What is the chest? |

Structure and Functions

The chest, or thorax, consists of those parts of the body lying between the diaphragm and the neck. Included here are the rib cage, diaphragm, heart, lungs, chest muscles, and breasts.

The skeletal support of the chest consists of the thoracic vertebrae and rib cage. In humans, there are usually twelve pairs of ribs and twelve thoracic vertebrae. Each thoracic vertebra consists of a cylindrical portion, the centrum or body, and a neural arch attached to the dorsal side of the centrum. The neural arch surrounds and protects the spinal column. A spinous process extends dorsally from the neural arch of each thoracic vertebra and serves as a site for muscle attachment. Near the base of each neural arch are two pairs of articular processes (zygapophyses). The superior pair of one vertebra face toward each other and articulate with the inferior articular processes of the adjacent vertebrae.

Attached to each thoracic vertebra is a rib. There are usually twelve pairs of ribs, but this number occasionally varies. Each rib consists of a bony portion and a cartilaginous extension, the costal cartilage. At its vertebral end, each rib has two articulating processes, the head (capitulum) and the tubercle (tuberculum). The costal cartilages of the first seven ribs (the number occasionally varies) extend all the way to the sternum. The next two or three ribs have costal cartilages that attach to the costal cartilage above them. The remaining ribs have costal cartilages that are “floating” and have no attachments. Together, the ribs make up a cagelike structure called the rib cage, which shapes the chest and protects the heart and lungs from injury.

The sternum, or breastbone, runs along the front of the chest in the midline of the body. It consists of a flattened top portion called the manubrium; a long, extended body (corpus); and an extension called the xiphoid process, which is made mostly of cartilage. The manubrium has notches for the attachment of the clavicle and the first rib on either side. The attachment for the second rib lies between the manubrium and corpus and is shared by both bones. The corpus of the sternum is formed by the fusion of five individual parts called sternebrae. The costal cartilages of the second through seventh ribs articulate with the corpus of the sternum and mark the boundaries between the individual sternebrae. Beyond the notch for the attachment of the seventh costal cartilage, the xiphoid process extends downward along the midline.

A muscular diaphragm marks the boundary between the chest cavity and the abdominal cavity. Although it is located at the lower end of the chest cavity, it originates in the neck region and derives its nerve supply, the phrenic nerve, from within the neck. The diaphragm is the principal muscle used in breathing. Normally dome-shaped and bowed upward, the diaphragm flattens when its muscles contract, expanding the chest cavity and resulting in the inhalation of air. Relaxation of the diaphragm returns the curvature of the dome upward, compressing the chest cavity and resulting in the exhalation of air. The diaphragm has openings for the passage of the esophagus and the major blood vessels, especially the descending aorta and inferior vena cava.

The heart and major blood vessels lie within the chest cavity and are protected by the rib cage. The heart
is a muscular pump that has four chambers (two atria and two ventricles). The right atrium receives oxygen-poor blood from the body’s organs via the superior vena cava (from the head and upper extremities) and the inferior vena cava (from the abdominal region, pelvic region, and lower extremities). Blood from the right atrium passes through the tricuspid valve to the right ventricle, from which it is pumped into the pulmonary artery. The pulmonary artery then divides in two branches that run separately to each lung. Oxygen-rich blood from the lungs returns to the heart by means of the pulmonary veins, which empty into the left atrium. Blood from the left atrium passes through the bicuspid valve and empties into the left ventricle, which has an extremely thick, muscular wall. Contraction of the left ventricle propels the blood out of the aorta and through the body via the arteries.

Contractions of the heart originate in a location known as the sinoatrial node, located on the surface of the right atrium. From this point, contractions spread to the atrioventricular node, located at the point where all four chambers meet. The wave of contraction then spreads rapidly down the septum between the two ventricles and up the side walls of each ventricle. A specialized bundle of cardiac muscle called the bundle of His, composed of modified muscle fibers (Purkinje fibers), is responsible for this rapid conduction.

Except for the pulmonary arteries, the arteries of the chest region are all branches of the aorta, the major artery that flows out from the left ventricle of the heart in an upward direction. The aorta can be subdivided into an initial portion (the ascending aorta), an aortic arch, and a longer descending aorta which extends from the thoracic region into the pelvis. The coronary arteries are small but important branches that arise from the ascending aorta as it leaves the heart. These arteries supply blood to the muscular wall of the heart itself. From the arch of the aorta, the most common pattern of branching is that of a brachiocephalic trunk, which then splits into a right common carotid and right subclavian artery, followed by a left common carotid artery and then a left subclavian artery. There is considerable variation, however, in this pattern of branchings. The carotid arteries run up the sides of the neck to supply blood to the head and neck.

The subclavian arteries of either side run first upward and then laterally through the chest cavity, continuing toward the upper extremity as the axillary artery. Along its course, each subclavian artery gives off the following branches: vertebral artery, thyrocervical trunk, internal thoracic artery, costocervical artery, and descending scapular artery. The vertebral artery, the largest branch, supplies blood to the vertebrae of the neck region and ultimately to the base of the brain. The short thyrocervical trunk divides almost immediately into three branches: an inferior thyroid artery to the larynx, trachea, esophagus, and surrounding muscles; a suprascapular artery to the subclavius and sternocleidomastoid muscles and to the overlying skin; and a transverse cervical artery to the muscles of the shoulder region.

The right and left internal thoracic arteries (also called internal mammary arteries) run along the ventral side of the chest, just beneath the costal cartilages and just to either side of the sternum. Each internal thoracic artery gives off branches to the diaphragm, the pleura, the pericardium, the thymus, the transverse thoracic muscle, the ribs and intercostal muscles, the pectoral muscles, and the mammary glands. Beyond the sixth rib, each internal thoracic artery divides into a musculophrenic branch to the last six ribs and the diaphragm and a superior epigastric artery, which descends along the abdominal surface, supplying the muscles of this region before meeting the inferior epigastric artery that ascends from the pelvic region.

The veins of the chest region include the external and internal jugular veins, draining the head and neck, and the subclavian veins, draining the upper extremities. These veins come together to form the right and left brachiocephalic veins, which then drain into the superior vena cava. The superior vena cava also receives several smaller tributaries, including the azygos vein, the paired internal thoracic and inferior thyroid veins, the highest intercostal vein, and several smaller veins of the vertebral column. The azygos vein (on the right) and the hemizygous vein (on the left) run parallel to each other on either side of the vertebral column along the dorsal or rear wall of the chest cavity, draining blood from the muscles of the back, the bronchi, the ribs, and the mediastinum. The right and left internal thoracic veins receive tributaries from the ribs and intercostal muscles as well as the diaphragm, pericardium, and mediastinum. The highest intercostal veins drain the first two or three intercostal spaces on either side, also receiving smaller tributaries from the bronchi and the upper portion of the diaphragm. The inferior thyroid veins drain the thyroid gland, esophagus, trachea, and larynx. In addition to the veins listed above, the veins of the heart muscle all drain into a coronary sinus, which runs between the left atrium and ventricle, then drains directly into the right atrium near the inferior vena cava.

The lungs, the principal organs of respiration, consist of several lobes. The right lung has superior, medial, and inferior lobes; the left lung has superior and inferior lobes only. Inhalation of air, or inspiration, is brought about by the lowering (contraction) of the diaphragm and by the raising and outward expansion of the rib cage. Exhalation of air, or expiration, is brought about by the raising (relaxation) of the diaphragm and by the relaxation of the intercostal muscles, lowering and contracting the rib cage. Under most conditions, inspiration is an active process requiring muscular contraction, while expiration takes place passively as the muscles relax.

Together, the heart, the lungs, and the thoracic portion of the esophagus occupy the thoracic cavity, or chest cavity. Each of these organs is surrounded by a thin membrane, the visceral pleura. This membrane is continuous with the parietal pleura, another thin membrane that lines the outer walls of the chest cavity. The right and left visceral pleura come together to form a septum called the mediastinum, which separates the bulk of the thoracic cavity into right and left pleural cavities, each containing one of the lungs. The pericardial cavity, containing the heart, is inserted between the layers of the mediastinum. Also occupying part of the thoracic cavity is a large mass of lymphoid tissue, the thymus body. The thymus is irregular in shape and occupies the highest portion of the thoracic cavity above the heart.

Muscles of the chest region may be divided into those associated developmentally with the upper extremity and those that are associated with the trunk of the body. One muscle, the trapezius, is a modified gill muscle that belongs developmentally to neither group. Fibers of the trapezius muscle originate from the cervical and thoracic vertebrae, including the adjoining ligaments and the adjacent part of the skull. These fibers converge onto the spine and acromion of the scapula and onto the clavicle. The muscles associated with the trunk of the body are called axial muscles. Of those in the chest or thoracic region, four are responsible primarily for movements of the shoulder blade (scapula), twelve for movements of the rib cage, and another eleven for movements of the vertebral column.

The levator scapula runs from the transverse processes of the first four cervical vertebrae to the vertebral border of the scapula; by contracting, it raises and rotates the scapula. The two rhomboid muscles run from the vertebral column to the vertebral border of the scapula. The rhomboideus minor originates from the spinous processes of the seventh cervical and the first thoracic vertebra and from the nuchal ligament that runs from these spinous processes to the skull. The rhomboideus major originates from the spinous processes of the second through fifth thoracic vertebrae. Both rhomboideus muscles run diagonally from the vertebral column to the vertebral border of the scapula, including the base of the scapular spine. The serratus anterior, also called serratus ventralis, is a sheetlike muscle that lies between the scapula and the rib cage. Its fibers originate from the ribs as a series of strips that converge slightly; they all insert onto the vertebral border of the scapula. The attachments of this muscle to the ribs resemble a series of angular sawteeth (serrations) that give the muscle its name. The four preceding muscles all share a common embryological origin, and all have a common nerve supply from the dorsal scapular nerve.

The axial muscles associated with movements of the rib cage include the scalenus anterior, scalenus medius, scalenus posterior, intercostals, subcostals, levatores costarum, transversus thoracis, serratus posterior superior, serratus posterior inferior, rectus abdominis, and diaphragm. The three scalene muscles, as their name implies, are all shaped like elongated scalene triangles (with three sides of different lengths). The scalenus anterior arises from the transverse processes of the third through sixth cervical vertebrae and inserts (attaches) onto the first rib. The largest of the scalene muscles is the scalenus medius, which runs from the transverse processes of the last six cervical vertebrae to an insertion on the first rib. The scalenus posterior arises from the transverse processes of the last two or three cervical vertebrae and inserts onto the second rib.

The intercostal muscles run between the ribs in two sets of fibers. The external intercostals run from each rib to the next in a diagonal direction; the upper end of each fiber is situated closer to the vertebral column than is the lower end. The internal intercostals also run diagonally from each rib to the next, but deep to the fibers of the external intercostals and perpendicular to them, so that the lower end of each fiber is closer to the vertebral end of each rib than is the upper end. Both sets of intercostals are broad, extending nearly along the entire extent of each rib, but the fibers are in each case short, extending only from one rib to the next. The subcostals are similar in position and orientation to the internal intercostals, except they are usually confined to the last few ribs and they span two or three intercostal spaces at a time. The levatores costarum are a continuation of the external intercostals onto the transverse processes of the vertebrae, from the last cervical vertebra to the eleventh thoracic vertebra. Each levator costarum is a triangular slip located in the angle between one of the ribs and the vertebra in front of it, running from the transverse process of the vertebra onto the rib.

The transversus thoracis is a flat muscle that covers part of the inside of the rib cage. Its fibers originate from the corpus and xiphoid process of the sternum; these fibers radiate both horizontally and diagonally upward to insert on the deep surfaces of the second through sixth ribs. The serratus posterior superior arises from the spinous processes of the first few thoracic vertebrae and the seventh cervical vertebra, as well as from the ligaments connecting these spinous processes with one another and with the skull. The fibers converge only slightly and are inserted in four separate slips onto the superior margins of the second through fifth ribs. The serratus posterior inferior is a similar but broader muscle located farther down the spine. It arises from the spinous processes of the last two thoracic and first few lumbar vertebrae, runs diagonally upward, and divides into four separate slips that insert onto the inferior margins of the last four ribs. The rectus abdominis, obliquus externus, obliquus internus, and transversus abdominis are abdominal muscles that pull down on the chest and particularly on the rib cage. The rectus abdominis consists of a strip of muscle fibers running vertically along the ventral midline. The other abdominal muscles are sheetlike and cover the majority of the abdominal surface. Contractions of these muscles generally pull downward on the ribs and oppose the expansion of the rib cage.

The diaphragm is also an axial muscle of the chest cavity. Its muscle fibers originate from the inside of the xiphoid process of the sternum (the sternal portion), from the inner surfaces of the last six ribs and their costal cartilages (the costal portion), and from two muscular arches and two tendinous crura that make up the lumbar portion. The medial lumbocostal arch forms a passage for the greater psoas muscle, while the lateral lumbocostal arch forms a passage for the lumbar quadrate muscle. The right and left crura arise from the ventral surfaces of the first few lumbar vertebrae. Together, the sternal, costal, and lumbar portions of the diaphragm converge upon a sheetlike central tendon, which is divided into large left and right leaflets and a small middle leaflet.

The axial muscles concerned with movements of the thoracic vertebrae include the longus and splenius muscles and the muscles of the erector spinae complex. The longus colli arises from the centra of the last few cervical and first few thoracic vertebrae along their ventral surfaces; it runs upward to insert onto the bodies of the first four cervical vertebrae and the transverse processes of the fifth and sixth cervical vertebrae. The splenius capitis originates from the spinous processes of the last cervical and the first three or four thoracic vertebrae and from the ligaments connecting these processes to one another and to the back of the skull. The muscle inserts onto the occipital and temporal bones on the back of the skull, including the mastoid process. The splenius cervicis arises from the spinous processes of the third through sixth thoracic vertebrae and runs to an insertion on the transverse processes of the first few cervical vertebrae. The muscles of the erector spinae complex include the iliocostalis, longissimus, spinalis, semispinalis, multifidius, rotatores, and intertransversarii. Collectively, these muscles are responsible for dorsal movements (extension) of the vertebral column throughout the lumbar, thoracic, and cervical regions.

The muscles associated developmentally with the extremities are called appendicular muscles. Appendicular muscles of the chest region include the pectoralis, latissimus dorsi, and subclavius. The pectoralis major is triangular in shape; it originates from the sternum, costal cartilages, and a portion of the clavicle, from which its fibers converge toward an insertion onto the greater tuberosity of the humerus. The pectoralis minor originates from the third through fifth ribs and inserts onto the coracoid process of the scapula. The latissimus dorsi is a broad, flat muscle that originates from the lower half of the vertebral column (and part of the ilium) by way of a tough tendinous sheet (the lumbar aponeurosis); it inserts high on the humerus. The subclavius muscle runs from the bottom surface of the clavicle diagonally onto the first rib. Upon contraction, this muscle helps pull the shoulder inward and the rib cage upward.

Each of the paired breasts consists of a mammary gland, nipple (papilla), areola, and surrounding fat tissue. The breasts are small in children and remain small in most adult men, but they become larger during puberty in women and enlarge even more during late pregnancy and throughout lactation. Toward the end of pregnancy, the gland begins to secrete milk, a white, nutritive fluid containing lactose (milk sugar), proteins, and some fats (more sugar and less fat than in cow’s milk). The secretion of milk is known as lactation. During lactation, the mammary gland continues to secrete milk as long as the baby continues nursing. When the child is weaned, the mammary gland undergoes a process of involution (shrinkage). The smaller ducts of the mammary gland collect into larger ducts, each draining a wedge-shaped section of the breast. These larger ducts converge toward a raised nipple (papilla) from which the milk exudes. The nipple is surrounded by a circular area, the areola, characterized by thin skin which is a bit more heavily pigmented (usually redder) than the remainder of the breast.

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