Divisions Health Dictionary

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Divisions: From 1 Different Sources

CEREBRUM This forms nearly 70 per cent of the brain and consists of two cerebral hemispheres which occupy the entire vault of the cranium and are incompletely separated from one another by a deep mid-line cleft, the longitudinal cerebral ?ssure. At the bottom of this cleft the two hemispheres are united by a thick band of some 200 million crossing nerve ?bres

– the corpus callosum. Other clefts or ?ssures (sulci) make deep impressions, dividing the cerebrum into lobes. The lobes of the cerebrum are the frontal lobe in the forehead region, the parietal lobe on the side and upper part of the brain, the occipital lobe to the back, and the temporal lobe lying just above the region of the ear. The outer 3 mm of the cerebrum is called the cortex, which consists of grey matter with the nerve cells arranged in six layers. This region is concerned with conscious thought, sensation and movement, operating in a similar manner to the more primitive areas of the brain except that incoming information is subject to much greater analysis.

Numbers of shallower infoldings of the surface, called furrows or sulci, separate raised areas called convolutions or gyri. In the deeper part, the white matter consists of nerve ?bres connecting di?erent parts of the surface and passing down to the lower parts of the brain. Among the white matter lie several rounded masses of grey matter, the lentiform and caudate nuclei. In the centre of each cerebral hemisphere is an irregular cavity, the lateral ventricle, each of which communicates with that on the other side and behind with the third ventricle through a small opening, the inter-ventricular foramen, or foramen of Monro.

BASAL NUCLEI Two large masses of grey matter embedded in the base of the cerebral hemispheres in humans, but forming the chief part of the brain in many animals. Between these masses lies the third ventricle, from which the infundibulum, a funnel-shaped process, projects downwards into the pituitary body, and above lies the PINEAL GLAND. This region includes the important HYPOTHALAMUS.

MID-BRAIN or mesencephalon: a stalk about 20 mm long connecting the cerebrum with the hind-brain. Down its centre lies a tube, the cerebral aqueduct, or aqueduct of Sylvius, connecting the third and fourth ventricles. Above this aqueduct lie the corpora quadrigemina, and beneath it are the crura cerebri, strong bands of white matter in which important nerve ?bres pass downwards from the cerebrum. The pineal gland is sited on the upper part of the midbrain.

PONS A mass of nerve ?bres, some of which run crosswise and others are the continuation of the crura cerebri downwards.

CEREBELLUM This lies towards the back, underneath the occipital lobes of the cerebrum.

MEDULLA OBLONGATA The lowest part of the brain, in structure resembling the spinal cord, with white matter on the surface and grey matter in its interior. This is continuous through the large opening in the skull, the foramen magnum, with the spinal cord. Between the medulla, pons, and cerebellum lies the fourth ventricle of the brain.

Structure The grey matter consists mainly of billions of neurones (see NEURON(E)) in which all the activities of the brain begin. These cells vary considerably in size and shape in di?erent parts of the brain, though all give o? a number of processes, some of which form nerve ?bres. The cells in the cortex of the cerebral hemispheres, for example, are very numerous, being set in layers ?ve or six deep. In shape these cells are pyramidal, giving o? processes from the apex, from the centre of the base, and from various projections elsewhere on the cell. The grey matter is everywhere penetrated by a rich supply of blood vessels, and the nerve cells and blood vessels are supported in a ?ne network of ?bres known as neuroglia.

The white matter consists of nerve ?bres, each of which is attached, at one end, to a cell in the grey matter, while at the other end it splits up into a tree-like structure around another cell in another part of the grey matter in the brain or spinal cord. The ?bres have insulating sheaths of a fatty material which, in the mass, gives the white matter its colour; they convey messages from one part of the brain to the other (association ?bres), or, grouped into bundles, leave the brain as nerves, or pass down into the spinal cord where they end near, and exert a control upon, cells from which in turn spring the nerves to the body.

Both grey and white matter are bound together by a network of cells called GLIA which make up 60 per cent of the brain’s weight. These have traditionally been seen as simple structures whose main function was to glue the constituents of the brain together. Recent research, however, suggests that glia are vital for growing synapses between the neurons as they trigger these cells to communicate with each other. So they probably participate in the task of laying down memories, for which synapses are an essential key. The research points to the likelihood that glial cells are as complex as neurons, functioning biochemically in a similar way. Glial cells also absorb potassium pumped out by active neurons and prevent levels of GLUTAMATE – the most common chemical messenger in the brain – from becoming too high.

The general arrangement of ?bres can be best understood by describing the course of a motor nerve-?bre. Arising in a cell on the surface in front of the central sulcus, such a ?bre passes inwards towards the centre of the cerebral hemisphere, the collected mass of ?bres as they lie between the lentiform nucleus and optic thalamus being known as the internal capsule. Hence the ?bre passes down through the crus cerebri, giving o? various small connecting ?bres as it passes downwards. After passing through the pons it reaches the medulla, and at this point crosses to the opposite side (decussation of the pyramids). Entering the spinal cord, it passes downwards to end ?nally in a series of branches (arborisation) which meet and touch (synapse) similar branches from one or more of the cells in the grey matter of the cord (see SPINAL CORD).

BLOOD VESSELS Four vessels carry blood to the brain: two internal carotid arteries in front, and two vertebral arteries behind. These communicate to form a circle (circle of Willis) inside the skull, so that if one is blocked, the others, by dilating, take its place. The chief branch of the internal carotid artery on each side is the middle cerebral, and this gives o? a small but very important branch which pierces the base of the brain and supplies the region of the internal capsule with blood. The chief importance of this vessel lies in the fact that the blood in it is under especially high pressure, owing to its close connection with the carotid artery, so that haemorrhage from it is liable to occur and thus give rise to stroke. Two veins, the internal cerebral veins, bring the blood away from the interior of the brain, but most of the small veins come to the surface and open into large venous sinuses, which run in grooves in the skull, and ?nally pass their blood into the internal jugular vein that accompanies the carotid artery on each side of the neck.

MEMBRANES The brain is separated from the skull by three membranes: the dura mater, a thick ?brous membrane; the arachnoid mater, a more delicate structure; and the pia mater, adhering to the surface of the brain and containing the blood vessels which nourish it. Between each pair is a space containing ?uid on which the brain ?oats as on a water-bed. The ?uid beneath the arachnoid membrane mixes with that inside the ventricles through a small opening in the fourth ventricle, called the median aperture, or foramen of Magendie.

These ?uid arrangements have a great in?uence in preserving the brain from injury.

Health Source: Medical Dictionary
Author: Health Dictionary

Bronchus

Bronchus, or bronchial tube, is the name applied to tubes into which the TRACHEA divides, one going to either lung. The name is also applied to the divisions of these tubes distributed throughout the lungs, the smallest being called bronchioles.... bronchus

Sympathetic Nervous System

One of the 2 divisions of the autonomic nervous system. In conjunction with the other division (the parasympathetic nervous system), this system controls many of the involuntary activities of the body’s glands and organs.... sympathetic nervous system

Meiosis

Meiosis, or reduction division, is the form of cell division that only occurs in the gonads (see GONAD) – that is, the testis (see TESTICLE) and the ovary (see OVARIES) – giving rise to the germ cells (gametes) of the sperms (see SPERMATOZOON) and the ova (see OVUM).

Two types of sperm cells are produced: one contains 22 autosomes and a Y sex chromosome (see SEX CHROMOSOMES); the other, 22 autosomes and an X sex chromosome. All the ova, however, produced by normal meiosis have 22 autosomes and an X sex chromosome.

Two divisions of the NUCLEUS occur (see also CELLS) and only one division of the chromosomes, so that the number of chromosomes in the ova and sperms is half that of the somatic cells. Each chromosome pair divides so that the gametes receive only one member of each pair. The number of chromosomes is restored to full complement at fertilisation so that the zygote has a complete set, each chromosome from the nucleus of the sperm pairing up with its corresponding partner from the ovum.

The ?rst stage of meiosis involves the pairing of homologous chromosomes which join together and synapse lengthwise. The chromosomes then become doubled by splitting along their length and the chromatids so formed are held together by centromeres. As the homologous chromosomes – one of which has come from the mother, and the other from the father – are lying together, genetic interchange can take place between the chromatids and in this way new combinations of GENES arise. All four chromatids are closely interwoven and recombination may take place between any maternal or any paternal chromatids. This process is known as crossing over or recombination. After this period of interchange, homologous chromosomes move apart, one to each pole of the nucleus. The cell then divides and the nucleus of each new cell now contains 23 and not 46 chromosomes. The second meiotic division then occurs, the centromeres divide and the chromatids move apart to opposite poles of the nucleus so there are still 23 chromosomes in each of the daughter nuclei so formed. The cell divides again so that there are four gametes, each containing a half number (haploid) set of chromosomes. However, owing to the recombination or crossing over, the genetic material is not identical with either parent or with other spermatozoa.... meiosis

Nervous System

This extensive, complex and ?nely tuned network of billions of specialised cells called neurones (see NEURON(E)) is responsible for maintaining the body’s contacts with and responses to the outside world. The network also provides internal communication links – in concert with HORMONES, the body’s chemical messengers – between the body’s diverse organs and tissues, and, importantly, the BRAIN stores relevant information as memory. Each neurone has a ?lamentous process of varying length called an AXON along which passes messages in the form of electrochemically generated impulses. Axons are bundled together to form nerves (see NERVE).

The nervous system can be likened to a computer. The central processing unit – which receives, processes and stores information and initiates instructions for bodily activities – is called the central nervous system: this is made up of the brain and SPINAL CORD. The peripheral nervous system – synonymous with the cables that transmit information to and from a computer’s processing unit – has two parts: sensory and motor. The former collects information from the body’s many sense organs. These respond to touch, temperature, pain, position, smells, sounds and visual images and the information is signalled to the brain via the sensory nerves. When information has been processed centrally, the brain and spinal cord send instructions for action via motor nerves to the ‘voluntary’ muscles controlling movements and speech, to the ‘involuntary’ muscles that operate the internal organs such as the heart and intestines, and to the various glands, including the sweat glands in the skin. (Details of the 12 pairs of cranial nerves and the 31 pairs of nerves emanating from the spinal cord are given in respective texts on brain and spinal cord.)

Functional divisions of nervous system As well as the nervous system’s anatomical divisions, the system is divided functionally, into autonomic and somatic parts. The autonomic nervous system, which is split into sympathetic and parasympathetic divisions, deals with the automatic or unconscious control of internal bodily activities such as heartbeat, muscular status of blood vessels, digestion and glandular functions. The somatic system is responsible for the skeletal (voluntary) muscles (see MUSCLE) which carry out intended movements initiated by the brain – for example, the activation of limbs, tongue, vocal cords (speech), anal muscles (defaecation), urethral sphincters (urination) or vaginal muscles (childbirth). In addition, many survival responses – the most powerfully instinctive animal drives, which range from avoiding danger and pain to shivering when cold or sweating when hot – are initiated unconsciously and automatically by the nervous system using the appropriate neural pathways to achieve the particular survival reaction required.

The complex functions of the nervous system include the ability to experience emotions, such as excitement and pleasure, anxiety and frustration, and to undertake intellectual activities. For these experiences an individual can utilise many built-in neurological programmes and he or she can enhance performance through learning – a vital human function that depends on MEMORY, a three stage-process in the brain of registration, storage and recall. The various anatomical and functional divisions of the nervous system that have been unravelled as science has strived to explain how it works may seem confusing. In practical terms, the nervous system works mainly by using automatic or relex reactions (see REFLEX ACTION) to various stimuli (described above), supplemented by voluntary actions triggered by the activity of the conscious (higher) areas of the brain. Some higher functions crucial to human activity – for example, visual perception, thought, memory and speech – are complex and subtle, and the mechanisms are not yet fully understood. But all these complex activities rest on the foundation of relatively simple electrochemical transmissions of impulses through the massive network of billions of specialised cells, the neurones.... nervous system

Alveolus

(1) The minute divisions of glands and the air sacs of the lungs.

(2) The sockets of the teeth in the jawbone.... alveolus

Arteries

Arteries are vessels which convey oxygenated blood away from the heart to the tissues of the body, limbs and internal organs. In the case of most arteries the blood has been puri?ed by passing through the lungs, and is consequently bright red in colour; but in the pulmonary arteries, which convey the blood to the lungs, it is deoxygenated, dark, and like the blood in veins.

The arterial system begins at the left ventricle of the heart with the AORTA, which gives o? branches that subdivide into smaller and smaller vessels. The ?nal divisions, called arterioles, are microscopic and end in a network of capillaries which perforate the tissues like the pores of a sponge and bathe them in blood that is collected and brought back to the heart by veins. (See CIRCULATORY SYSTEM OF THE BLOOD.)

The chief arteries after the aorta and its branches are:

(1) the common carotid, running up each side of the neck and dividing into the internal carotid to the brain, and external carotid to the neck and face;

(2) the subclavian to each arm, continued by the axillary in the armpit, and the brachial along the inner side of the arm, dividing at the elbow into the radial and the ulnar,

which unite across the palm of the hand in arches that give branches to the ?ngers;

(3) the two common iliacs, in which the aorta ends, each of which divides into the internal iliac to the organs in the pelvis, and the external iliac to the lower limb, continued by the femoral in the thigh, and the popliteal behind the knee, dividing into the anterior and posterior tibial arteries to the front and back of the leg. The latter passes behind the inner ankle to the sole of the foot, where it forms arches similar to those in the hand, and supplies the foot and toes by plantar branches.

Structure The arteries are highly elastic, dilating at each heartbeat as blood is driven into them, and forcing it on by their resiliency (see PULSE). Every artery has three coats: (a) the outer or adventitia, consisting of ordinary strong ?brous tissue; (b) the middle or media, consisting of muscular ?bres supported by elastic ?bres, which in some of the larger arteries form distinct membranes; and (c) the inner or intima, consisting of a layer of yellow elastic tissue on whose inner surface rests a layer of smooth plate-like endothelial cells, over which ?ows the blood. In the larger arteries the muscle of the middle coat is largely replaced by elastic ?bres, which render the artery still more expansile and elastic. When an artery is cut across, the muscular coat instantly shrinks, drawing the cut end within the ?brous sheath that surrounds the artery, and bunching it up, so that a very small hole is left to be closed by blood-clot. (See HAEMORRHAGE.)... arteries

Autonomic Nervous System

Part of the nervous system which regulates the bodily functions that are not under conscious control: these include the heartbeat, intestinal movements, salivation, sweating, etc. The autonomic nervous system consists of two main divisions – the SYMPATHETIC NERVOUS SYSTEM and the PARASYMPATHETIC NERVOUS SYSTEM. The smooth muscles, heart and most glands are connected to nerve ?bres from both systems and their proper functioning depends on the balance between these two. (See also NERVES; NERVOUS SYSTEM.)... autonomic nervous system

Bronchioles

The term applied to the ?nest divisions of the bronchial tubes of the LUNGS.... bronchioles

Class

The total number of observations of a particular variable may be grouped according to convenient divisions of the variable range. A group so determined is called a class.... class

Embryo Research

When a woman is treated for infertility it is necessary to nurture human embryos for a few days (until the ?rst cell divisions of the fertilised egg have occurred) in a specialised laboratory. More eggs are fertilised than are usually needed because not all fertilisations are successful. Surplus embryos may be frozen for use in later attempts to implant an embryo in the womb. Research has been done on very early embryos but the practice is controversial and some countries have either forbidden it or imposed tight restrictions. In the UK such research is controlled by the government Human Fertilisation & Embryology Authority (see ASSISTED CONCEPTION).... embryo research

Lobe

The term applied to the larger divisions of various organs, such as to the four lobes of the LIVER, the three lobes of the right and the two lobes of the left lung, which are separated by ?ssures from one another (see LUNGS), and to the lobes or super?cial areas into which the BRAIN is divided. The term lobar is applied to structures which are connected with lobes of organs, or to diseases which have a tendency to be limited by the boundaries of lobes, such as lobar PNEUMONIA.... lobe

Schizonts

Stage in the life cycle of opicomplexan protozoa in which there is multiple asexual divisions (e.g. in malarial parasites).... schizonts

Septum

A dividing wall within a structure in the body. Examples are the divisions between the chambers of the heart, and the layer of bone and cartilage that separates the two nostrils of the nose.... septum

Parasuicide

See suicide, attempted.

parasympathetic nervous system

One of the 2 divisions of the autonomic nervous system.... parasuicide

Calyx

n. (pl. calyces) a cup-shaped part, especially any of the divisions of the pelvis of the *kidney. Each calyx receives urine from the urine-collecting tubes in one sector of the kidney.... calyx

Intestine

All the alimentary canal beyond below the stomach. In it, most DIGESTION is carried on, and through its walls all the food material is absorbed into the blood and lymph streams. The length of the intestine in humans is about 8·5–9 metres (28–30 feet), and it takes the form of one continuous tube suspended in loops in the abdominal cavity.

Divisions The intestine is divided into small intestine and large intestine. The former extends from the stomach onwards for 6·5 metres (22 feet) or thereabouts. The large intestine is the second part of the tube, and though shorter (about 1·8 metres [6 feet] long) is much wider than the small intestine. The latter is divided rather arbitrarily into three parts: the duodenum, consisting of the ?rst 25–30 cm (10–12 inches), into which the ducts of the liver and pancreas open; the jejunum, comprising the next 2·4–2·7 metres (8–9 feet); and ?nally the ileum, which at its lower end opens into the large intestine.

The large intestine begins in the lower part of the abdomen on the right side. The ?rst part is known as the caecum, and into this opens the appendix vermiformis. The appendix is a small tube, closed at one end and about the thickness of a pencil, anything from 2 to 20 cm (average 9 cm) in length, which has much the same structure as the rest of the intestine. (See APPENDICITIS.) The caecum continues into the colon. This is subdivided into: the ascending colon which ascends through the right ?ank to beneath the liver; the transverse colon which crosses the upper part of the abdomen to the left side; and the descending colon which bends downwards through the left ?ank into the pelvis where it becomes the sigmoid colon. The last part of the large intestine is known as the rectum, which passes straight down through the back part of the pelvis, to open to the exterior through the anus.

Structure The intestine, both small and large, consists of four coats, which vary slightly in structure and arrangement at di?erent points but are broadly the same throughout the entire length of the bowel. On the inner surface there is a mucous membrane; outside this is a loose submucous coat, in which blood vessels run; next comes a muscular coat in two layers; and ?nally a tough, thin peritoneal membrane. MUCOUS COAT The interior of the bowel is completely lined by a single layer of pillar-like cells placed side by side. The surface is increased by countless ridges with deep furrows thickly studded with short hair-like processes called villi. As blood and lymph vessels run up to the end of these villi, the digested food passing slowly down the intestine is brought into close relation with the blood circulation. Between the bases of the villi are little openings, each of which leads into a simple, tubular gland which produces a digestive ?uid. In the small and large intestines, many cells are devoted to the production of mucus for lubricating the passage of the food. A large number of minute masses, called lymph follicles, similar in structure to the tonsils are scattered over the inner surface of the intestine. The large intestine is bare both of ridges and of villi. SUBMUCOUS COAT Loose connective tissue which allows the mucous membrane to play freely over the muscular coat. The blood vessels and lymphatic vessels which absorb the food in the villi pour their contents into a network of large vessels lying in this coat. MUSCULAR COAT The muscle in the small intestine is arranged in two layers, in the outer of which all the ?bres run lengthwise with the bowel, whilst in the inner they pass circularly round it. PERITONEAL COAT This forms the outer covering for almost the whole intestine except parts of the duodenum and of the large intestine. It is a tough, ?brous membrane, covered upon its outer surface with a smooth layer of cells.... intestine

Lungs

Positioned in the chest, the lungs serve primarily as respiratory organs (see RESPIRATION), also acting as a ?lter for the blood.

Form and position Each lung is a sponge-like cone, pink in children and grey in adults. Its apex projects into the neck, with the base resting on the DIAPHRAGM. Each lung is enveloped by a closed cavity, the pleural cavity, consisting of two layers of pleural membrane separated by a thin layer of ?uid. In healthy states this allows expansion and retraction as breathing occurs.

Heart/lung connections The HEART lies in contact with the two lungs, so that changes in lung volume inevitably affect the pumping action of the heart. Furthermore, both lungs are connected by blood vessels to the heart. The pulmonary artery passes from the right ventricle and divides into two branches, one of which runs straight outwards to each lung, entering its substance along with the bronchial tube at the hilum or root of the lung. From this point also emerge the pulmonary veins, which carry the blood oxygenated in the lungs back to the left atrium.

Fine structure of lungs Each main bronchial tube, entering the lung at the root, divides into branches. These subdivide again and again, to be distributed all through the substance of the lung until the ?nest tubes, known as respiratory bronchioles, have a width of only 0·25 mm (1/100 inch). All these tubes consist of a mucous membrane surrounded by a ?brous sheath. The surface of the mucous membrane comprises columnar cells provided with cilia (hair-like structures) which sweep mucus and unwanted matter such as bacteria to the exterior.

The smallest divisions of the bronchial tubes, or bronchioles, divide into a number of tortuous tubes known as alveolar ducts terminating eventually in minute sacs, known as alveoli, of which there are around 300 million.

The branches of the pulmonary artery accompany the bronchial tubes to the furthest recesses of the lung, dividing like the latter into ?ner and ?ner branches, and ending in a dense network of capillaries. The air in the air-vesicles is separated therefore from the blood only by two delicate membranes: the wall of the air-vesicle, and the capillary wall, through which exchange of gases (oxygen and carbon dioxide) readily takes place. The essential oxygenated blood from the capillaries is collected by the pulmonary veins, which also accompany the bronchi to the root of the lung.

The lungs also contain an important system of lymph vessels, which start in spaces situated between the air-vesicles and eventually leave the lung along with the blood vessels, and are connected with a chain of bronchial glands lying near the end of the TRACHEA.... lungs

Cleavage

n. (in embryology) the process of repeated cell division of the fertilized egg to form a ball of cells that becomes the *blastocyst. The cells (blastomeres) do not grow between divisions and so they decrease in size.... cleavage

Digit

n. any one of the terminal divisions of a limb: a finger or toe.... digit

Leukaemia

Leukaemia is an umbrella term for several malignant disorders of white blood cells in which they proliferate in a disorganised manner. The disease is also characterised by enlargement of the SPLEEN, changes in the BONE MARROW, and by enlargement of the LYMPH glands all over the body. The condition may be either acute or chronic.

According to the type of cells that predominate, leukaemia may be classi?ed as acute or chronic lymphoblastic leukaemia or myeloid leukaemia. Acute lymphoblastic leukaemia (ALL) is mostly a disease of childhood and is rare after the age of 25. Acute myeloid leukaemia is most common in children and young adults, but may occur at any age. Chronic lymphatic leukaemia occurs at any age between 35 and 80, most commonly in the 60s, and is twice as common in men as in women. Chronic myeloid leukaemia is rare before the age of 25, and most common between the ages of 30 and 65; men and women are equally affected. Around 2,500 patients with acute leukaemia are diagnosed in the United Kingdom, with a similar number annually diagnosed with chronic leukaemia.

Cause Both types of acute leukaemia seem to arise from a MUTATION in a single white cell. The genetically changed cell then goes through an uncontrolled succession of divisions resulting in many millions of abnormal white cells in the blood, bone marrow and other tissues. Possible causes are virus infection, chemical exposure, radiation and genetic background. The cause of chronic lymphocytic leukaemia is not known; the chronic myeloid version may have a genetic background.

Symptoms In acute cases the patient is pale due to anaemia, may have a purpuric rash due to lack of platelets, and may have enlarged lymphatic glands and spleen. The temperature is raised, and the condition may be mistaken for an acute infection (or may ?rst become apparent because the patient develops a severe infection due to a lack of normal white blood cells).

In the chronic type of the disease the onset is gradual, and the ?rst symptoms which occasion discomfort are either swelling of the abdomen and shortness of breath, due to painless enlargement of the spleen; or the enlargement of glands in the neck, armpits and elsewhere; or the pallor, palpitation, and other symptoms of anaemia which often accompany leukaemia. Occasional bleeding from the nose, stomach, gums or bowels may occur, and may be severe. Generally, there is a slight fever.

When the blood is examined microscopically, not only is there an enormous increase in the number of white cells, which may be multiplied 30- or 60-fold, but various immature forms are also found. In the lymphatic form of the disease, most white cells resemble lymphocytes, which, in healthy blood, are present only in small numbers. In the myeloid form, myelocytes, or large immature cells from the bone marrow, which are never present in healthy blood, appear in large numbers, and there may also be large numbers of immature, nucleated erythrocytes.

Treatment This varies according to the type of leukaemia and to the particular condition of the patient. Excellent results are being obtained in the control of ALL using blood transfusions, CHEMOTHERAPY, RADIOTHERAPY and bone-marrow TRANSPLANTATION. In the case of acute leukaemia, the drugs now being used include MERCAPTOPURINE, METHOTREXATE and CYCLOPHOSPHAMIDE. Blood transfusion and CORTICOSTEROIDS play an important part in controlling the condition during the period before a response to chemotherapy can be expected. Chemotherapy has almost completely replaced radiotherapy in the treatment of chronic leukaemia. For the myeloid form, BUSULFAN is the most widely used drug, replaced by hydroxyurea, mercaptopurine, or one of the nitrogen mustard (see NITROGEN MUSTARDS) derivatives in the later stages of the disease. For the lymphatic form, the drugs used are CHLORAMBUCIL, CYCLOPHOSPHAMIDE, and the nitrogen mustard derivatives.

Prognosis Although there is still no guaranteed cure, the outlook in both acute and chronic leukaemia has greatly improved – particularly for the acute form of the disease. Between 70 and 80 per cent of children with acute lymphoblastic leukaemia may be cured; between 20 and 50 per cent of those with acute myeloid leukaemia now have much-improved survival rates. Prognosis of patients with chronic lymphocytic leukaemia is often good, depending on early diagnosis.... leukaemia

Dissection

n. the cutting apart and separation of the body tissues along the natural divisions of the organs and different tissues in the course of an operation. In *interventional radiology, dissection refers specifically to pathological dissection of the intimal layer of an aorta or artery that allows blood to pass through *subintimal space. This may result in occlusion of the vessel branches. Dissection of corpses is carried out for the study of anatomy.... dissection

Liver

The liver is the largest gland in the body, serving numerous functions, chie?y involving various aspects of METABOLISM.

Form The liver is divided into four lobes, the greatest part being the right lobe, with a small left lobe, while the quadrate and caudate lobes are two small divisions on the back and undersurface. Around the middle of the undersurface, towards the back, a transverse ?ssure (the porta hepatis) is placed, by which the hepatic artery and portal vein carry blood into the liver, and the right and left hepatic ducts emerge, carrying o? the BILE formed in the liver to the GALL-BLADDER attached under the right lobe, where it is stored.

Position Occupying the right-hand upper part of the abdominal cavity, the liver is separated from the right lung by the DIAPHRAGM and the pleural membrane (see PLEURA). It rests on various abdominal organs, chie?y the right of the two KIDNEYS, the suprarenal gland (see ADRENAL GLANDS), the large INTESTINE, the DUODENUM and the STOMACH.

Vessels The blood supply di?ers from that of the rest of the body, in that the blood collected from the stomach and bowels into the PORTAL VEIN does not pass directly to the heart, but is ?rst distributed to the liver, where it breaks up into capillary vessels. As a result, some harmful substances are ?ltered from the bloodstream and destroyed, while various constituents of the food are stored in the liver for use in the body’s metabolic processes. The liver also receives the large hepatic artery from the coeliac axis. After circulating through capillaries, the blood from both sources is collected into the hepatic veins, which pass directly from the back surface of the liver into the inferior vena cava.

Minute structure The liver is enveloped in a capsule of ?brous tissue – Glisson’s capsule – from which strands run along the vessels and penetrate deep into the organ, binding it together. Subdivisions of the hepatic artery, portal vein, and bile duct lie alongside each other, ?nally forming the interlobular vessels,

which lie between the lobules of which the whole gland is built up. Each is about the size of a pin’s head and forms a complete secreting unit; the liver is built up of hundreds of thousands of such lobules. These contain small vessels, capillaries, or sinusoids, lined with stellate KUPFFER CELLS, which run into the centre of the lobule, where they empty into a small central vein. These lobular veins ultimately empty into the hepatic veins. Between these capillaries lie rows of large liver cells in which metabolic activity occurs. Fine bile capillaries collect the bile from the cells and discharge it into the bile ducts lying along the margins of the lobules. Liver cells are among the largest in the body, each containing one or two large round nuclei. The cells frequently contain droplets of fat or granules of GLYCOGEN – that is, animal starch.

Functions The liver is, in e?ect, a large chemical factory and the heat this produces contributes to the general warming of the body. The liver secretes bile, the chief constituents of which are the bile salts (sodium glycocholate and taurocholate), the bile pigments (BILIRUBIN and biliverdin), CHOLESTEROL, and LECITHIN. These bile salts are collected and formed in the liver and are eventually converted into the bile acids. The bile pigments are the iron-free and globin-free remnant of HAEMOGLOBIN, formed in the Kup?er cells of the liver. (They can also be formed in the spleen, lymph glands, bone marrow and connective tissues.) Bile therefore serves several purposes: it excretes pigment, the breakdown products of old red blood cells; the bile salts increase fat absorption and activate pancreatic lipase, thus aiding the digestion of fat; and bile is also necessary for the absorption of vitamins D and E.

The other important functions of the liver are as follows:

In the EMBRYO it forms red blood cells, while the adult liver stores vitamin B12, necessary for the proper functioning of the bone marrow in the manufacture of red cells.

It manufactures FIBRINOGEN, ALBUMINS and GLOBULIN from the blood.

It stores IRON and copper, necessary for the manufacture of red cells.

It produces HEPARIN, and – with the aid of vitamin K – PROTHROMBIN.

Its Kup?er cells form an important part of the RETICULO-ENDOTHELIAL SYSTEM, which breaks down red cells and probably manufactures ANTIBODIES.

Noxious products made in the intestine and absorbed into the blood are detoxicated in the liver.

It stores carbohydrate in the form of glycogen, maintaining a two-way process: glucose

glycogen.

CAROTENE, a plant pigment, is converted to vitamin A, and B vitamins are stored.

It splits up AMINO ACIDS and manufactures UREA and uric acids.

It plays an essential role in the storage and metabolism of FAT.... liver

Interkinesis

n. 1. the resting stage between the two divisions of *meiosis. 2. see interphase.... interkinesis

National Patient Safety Agency

(NPSA) formerly, a special health authority that led and coordinated work to improve all aspects of patient safety in England. The NPSA comprised three divisions: the National Reporting and Learning Service, the National Research Ethics Service, and the National Clinical Assessment Service. It closed in 2012, with its key functions transferred to *NHS England. In 2016 the same functions were transferred from NHS England to the newly formed *NHS Improvement.... national patient safety agency

Parasympathetic Nervous System

one of the two divisions of the *autonomic nervous system, having fibres that leave the central nervous system from the brain and the lower portion of the spinal cord and are distributed to blood vessels, glands, and the majority of internal organs. The system works in balance with the *sympathetic nervous system, the actions of which it frequently opposes.... parasympathetic nervous system

Parietal Lobe

one of the major divisions of each cerebral hemisphere (see cerebrum), lying behind the frontal lobe, above the temporal lobe, and in front of the occipital lobe. It is thus beneath the crown of the skull. It contains the *sensory cortex and *association areas.... parietal lobe

Pyramid

n. 1. one of the conical masses that make up the medulla of the *kidney, extending inwards from a base inside the cortex towards the pelvis of the kidney. 2. one of the elongated bulging areas on the anterior surface of the *medulla oblongata in the brain, extending downwards to the spinal cord. 3. one of the divisions of the vermis of the *cerebellum in the middle lobe. 4. a protrusion of the medial wall of the vestibule of the middle ear.... pyramid

Trigeminal Nerve

The ?fth cranial nerve (arising from the BRAIN). It consists of three divisions: (1) the ophthalmic nerve, which is purely sensory in function, being distributed mainly over the forehead and front part of the scalp; (2) the maxillary nerve, which is also sensory and distributed to the skin of the cheek, the mucous membrane of the mouth and throat, and the upper teeth; and (3) the mandibular nerve, which is the nerve of sensation to the lower part of the face, the tongue and the lower teeth, as well as being the motor nerve to the muscles concerned in chewing. The trigeminal nerve is of special interest, owing to its liability to NEURALGIA – TRIGEMINAL NEURALGIA, or tic douloureux as it is also known, being the most painful form known.... trigeminal nerve

Somite

n. any of the paired segmented divisions of *mesoderm that develop along the length of the early embryo. The somites differentiate into voluntary muscle, bones, connective tissue, and the deeper layers of the skin (see dermatome; myotome; sclerotome).... somite

Sporozoite

n. one of the many cells formed as a result of *sporogony during the life cycle of a sporozoan. In *Plasmodium sporozoites are formed by repeated divisions of the contents of the *oocyst inside the body of the mosquito. The released sporozoites ultimately pass into the insect’s salivary glands and await transmission to a human host at the next blood meal.... sporozoite

Telomere

n. the end of a chromosome, which consists of repeated sequences of DNA that perform the function of ensuring that each cycle of DNA replication has been completed. Each time a cell divides some sequences of the telomere are lost; eventually (after 60–100 divisions in an average cell) the cell dies. Replication of telomeres is directed by telomerase, an enzyme consisting of RNA and protein that is inactive in normal cells. Its presence in tumours is linked to the uncontrolled multiplication of cancer cells.... telomere

Telophase

n. the final stage of *mitosis and of each of the divisions of *meiosis, in which the chromosomes at each end of the cell become long and thin and the nuclear membrane reforms around them. The cytoplasm begins to divide.... telophase

Temporal Lobe

one of the main divisions of the *cerebral cortex in each hemisphere of the brain, lying at the side within the temple of the skull and separated from the frontal lobe by a cleft, the lateral sulcus. Areas of the cortex in this lobe are concerned with the appreciation of sound and spoken language.... temporal lobe

Respiration

The process in which air passes into and out of the lungs so that the blood can absorb oxygen and give o? carbon dioxide and water. This occurs 18 times a minute in a healthy adult at rest and is called the respiratory rate. An individual breathes more than 25,000 times a day and during this time inhales around 16 kg of air.

Mechanism of respiration For the structure of the respiratory apparatus, see AIR PASSAGES; CHEST; LUNGS. The air passes rhythmically into and out of the air passages, and mixes with the air already in the lungs, these two movements being known as inspiration and expiration. INSPIRATION is due to a muscular e?ort which enlarges the chest, so that the lungs have to expand in order to ?ll up the vacuum that would otherwise be left, the air entering these organs by the air passages. The increase of the chest in size from above downwards is mainly due to the diaphragm, the muscular ?bres of which contract and reduce its domed shape and cause it to descend, pushing down the abdominal organs beneath it. EXPIRATION is an elastic recoil, the diaphragm rising and the ribs sinking into the position that they naturally occupy, when muscular contraction is ?nished. Occasionally, forced expiration may occur, involving powerful muscles of the abdomen and thorax; this is typically seen in forcible coughing.

Nervous control Respiration is usually either an automatic or a REFLEX ACTION, each expiration sending up sensory impulses to the CENTRAL NERVOUS SYSTEM, from which impulses are sent down various other nerves to the muscles that produce inspiration. Several centres govern the rate and force of the breathing, although all are presided over by a chief respiratory centre in the medulla oblongata (see under BRAIN – Divisions). This in turn is controlled by the higher centres in the cerebral hemispheres, so that breathing can be voluntarily stopped or quickened.

Quantity of air The lungs do not completely empty themselves at each expiration and re?ll at each inspiration. With each breath, less than one-tenth of the total air in the lungs passes out and is replaced by the same quantity of fresh air, which mixes with the stale air in the lungs. This renewal, which in quiet breathing amounts to about 500 millilitres, is known as the tidal air. By a special inspiratory e?ort, an individual can draw in about 3,000 millilitres, this amount being known as complemental air. By a special expiratory e?ort, too, after an ordinary breath one can expel much more than the tidal air from the lungs – this extra amount being known as the supplemental or reserve air, and amounting to about 1,300 millilitres. If an individual takes as deep an inspiration as possible and then makes a forced expiration, the amount expired is known as the vital capacity, and amounts to around 4,000 millilitres in a healthy adult male of average size. Figures for women are about 25 per cent lower. The vital capacity varies with size, sex, age and ethnic origin.

Over and above the vital capacity, the lungs contain air which cannot be expelled; this is known as residual air, and amounts to another 1,500 millilitres.

Tests of respiratory e?ciency are used to assess lung function in health and disease. Pulmonary-function tests, as they are known, include spirometry (see SPIROMETER), PEAK FLOW METER (which measures the rate at which a person can expel air from the lungs, thus testing vital capacity and the extent of BRONCHOSPASM), and measurements of the concentration of oxygen and carbon dioxide in the blood. (See also LUNG VOLUMES.)

Abnormal forms of respiration Apart from mere changes in rate and force, respiration is modi?ed in several ways, either involuntarily or voluntarily. SNORING, or stertorous breathing, is due to a ?accid state of the soft palate causing it to vibrate as the air passes into the throat, or simply to sleeping with the mouth open, which has a similar e?ect. COUGH is a series of violent expirations, at each of which the larynx is suddenly opened after the pressure of air in the lungs has risen considerably; its object is to expel some irritating substance from the air passages. SNEEZING is a single sudden expiration, which di?ers from coughing in that the sudden rush of air is directed by the soft palate up into the nose in order to expel some source of irritation from this narrow passage. CHEYNE-STOKES BREATHING is a type of breathing found in persons suffering from stroke, heart disease, and some other conditions, in which death is impending; it consists in an alternate dying away and gradual strengthening of the inspirations. Other disorders of breathing are found in CROUP and in ASTHMA.... respiration

Trigeminal Neuralgia

Also called tic douloureux, this is one of the most severe forms of NEURALGIA. It affects the main sensory nerve in the face (TRIGEMINAL NERVE), and may occur in one or more of the three divisions in which the nerve is distributed.

It is usually con?ned to one side. It is more common in women than in men, usually occurring over the age of 50. The attack is often precipitated by movements of the jaw, as in talking or eating, or by tactile stimuli such as a cold wind or washing the face. When the ?rst or upper division of the nerve is involved, the pain is mostly felt in the forehead and side of the head. It is usually of an intensely sharp, cutting, or burning character, either constant or with exacerbations each day while the attack continues. There is also pain in the eyelid, redness of the eye and increased ?ow of tears. When the second division of the nerve is affected, the pain is chie?y in the cheek and upper jaw. When the third division of the nerve suffers, the pain affects the lower jaw. Attacks may recur for years; and, although interfering with sleeping and eating, they rarely appear to lead to any serious results. Nevertheless, the pain may become intolerable.

Treatment The outlook in trigeminal neuralgia was radically altered by the introduction of the drug CARBAMAZEPINE, which usually relieves the pain. If the side-effects – for example, dizziness, headache, nausea or drowsiness – are unacceptable or pain not relieved, PHENYTOIN SODIUM may help. Otherwise, surgery is needed in the shape of controlled, radio-frequency heat damage to the appropriate part of the trigeminal nerve.... trigeminal neuralgia

Ulnar Nerve

one of the major nerves of the arm. It originates in the neck, from spinal roots of the last cervical and first thoracic divisions, and runs down the inner side of the upper arm to behind the elbow. In the forearm it supplies the muscles with motor nerves; lower down it divides into branches that supply the skin of the palm and fourth and fifth fingers.... ulnar nerve

Vernier

n. a device for obtaining accurate measurements of length, to 1/10th, 1/100th or smaller fractions of a unit. It consists of a fixed graduated main scale against which a shorter vernier scale slides. The vernier scale is graduated into divisions equal to nine-tenths of the smallest unit marked on the main scale. The vernier scale is often adjusted by means of a screw thread. A reading is taken by observing which of the markings on the scales coincide.... vernier

Unconsciousness

The BRAIN is the organ of the mind. Normal conscious alertness depends upon its continuous adequate supply with oxygen and glucose, both of which are essential for the brain cells to function normally. If either or both of these are interrupted, altered consciousness results. Interruption may be caused by three broad types of process affecting the brain stem: the reticular formation (a network of nerve pathways and nuclei-connecting sensory and motor nerves to and from the cerebrum, cerebellum, SPINAL CORD and cranial nerves) and the cerebral cortex. The three types are di?use brain dysfunction – for example, generalised metabolic disorders such as URAEMIA or toxic disorders such as SEPTICAEMIA; direct effects on the brain stem as a result of infective, cancerous or traumatic lesions; and indirect effects on the brain stem such as a tumour or OEDEMA in the cerebrum creating pressure within the skull. Within these three divisions are a large number of speci?c causes of unconsciousness.

Unconsciousness may be temporary, prolonged or inde?nite (see PERSISTENT VEGETATIVE STATE (PVS)), depending upon the severity of the initiating incident. The patient’s recovery depends upon the cause and success of treatment, where given. MEMORY may be affected, as may motor and sensory functions; but short periods of unconsciousness as a result, say, of trauma have little obvious e?ect on brain function. Repeated bouts of unconsciousness (which can happen in boxing) may, however, have a cumulatively damaging e?ect, as can be seen on CT (COMPUTED TOMOGRAPHY) scans of the brain.

POISONS such as CARBON MONOXIDE (CO), drug overdose, a fall in the oxygen content of blood (HYPOXIA) in lung or heart disease, or liver or kidney failure harm the normal chemical working or metabolism of nerve cells. Severe blood loss will cause ANOXIA of the brain. Any of these can result in altered brain function in which impairment of consciousness is a vital sign.

Sudden altered consciousness will also result from fainting attacks (syncope) in which the blood pressure falls and the circulation of oxygen is thereby reduced. Similarly an epileptic ?t causes partial or complete loss of consciousness by causing an abrupt but temporary disruption of the electrical activity in the nerve cells in the brain (see EPILEPSY).

In these events, as the brain’s function progressively fails, drowsiness, stupor and ?nally COMA ensue. If the cause is removed (or when the patient spontaneously recovers from a ?t or faint), normal consciousness is usually quickly regained. Strokes (see STROKE) are sometimes accompanied by a loss of consciousness; this may be immediate or come on slowly, depending upon the cause or site of the strokes.

Comatose patients are graded according to agreed test scales – for example, the GLASGOW COMA SCALE – in which the patient’s response to a series of tests indicate numerically the level of coma.

Treatment of unconscious patients depends upon the cause, and range from ?rst-aid care for someone who has fainted to hospital intensive-care treatment for a victim of a severe head injury or massive stroke.... unconsciousness

Blood Cells

Cells, also called blood corpuscles, present in blood for most or part of their lifespan. They include red blood cells, which make up about 45 per cent by volume of normal blood, white blood cells, and platelets. Blood cells are made in the bone marrow by a series of divisions from stem cells.

Red blood cells (also known as RBCs, red blood corpuscles, or erythrocytes) transport oxygen from the lungs to the tissues (see respiration). Each is packed with haemoglobin, enzymes, minerals, and sugars. Abnormalities can occur in the rate at which RBCs are either produced or destroyed, in their numbers, and in their shape, size, and haemoglobin content, causing forms of

anaemia and polycythaemia (see blood, disorders of).

White blood cells (also called WBCs, white blood corpuscles, or leukocytes) protect the body against infection and fight infection when it occurs. The 3 main types of are granulocytes (also called polymorphonuclear leukocytes), monocytes, and lymphocytes. Granulocytes are further classified as neutrophils, eosinophils, or basophils, and each type of granulocyte has a role in either fighting infection or in inflammatory or allergic reactions. Monocytes and lymphocytes also play an important part in the immune system. Lymphocytes are usually formed in the lymph nodes. One type, a T-lymphocyte, is responsible for the delayed hypersensitivity reactions

White (see allergy) and Red blood blood cell is also involved in cell (neutrophil) protection against cancer. T-lymphocytes manufacture chemicals, known as lymphokines, which affect the function of other cells. In addition, the T-cells moderate the activity of B-lymphocytes, which form the antibodies that can prevent a second attack of certain infectious diseases. Platelets (also known as thrombocytes), are the smallest blood cells and are important in blood clotting.

The numbers, shapes, and appearance of the various types of blood cell are of great value in the diagnosis of disease (see blood count; blood film).... blood cells

Chromosomal Abnormalities

Variations from normal in the number or structure of chromosomes contained in a person’s cells. The cause is generally a fault in the process of chromosome division, either during the formation of an egg or sperm, or during the first few divisions of a fertilized egg. Chromosomal abnormalities are classified according to whether they involve the 44 autosomes or the 2 X and Y sex chromosomes. A complete extra set of chromosomes per cell is called polyploidy and is lethal.

Autosomal abnormalities cause physical and mental defects of varying severity. Some types of autosomal abnormality, known as trisomy, consist of an extra chromosome on 1 of the 22 pairs of autosomes. The most common trisomy is Down’s syndrome. Sometimes, part of a chromosome is missing, as in cri du chat syndrome. In translocation, a part of a chromosome is joined to another, causing no ill effects in the person but a risk of abnormality in his or her children.

Sex chromosome abnormalities include Turner’s syndrome, in which a girl is born with a single X chromosome in her

cells instead of 2, causing physical abnormalities, defective sexual development, and infertility. A boy with 1 or more extra X chromosomes has Klinefelter’s syndrome, which causes defective sexual development and infertility. The presence of an extra X chromosome in women or an extra Y chromosome in men normally has no physical effect but increases the risk of mild mental handicap.

Chromosomal abnormalities are diagnosed by chromosome analysis in early pregnancy, using amniocentesis or chorionic villus sampling.... chromosomal abnormalities

Fertilization

The union of a sperm and an ovum. In natural fertilization, the sperm and ovum unite in the fallopian tube of the woman following sexual intercourse. A single sperm penetrates the ovum by releasing enzymes that can dissolve the outer layers of the ovum. Once inside, the sperm’s nucleus fuses with that of the ovum, and its empty body shell and tail drop off. Then, the newly fertilized ovum, called a zygote, forms an outer layer that is impenetrable to other sperm. The zygote undergoes repeated cell divisions as it passes down the fallopian tube to the uterus, where it implants and will eventually grow into an embryo.

Fertilization may also occur as a result of semen being artificially introduced into the cervix (see artificial insemination) or may take place in a laboratory (see in vitro fertilization).

fetal alcohol syndrome A rare condition consisting of a combination of congenital defects that result from the continuous consumption of excessive amounts of alcohol by the mother throughout pregnancy. The affected baby has diminished growth, delayed mental development, a small head, a small brain, and small eyes. He or she may have a cleft palate, a small jaw, heart defects, and joint abnormalities. As a newborn, the baby sucks poorly, sleeps badly, and is irritable as a result of alcohol withdrawal. Almost one-fifth of affected babies die during the first few weeks of life; and many who survive are, to some degree, mentally and physically handicapped.... fertilization

Stem Cell

an undifferentiated cell that is able to renew itself and produce specialized cells. Embryonic stem cells at the *blastocyst stage of development can differentiate into almost any cell type (except placental cells); they are described as pluripotent. Embryonic cells preceding the blastocyst, produced by the first 3–4 divisions of the fertilized egg, are capable of producing all the different cell types required by the developing embryo (i.e. they are totipotent). Adult stem cells (also known as somatic stem cells) occur in many tissues and organs, including bone marrow (see haemopoietic stem cell), muscle, liver, pancreas, etc., and can produce the specialized cells needed in the particular tissue or organ in which they arise (i.e. they are multipotent). See also umbilical cord blood banked stem cells.... stem cell

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