The Brain

The brain consists of five main divisions, the cerebrum or fore brain, the basal ganglia, the mid brain, the cerebellum or hind brain, and the medulla oblongata. It is covered and protected by three membranes, the meninges, which are separated by fluid. These three membranes are known as the dura mater, the arachnoid, and the pia mater.

The Cerebrum is the upper and larger portion of the brain, and occupies the whole of the vault or dome of the cranium. It is divided into two cerebral hemispheres by a deep cleft or fissure; a portion of the dura mater dips into this cleft and from its resemblance to a sickle is called the falx cerebri. It thus supports each half of the brain and prevents it from pressing upon the other half when the head inclines to one side. The undulating surface of the cerebrum is formed by what are called convolutions or gyri, with their intervening sulci or grooves. The cerebrum is divided into four main lobes, the anterior or frontal, the middle or parietal, the posterior or occipital, and the lateral or temporal lobes.

The surface layer of the cerebrum is called the cortex and is of a grey colour, being cellular; the central portion called the medulla is white and consists of nerve fibres. Deeply embedded in the white matter are masses of grey matter known as basal ganglia, the largest being the thalamus which is the main subconscious centre. The cerebrum is associated with the intellectual faculties and controls the other parts of the central nervous system and the muscular system.

The Cerebellum is about one-sixth the size of the cerebrum. It lies just under the posterior lobe of the cerebrum, and is separated from it by an extension of the dura mater called the tentorium cerebelli. It is composed of white and grey matter; when the former is cut it resembles the trunk and branches of a tree, and is called the arbor vita. The cerebellum regulates the muscular movements and postural equilibrium.

The Medulla Oblongata is the lowest pyramidal part of the brain, and in structure resembles the spinal cord with which it is continuous. It is about one and a quarter inches long and contains centres which control various important functions such as respiration, digestion, and the action of the heart.

The Meninges

The Dura Mater is a strong fibrous membrane which lines the skull and spinal column, and sends folds or processes inward to support the different lobes of the brain and the nerves which go out from the brain and spinal cord.

The Arachnoid Mater is a serous membrane and, like all other serous membranes, is a closed sac. It is reflected upon the inner surface of the dura mater.

The Pia Mater is a vascular membrane and lies next to and invests the whole surface of the brain, enfolding its convolutions. It transmits blood vessels to the brain.

The Cranial Nerves

The cranial nerves pass out from the brain in twelve pairs. In reading a description of them, let the reader keep his eye.

The First Pair, the olfactory nerves (6), pass through several small openings in the ethmoid bone, and are distributed to the mucous membrane which lines the nose. If they are cut or destroyed the sense of smell is lost.

The Second Pair, the optic nerves (7), pass through the base of the skull, and enter the orbit where they spread out over the inner surface of the wall of the eye, namely the retina. Disease of these nerves may impair sight, or cause total blindness.

The Third Pair, the oculomotor nerves (9), pass through the sphenoid bone to the muscles of the eye and control their movements.

The Fourth Pair, or trochlear nerves (10), pass to the superior oblique muscle of the eye.

The Fifth Pair, the trigeminal nerves (11), like the spinal nerves have two roots, and each nerve divides into three branches, one going to the eye, forehead, and nose, called the ophthalmic branch; another supplies the eye, the teeth of the upper jaw, etc., and is called the superior maxillary nerve; and the third goes to the ear, the tongue, and the teeth of the lower jaw, and is called the inferior maxillary nerve. Disease of the branches of the fifth pair gives rise to the neuralgic affection called tic douloureux.

The Sixth Pair, the abducent nerves (12), pass through the opening by which the carotid artery enters the cavity of the skull, and go to the external rectus muscles of the eye.

The Seventh Pair, the facial nerves (13), are distributed over the face and send filaments to the muscles.

The Eighth Pair
, the auditory nerves (14), supply the structures in the internal ear.

The Ninth Pair, or glosso-pharyngeal nerves (15), pass through the same opening as the jugular vein and are distributed upon the mucous membrane of the tongue and throat.

The Tenth Pair, the vagi (16), supply the pharynx, larynx, gullet, lungs, spleen, pancreas, liver, stomach and bowels.

The Eleventh Pair, or spinal accessory nerves (8), connect with the ninth and tenth pairs, and are distributed to the muscles of the neck.

The Twelfth Pair, the hypoglossal nerves (17), go to the tongue and are the motor nerves of this organ.

The ‘Vital’ Centres. There seem to be levels of organisation in the brain. The most primitive parts are the medulla and midbrain. Here there are collections of nerve cells, or ‘centres’, controlling respiration, heart rate and blood pressure. These centres are vital, or essential to life.

The medullary vital centres, in addition, respond to messages from other parts of the body. Sensory stimuli can alter the heart rate, blood pressure and respiration. For example, when a person gets out of bed in the morning, both the heart rate and blood pressure alter. The change in posture, from lying down to standing up, sets off reflexes which make the heart beat faster and the blood pressure rise. The medullary centres are involved in both these reflexes.

When a person exercises, his respiration increases. The demand for oxygen and the need to get rid of the extra carbon dioxide stimulates the respiratory centre. This in turn makes the person breathe more quickly and deeply. He takes more fresh air into his lungs, and so satisfies his need for oxygen.

The Cerebellum springs from the roof of the medulla. It helps other parts of the nervous system to control limb movements.

The Hypothalamus. This forms the floor of the most forward part of the brain. There are several nuclei in the hypothalamus, which regulates many functions. The hypothalamus seems to control the intake and loss of water from the body as well as the appetite and food intake; it also regulates body temperature. It probably also controls some of the functions of the anterior pituitary gland.

The Cerebrum. The two cerebral hemispheres form the largest part of the brain. The cerebrum is best developed in man and apes - the most intelligent members of the animal kingdom.

The Cortex. The surface of the cerebrum, or the cerebral cortex, is divided by folds, or sulci. One very large sulcus runs transversely across each hemisphere dividing it roughly into front and back portions.

The part behind the central sulcus is concerned with sensation. There are definite areas associated with special sense organs. The occipital cortex has something to do with vision, and the temporal cortex with hearing.

In front of the central sulcus lie the great motor areas. The nerve cells which command voluntary movement lie in the ridge just in front of the central sulcus. There is a special centre for speech, another for eye movements, and so on.

It is very difficult to discover what the cerebral cortex actually does. We know that damage to certain parts of it interferes with normal function.

If the occipital cortex is damaged, the person may not be able to see, even though his eyes are quite healthy. This, of course, tells us nothing of how the cortex is concerned in vision. Sometimes injury to a part of the cerebral cortex produces no obvious change in the animal. This may be because one cerebral hemisphere can completely take over the function of the other.

The Forebrain. The most forward part of the cerebral cortex is the frontal area. It is probably concerned with social behaviour. Occasionally a patient is in a state of severe nervous tension which cannot be relieved by medical treatment. The surgeon may decide to cut some of the fibres running from the frontal cortex to the deeper parts of the brain. This is the operation of frontal leucotomy.

After the operation, the nervous tension goes, but the patient is often indifferent to the needs of the people around him. He may be unable to plan and organise his day. This disability may of course be preferable to the nervous tension. But since such an operation is irreversible, the advantages and disadvantages must be carefully weighed before it is done.

The cerebral cortex is probably concerned with adding together and correlating information from all levels of the nervous system. It may act as the final regulator of all that we do.

Submitted By

Dr. Naimul Islam Ph.D
The author is a professor of Physiology at the University of Waterloo, Ontario, Canada.

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