each part consists chiefly of nerve cells, called neurons, and supporting cells, called glia. 

Each hemisphere, in turn, is divided into four lobes (regions). Each lobe has the same name as the bone of the skull that lies above it. The lobes are: (1) the frontal lobe, at the front; (2) the temporal lobe, at the lower side; (3) the parietal lobe, in the middle; and (4) the occipital lobe, at the rear. Fissures in the cerebral cortex form the boundaries between the lobes. The two major fissures are the central fissure and the lateral fissure. 

A thin layer of nerve cell bodies called the cerebral cortex or cortex forms the outermost part of the cerebrum. Most of the cerebrum beneath the cortex consists of nerve cell fibres. Some of these fibres connect parts of the cortex. Others link the cortex with the cerebellum, brain stem, and spinal cord. 

The cerebral cortex is folded into a surface with many ridges and grooves. This folding greatly increases the surface area of the cortex and the number of nerve cells it contains within the limited space of the skull. Some areas of the cortex, called the sensory cortex, receive messages from the sense organs as well as messages of touch and temperature from throughout the body. Areas in the frontal lobes called the motor cortex send out nerve impulses that control the voluntary movements of all the skeletal muscles. 
 

The largest portion of the cortex is the association cortex. Every lobe of the brain has areas of association cortex that analyse, process, and store information. These association areas make possible all of our higher mental abilities, such as thinking, speaking, and remembering. 

Nerve pathways connect the right half of the cerebellum with the left cerebral hemisphere and the right side of the body. Pathways from the left half connect with the right cerebral hemisphere and the left side of the body. 

The brain stem is a stalklike structure that connects the cerebrum with the spinal cord. The bottom part of the brain stem is called the medulla oblongata or medulla. The medulla has nerve centres that control breathing, heartbeat, and many other body processes essential to life. 

Just above the medulla is the pons, which connects the hemispheres of the cerebellum. The pons also contains nerve fibres that link the cerebellum and the cerebrum. Above the pons lies the lic> Nerve centres in the midbrain help control movements of the eyes and the size of the pupils. 

At the upper end of the brain stem are the hypothalamus and the thalamus. There are actually two thalami, one on the left side of the brain stem and one on the right side. Each thalamus receives nerve impulses from various parts of the body and routes them to the appropriate areas of the cerebral cortex. The thalami also relay impulses from one part of the brain to another. The hypothalamus regulates body temperature, hunger, and other internal conditions. It also controls the activity of the nearby pituitary gland, the master gland of the body. 

A network of nerve fibres called the reticular formation lies deep within the brain stem. The reticular formation helps regulate and maintain the brain's level of awareness. Sensory messages that pass through the brain stem stimulate the reticular formation, which in turn stimulates alertness and activity throughout the cerebral cortex. 
 

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