Thorax Health Dictionary

The chest, or THORAX, is the upper part of the trunk. It is enclosed by the breastbone (sternum) and the 12 ribs which join the sternum by way of cartilages and are attached to the spine behind. At the top of the thorax, the opening in between the ?rst ribs admits the windpipe (TRACHEA), the gullet (OESOPHAGUS) and the large blood vessels. The bottom of the thorax is separated from the abdomen below by the muscular DIAPHRAGM which is the main muscle of breathing. Other muscles of respiration, the intercostal muscles, lie in between the ribs. Overlying the ribs are layers of muscle and soft tissue including the breast tissue.

Contents The trachea divides into right and left main bronchi which go to the two LUNGS. The left lung is slightly smaller than the right. The right has three lobes (upper, middle and lower) and the left lung has two lobes (upper and lower). Each lung is covered by two thin membranes lubricated by a thin layer of ?uid. These are the pleura; similar structures cover the heart (pericardium). The heart lies in the middle, displaced slightly to the left. The oesophagus passes right through the chest to enter the stomach just below the diaphragm. Various nerves, blood vessels and lymph channels run through the thorax. The thoracic duct is the main lymphatic drainage channel emptying into a vein on the left side of the root of the neck. (For diseases affecting the chest and its contents, see HEART, DISEASES OF; LUNGS, DISEASES OF; CHEST, DEFORMITIES OF.)... chest

The lower part of the trunk. Above, and separated from it by the diaphragm, lies the thorax or chest, and below lies the PELVIS, generally described as a separate cavity though continuous with that of the abdomen. Behind are the SPINAL COLUMN and lower ribs, which come within a few inches of the iliac bones. At the sides the contained organs are protected by the iliac bones and down-sloping ribs, but in front the whole extent is protected only by soft tissues. The latter consist of the skin, a varying amount of fat, three layers of broad, ?at muscle, another layer of fat, and ?nally the smooth, thin PERITONEUM which lines the whole cavity. These soft tissues allow the necessary distension when food is taken into the STOMACH, and the various important movements of the organs associated with digestion. The shape of the abdomen varies; in children it may protrude considerably, though if this is too marked it may indicate disease. In healthy young adults it should be either slightly prominent or slightly indrawn, and should show the outline of the muscular layer, especially of the pair of muscles running vertically (recti), which are divided into four or ?ve sections by transverse lines. In older people fat is usually deposited on and inside the abdomen. In pregnancy the abdomen enlarges from the 12th week after conception as the FETUS in the UTERUS grows (see PREGNANCY AND LABOUR; ANTENATAL CARE).

Contents The principal contents of the abdominal cavity are the digestive organs, i.e. the stomach and INTESTINE, and the associated glands, the LIVER and PANCREAS. The position

of the stomach is above and to the left when the individual is lying down, but may be much lower when standing. The liver lies above and to the right, largely under cover of the ribs, and occupying the hollow of the diaphragm. The two KIDNEYS lie against the back wall on either side, protected by the last two ribs. From the kidneys run the URETERS, or urinary ducts, down along the back wall to the URINARY BLADDER in the pelvis. The pancreas lies across the spine between the kidneys, and on the upper end of each kidney is a suprarenal gland

(see ADRENAL GLANDS). The SPLEEN is positioned high up on the left and partly behind the stomach. The great blood vessels and nerves lie on the back wall, and the remainder of the space is taken up by the intestines or bowels (see INTESTINE). The large intestine lies in the ?anks on either side in front of the kidneys, crossing below the stomach from right to left, while the small intestine hangs from the back wall in coils which ?ll up the spaces between the other organs. Hanging down from the stomach in front of the bowels is the OMENTUM, or apron, containing much fat and helping to protect the bowels. In pregnancy the UTERUS, or womb, rises up from the pelvis into the abdomen as it increases in size, lifting the coils of the small intestine above it.

The PELVIS is the part of the abdomen within the bony pelvis (see BONE), and contains the rectum or end part of the intestine, the bladder, and in the male the PROSTATE GLAND; in the female the uterus, OVARIES, and FALLOPIAN TUBES.... abdomen

This is the bodies’ main lymph collecting vessel. It starts in the little collecting bladder in the abdomen (the cisterna chyli), moves up the center of the body in front of the spinal chord, alongside the esophagus and aorta to the neck, where it drains into the left subclavian vein. It drains the lymph from the entire body, except the head, right thorax and arm, which collects lymph separately and drains into the right subclavian vein. Lacking the ability to contract and expand, the thoracic duct relies on its valves and the kinetic energy of breathing and nearby arterial pumping to drain lymph upwards.... thoracic duct

The adult (imago) is a slender, delicate insect with six comparatively long, thin legs. The outer covering of the body is composed of a tough substance called chitin. The body is divided into three distinct parts: head, thorax and abdomen.... adult mosquito

The common name for the sternum, the front part of the thorax.... breastbone

(abdomino-) combining form denoting the abdomen. Examples: abdominalgia (pain in the abdomen); abdominothoracic (relating to the abdomen and thorax).... abdomin

n. a genus of widely distributed mosquitoes occurring throughout the tropics and subtropics. Most species are black with distinct white or silvery-yellow markings on the legs and thorax. Aëdes species are not only important as vectors of *dengue, *yellow fever, *filariasis, and Group B viruses causing encephalitis but also constitute a serious biting nuisance. A. aegypti is the principal vector of dengue and yellow fever.... aëdes

(artificial ventilation) an emergency procedure for maintaining a flow of air into and out of a patient’s lungs when the natural breathing reflexes are absent or insufficient. This may occur after drowning, poisoning, etc., or during a surgical operation on the thorax or abdomen when muscle-relaxing drugs are administered. The simplest and most efficient method is *mouth-to-mouth resuscitation. In hospital the breathing cycle is maintained by means of a *ventilator.... artificial respiration

Immature developing (L1 and L2) and infective (L3) stages of nematode larvae, e.g. filariasis, hookworm etc.... In filariasis, after an infective blood meal, microfilariae exsheath, penetrate the stomach wall and pass into the haematocoele, from where they migrate to the thoracic muscles of the mosquito. In the thorax, the small larvae become more or less inactive, grow shorter but considerably fatter and develop, after 2 days, into “sausageshaped” forms (L1). They undergo two (2) moults and the resultant third stage larvae (L3) become active. This is the infective stage and is formed some 10 days or more after the microfilariae have been ingested with a blood meal.... l1, l2, and l3

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

an unpaired vein that arises from the inferior vena cava and drains into the superior vena cava, returning blood from the thorax and abdominal cavities.

b... azygos vein

n. 1. the mammary gland of a woman: one of two compound glands that produce milk. Each breast consists of glandular lobules – the milk-secreting areas – embedded in fatty tissue (see illustration). The milk passes from the lobules into ducts, which join up to form 15–20 lactiferous ducts. Near the front of the breast the lactiferous ducts are dilated into ampullae, which act as reservoirs for the milk. Each lactiferous duct discharges through a separate orifice in the nipple. The dark area around the nipple is called the areola. See also lactation. Anatomical name: mamma. 2. the front part of the chest (thorax).... breast

n. a growth found incidentally on (usually) an adrenal gland (adrenal incidentaloma) during CT or MRI scanning of the abdomen or thorax for other clinical reasons. These growths are rarely significant, particularly if small, but they may be pathological if the patient has symptoms (e.g. hypertension, flushing) that could be attributable to an adrenal tumour.... incidentaloma

n. inflammation of the midline partition of the chest cavity (mediastinum), usually complicating a rupture of the oesophagus (gullet). Sclerosing mediastinitis often leads to *fibrosis, which may cause compression of other structures in the thorax, such as the superior vena cava, the bronchial tree, or the oesophagus.... mediastinitis

Also known as the spine, this forms an important part of the skeleton, acting both as the rigid pillar which supports the upper parts of the body and as a protection to the SPINAL CORD and nerves arising from it. The spinal column is built up of a number of bones placed one upon another, which, in consequence of having a slight degree of turning-movement, are known as the vertebrae. The possession of a spinal cord supported by a vertebral column distinguishes the higher animals from the lower types, and is why they are called vertebrates. Of the vertebrates, humans alone stand absolutely erect, and this erect carriage of the body gives to the skull and vertebral column certain distinctive characters.

The human backbone is about 70 cm (28

inches) in length, and varies little in full-grown people; di?erences in height depend mainly upon the length of the lower limbs. The number of vertebrae is 33 in children, although in adult life ?ve of these fuse together to form the sacrum, and the lowest four unite in the coccyx, so that the number of separate bones is reduced to 26. Of these there are seven in the neck, known as cervical vertebrae; 12 with ribs attached, in the region of the thorax known as thoracic or dorsal vertebrae; ?ve in the loins, called lumbar vertebrae; ?ve fused to form the sacrum; and four joined in the coccyx. These numbers are expressed in a formula thus: C7, D12, L5, S5, Coc4=33.

Although the vertebrae in each of these regions have distinguishing features, all the vertebrae are constructed on the same general plan. Each has a thick, rounded, bony part in front, known as the body, and these bodies form the main thickness of the column. Behind the body of each is a ring of bone, the neural ring, these rings placed one above another forming the bony canal which lodges the spinal cord. From each side of the ring a short process of bone known as the transverse process stands out, and from the back of the ring a larger process, the spinous process, projects. These processes give attachment to the strong ligaments and muscles which unite, support, and bend the column. The spines can be seen or felt beneath the skin of the back lying in the centre of a groove between the muscular masses of the two sides, and they give to the column its name of the spinal column. One of these spines, that of the seventh cervical vertebra, is especially large and forms a distinct bony prominence, where the neck joins the back. Between the bodies of the vertebrae lies a series of thick discs of ?brocartilage known as intervertebral discs. Each disc consists of an outer portion, known as the annulus ?brosus, and an inner core, known as the nucleus pulposus. These 23 discs provide the upper part of the spine with pliability and resilience.

The ?rst and second cervical vertebrae are specially modi?ed. The ?rst vertebra, known as the atlas, is devoid of a body, but has a specially large and strong ring with two hollows upon which the skull rests, thus allowing forward and backward movements (nodding). The second vertebra, known as the axis, has a pivot on its body which ?ts into the ?rst vertebra and thus allows free rotation of the head from side to side. The spinal column has four natural curves (see diagram) which help to cushion the shocks of walking and running.

The neural rings of the vertebrae form a canal, which is wide in the neck, smaller and almost round in the dorsal region, and wide again in the lumbar vertebrae. Down the canal runs the spinal cord, and the nerves leaving the cord do so through openings between the vertebrae which are produced by notches on the upper and lower margins of each ring. The intervertebral foramina formed by these notches are so large in comparison with the nerves passing through them that there is no chance of pressure upon the latter, except in very serious injuries which dislocate and fracture the spine.... spinal column

The process by which air passes into and out of the lungs to allow the blood to take up oxygen and dispose of carbon dioxide. Breathing is controlled by the respiratory centre in the brainstem. When air is inhaled, the diaphragm contracts and flattens. The intercostal muscles (muscles between the ribs) contract and pull the ribcage upwards and outwards. The resulting increase in chest volume causes the lungs to expand, and the reduced pressure draws air into the lungs. When air is exhaled, the chest muscles and diaphragm relax, causing the ribcage to sink and the lungs to contract, squeezing air out.

In normal, quiet breathing, only about a 10th of the air in the lungs passes out to be replaced by the same amount of fresh air (tidal volume). This new air mixes with the stale air (residual volume) already held in the lungs. The normal breathing rate for an adult at rest is 13–17 breaths per minute. (See also respiration.)breathing difficulty Laboured or distressed breathing that includes a change in the rate and depth of breathing or a feeling of breathlessness. Some degree of breathlessness is normal after exercise, particularly in unfit or overweight people. Breathlessness at rest is always abnormal and is usually due to disorders that affect the airways (see asthma), lungs (see pulmonary disease, chronic obstructive), or cardiovascular system (see heart failure). Severe anxiety can result in breathlessness, even when the lungs are normal (see hyperventilation). Damage to the breathing centre in the brainstem due to a stroke or head injury can affect breathing. This may also happen as a side effect of certain drugs. Ventilator assistance is sometimes needed.

At high altitudes, the lungs have to work harder in order to provide the body with sufficient oxygen (see mountain sickness). Breathlessness may occur in severe anaemia because abnormal or low levels of the oxygen-carrying pigment haemoglobin means that the lungs need to work harder to supply the body with oxygen. Breathing difficulty that intensifies on exertion may be caused by reduced circulation of blood through the lungs. This may be due to heart failure, pulmonary embolism, or pulmonary hypertension. Breathing difficulty due to air-flow obstruction may be caused by chronic bronchitis, asthma, an allergic reaction, or lung cancer. Breathing difficulty may also be due to inefficient transfer of oxygen from the lungs into the bloodstream. Temporary damage to lung tissue may be due to pneumonia, pneumothorax, pulmonary oedema, or pleural effusion. Permanent lung damage may be due to emphysema. Chest pain (for example, due to a broken rib) that is made worse by chest or lung movement can make normal breathing difficult and painful, as can pleurisy, which is associated with pain in the lower chest and often in the shoulder tip of the affected side.

Abnormalities of the skeletal structure of the thorax (chest), such as severe scoliosis or kyphosis, may cause difficulty in breathing by impairing normal movements of the ribcage.... breathing

n. the space in the thorax (chest cavity) between the two pleural sacs. The mediastinum contains the heart, aorta, trachea, oesophagus, and thymus gland and is divided into anterior, middle, posterior, and superior regions.... mediastinum

n. a member of a large group of mainly land-dwelling *arthropods. The body of the adult is divided into a head, thorax, and abdomen. The head bears a single pair of sensory antennae; the thorax bears three pairs of legs and, in most insects, wings (these are absent in some parasitic groups, such as lice and fleas). Some insects are of medical importance. Various bloodsucking insects transmit tropical diseases, for example the female Anopheles mosquito transmits malaria and the tsetse fly transmits sleeping sickness. The bites of lice can cause intense irritation and, secondarily, bacterial infection. The organisms causing diarrhoea and dysentery can be conveyed to food on the bodies of flies. See also myiasis.... insect

n. a free-living or parasitic arthropod belonging to a group (Acarina) that also includes the *ticks. Most mites are small, averaging 1 mm or less in length. A mite has no antennae or wings, and its body is not divided into a distinct head, thorax, and abdomen. Medically important mites include the many species causing dermatitis (e.g. Dermatophagoides) and the harvest mite (see Trombicula), which transmits scrub typhus.... mite

n. the region of the thorax immediately over the heart. —precordial adj.... precordium

n. a curved, slightly twisted, strip of bone forming part of the skeleton of the thorax, which protects the heart and lungs. There are 12 pairs of ribs. The head of each rib articulates with one of the 12 thoracic vertebrae of the backbone; the other end is attached to a section of cartilage (see costal cartilage). The first seven pairs – the true ribs – are connected directly to the sternum by their costal cartilages. The next three pairs – the false ribs – are attached indirectly: each is connected by its cartilage to the rib above it. The last two pairs of ribs – the floating ribs – end freely in the muscles of the body wall. Anatomical name: costa.... rib

n. any of several muscles arising from or inserted by a series of processes that resemble the teeth of a saw. An example is the serratus anterior, a muscle situated between the ribs and shoulder blade in the upper and lateral parts of the thorax. It is the chief muscle responsible for pushing and punching movements.... serratus

a star-shaped collection of sympathetic nerve cell bodies in the root of the neck, from which sympathetic nerve fibres are distributed to the face and neck and to the blood vessels and organs of the thorax.... stellate ganglion

n. (pl. sterna) the breastbone: a flat bone, 15–20 cm long, extending from the base of the neck to just below the diaphragm and forming the front part of the skeleton of the thorax. The sternum articulates with the collar bones (see clavicle) and the costal cartilages of the first seven pairs of ribs. It consists of three sections: the middle and longest section – the body or gladiolus – is attached to the *manubrium at the top and the *xiphoid (or ensiform) process at the bottom. The manubrium slopes back from the body so that the junction between the two parts forms an angle (angle of Louis or sternal angle). —sternal adj.... sternum

(thoraco-) combining form denoting the thorax or chest.... thorac

the chest cavity. See thorax.... thoracic cavity

(pl. venae cavae) either of the two main veins, conveying deoxygenated blood from the other veins to the right atrium of the heart. The inferior vena cava, formed by the union of the right and left common iliac veins, receives blood from parts of the body below the diaphragm. The superior vena cava, originating at the junction of the two innominate veins, drains blood from the head, neck, thorax, and arms.... vena cava