Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Thorax: overview and surface anatomy
The thorax is the upper part of the trunk. It consists of an external musculoskeletal cage, the thoracic wall and an internal cavity that contains the heart, lungs, oesophagus, trachea and main bronchi, thymus, vagus and phrenic nerves, sympathetic trunks and ganglia, thoracic duct, lymph nodes, and major systemic and pulmonary vasculature ( Fig. 52.1 ). Superiorly, the thoracic cavity communicates with the neck and the upper limbs via the superior thoracic aperture. Inferiorly, it is separated from the abdominal cavity by the respiratory diaphragm. The thoracic cage offers protection to some of the superiorly located abdominal viscera: the majority of the liver lies inferior to the right dome (cupula) of the respiratory diaphragm; the stomach, spleen and left lobe of the liver lie inferior to the left dome of the diaphragm; and the posterior surfaces of the suprarenal glands and kidneys contact the anteroinferior surface of the lumbar part of the diaphragm.
The thoracic cage and thoracic cavity; showing the lungs, pleura, pleural cavity, diaphragm and mediastinum.
With permission from Drake RL, Vogl AW, Mitchell A et al (eds) Gray’s Atlas of Anatomy, 2nd ed. Elsevier, Churchill Livingstone. Copyright 2015.
The shape, dimensions and proportions of the thoracic cage vary between individuals, particularly with age and sex ( ). In an adult, the dimensions of the thoracic cavity change continuously with the movements of the ribs and diaphragm during respiration ( Ch. 55 ) and also with distension of the abdominal viscera and movement of the vertebral column. In a normal healthy adult, both the thoracic cage and abdominal wall move outwards on inspiration and inwards on expiration. Changes to this pattern, including paradoxical movements, may be seen in respiratory disease ( ). Thoracic capacity is less in females than it is in males, both absolutely and proportionately; in the female, the sternum is normally shorter and the thoracic inlet more oblique.
In neonates and young infants, the thoracic cage has the shape of a truncated cone and is relatively round in cross-section, the ribs have an almost horizontal alignment, and the domes of the respiratory diaphragm and sternal ends of the clavicles sit in a more superior position than is the case in older children and adults ( ). Thoracic expansion is consequently limited and contributes little to breathing, which is reliant on the respiratory diaphragm. In a neonate, diaphragmatic contraction during inspiration pulls the thoracic cage inwards and moves the abdomen outwards, the thoraco-abdominal paradox. The neonatal thoracic wall is highly compliant and is easily deformed. By 2 years of age, as standing and walking begin, adult thoracic proportions and geometry have developed. These include downward sloping ribs, an oval cross-sectional shape to the thoracic cage, and a more inferior position of the domes of the respiratory diaphragm and the sternal ends of the clavicles ( , ). This means that the thorax is able to move outwards during inspiration and the thoraco-abdominal paradox diminishes until it is no longer observed.
Musculoskeletal Framework
Bones and joints
The thoracic cage consists of 12 thoracic vertebrae and the intervening intervertebral discs, 12 pairs of ribs and their anteriorly located costal cartilages, and the sternum ( Fig. 52.2 ). When articulated, they form an irregularly shaped osseocartilaginous frame (see Fig. 53.8 ). Laterally, the outer surface of the thoracic cage is convex and is formed by the ribs; anteriorly, it is slightly convex and is formed by the sternum and the distal parts of the ribs and their costal cartilages. The costal cartilages of the first seven pairs of ribs are connected to the sternum at the sternocostal joints, the costal cartilages of the eighth to tenth ribs usually join the inferior border of the superiorly located costal cartilage at the interchondral joints, and the eleventh and twelfth (sometimes the tenth) ribs and their cartilage are free (floating) at their anterior ends and project into the musculature of the abdominal wall. The posterolateral curvature of the ribs, from their head (vertebral end) to their angle (see Fig. 53.9 ), produces a deep internal groove, the paravertebral gutter, located on either side of the vertebral column. The ribs and costal cartilages are separated by intercostal spaces, which are deeper anteriorly and between the more superiorly located ribs. Each space is occupied by two or three layers of flat muscles and their aponeuroses (membranes), neurovascular bundles and lymphatic channels. FLOAT NOT FOUND
The superior thoracic aperture (thoracic inlet) typically measures 5 cm anteroposteriorly and 10 cm transversely (see Fig. 52.1 ). It is bounded by the body of the first thoracic vertebra posteriorly, the internal (medial) borders of the left and right first ribs, and the superior border (jugular notch) of the manubrium of the sternum anteriorly. It slopes anteriorly and inferiorly from the vertebral column, so that the apex of the lung is located in the inferior neck, above the level of the first rib and the medial one-third of the clavicle, and level with the body of the seventh cervical or first thoracic vertebra, or the intervening C7–T1 intervertebral disc; structures that pass between the thorax and the upper limb therefore pass over the first rib and the suprapleural membrane covering the pleurae and lung apex.
The inferior thoracic aperture (thoracic outlet) is bordered posteriorly by the body of the twelfth thoracic vertebra, posterolaterally by the twelfth rib and its costal cartilage and the distal end of the eleventh rib and its costal cartilage, anterolaterally by the cartilaginous ends of the seventh to tenth ribs (which unite and ascend as the costal arch or margin), and anteriorly by the xiphoid process of the sternum (see Fig. 52.2 ). In adults the dimension of the transverse plane of the aperture is larger than the sagittal plane, and the aperture slopes obliquely inferiorly and posteriorly towards the vertebral column, so that the thoracic cavity is deeper posteriorly than anteriorly.
The sternum is formed from three parts (manubrium, body and xiphoid process) that articulate at the manubriosternal and xiphisternal joints. The sternal plane passes horizontally through the manubriosternal joint (sternal angle, angle of Louis) at the level of the second costal cartilage anteriorly, intersecting the vertebral column posteriorly in a region that extends from the body of the fourth thoracic vertebra to the superior half of the body of the fifth thoracic vertebra ( ). The sternal plane separates the superior mediastinum from the inferior mediastinum and can be used as a reference plane to position intrathoracic mediastinal structures.
Muscles
Intrinsic and extrinsic muscles
The intrinsic muscles of the chest wall are the intercostal muscles, subcostalis, transversus thoracis, levatores costarum and serratus posterior superior and inferior (see Figs 46.71 , 53.15 ). Transversus thoracis may mimic pleural thickening on CT imaging: the uncommon variants, sternalis and rectus thoracis bifurcalis, are important clinically because they may mimic a focal density in medial breast craniocaudal mammograms and may be encountered during reconstructive surgery of the breast and chest wall.
The intercostal muscles are arranged in consecutive layers in each intercostal space and are named according to their positional relationship to each other, i.e. external, internal and innermost (see Fig. 53.17 ). All the intrinsic muscles except levatores costarum are innervated segmentally by the intercostal nerves derived from the ventral rami of the thoracic nerves. Levatores costarum are innervated segmentally by lateral branches of the dorsal rami of the thoracic spinal nerves. The intrinsic muscles can elevate or depress a rib depending upon their location and are particularly active in forced (deep) breathing; some also help to stiffen the chest wall, thereby preventing paradoxical movement during inspiration and optimizing inspiratory/expiratory reserve volume.
The skeletal framework of the thoracic wall provides attachment sites for muscles associated functionally with the neck, back, upper limbs and abdomen. Some (e.g., scalenes, infrahyoid strap muscles, sternocleidomastoid, serratus anterior, pectoralis major and minor, external and internal abdominal obliques and rectus abdominis) may also function as accessory muscles of respiration; they are usually active only during forced breathing. Muscles with thoracic attachment are described in more detail in Chapters 35 , 48 and 60 .
Respiratory diaphragm
The respiratory diaphragm is a curved musculotendinous sheet attached to the circumference of the inferior thoracic aperture and the bodies of the superior two or three lumbar vertebrae. It forms the inferior boundary (floor) of the thoracic cavity and separates it from the abdominal cavity ( Ch. 55 ). The diaphragm is relatively flat centrally, in the region inferior to the heart, and domed peripherally, rising higher on the right side than on the left, an asymmetry reflecting the differences in relative densities of the underlying abdominal viscera. From its most superior point on each side, the diaphragm slopes inferiorly to its peripheral costovertebral attachments. Posteriorly, the space between the diaphragm and the thoracic wall (costodiaphragmatic recess) is long and narrow. The position of the domes varies with body position; they are higher in the supine compared to the erect position. During quiet respiration in the erect position, diaphragmatic excursion is about 2 cm in both sexes, increasing to around 7 cm during deep breathing ( ); the range of excursion is often greater in men and may increase further whilst in the supine position ( ).
Diaphragmatic openings
The respiratory diaphragm is traversed by structures that pass between the thorax and abdomen, including the right and left vagus, phrenic, thoracic splanchnic and subcostal nerves, the thoracic duct, sympathetic trunks, right and left superior epigastric arteries (continuations of the internal thoracic arteries), and the azygos and hemiazygos veins. The inferior vena cava and oesophagus pass through the tendinous and muscular parts of the diaphragm, respectively (see Fig. 55.1 ). CT-based studies have shown the vertebral level at which they pass through the diaphragm is more inferior than reported conventionally.
Thoracic Cavity
Fascia and spaces
The endothoracic fascia (parietal fascia of the thorax) is a mixture of tissue types including connective, adipose, lymphoid and smooth muscle. It lines the inner surface of the thoracic wall and the superior surface of the respiratory diaphragm, separating them from the respective region (part) of the parietal pleura. It is attached posteriorly to the periosteum of the thoracic vertebrae and becomes continuous with the thoracic prevertebral fascia; superiorly it forms the suprapleural membrane; inferiorly it is continuous with the endo-abdominal fascia via apertures in the respiratory diaphragm, e.g., the aortic hiatus ( ).
More recent descriptions define the endothoracic fascia as the outermost connective tissue layer within the extrapleural space, a space situated between the parietal pleura and the inner surface of the thoracic wall and superior (thoracic) surface of the respiratory diaphragm ( ). The extrapleural space also contains a layer of adipose tissue in contact with the parietal pleura, loose connective tissue, vessels and lymphoid tissue. A range of pathologies may occur in this tissue plane, including liposarcoma, leiomyosarcoma, fibrosarcoma and lymphoma ( ). An increased accumulation of extrapleural fat may be confused for pleural thickening on CT imaging ( ).
The superficial subcutaneous fascia of the thorax, formed mainly by fat, is attached loosely to the skin and includes the breast tissue. Deeper fascial layers include those surrounding the muscles of the thoracic wall: some layers are dense and supportive, e.g. the clavipectoral fascia ( Ch. 49 ) and the thoracolumbar fascia ( Ch. 46 ).
The extrathoracic fascia is located posteriorly between the muscles that attach the scapula to the posterior thoracic wall and the underlying posterior thoracic wall ( ). The extrafascial space is superficial to the extrathoracic fascia and is relatively sealed, whereas the subfascial space deep to the extrathoracic fascia communicates freely with the cervical region superiorly and with the lumbar region inferiorly.
Pleural cavities
The right and left pleural cavities are separate compartments within the thoracic cavity, located on either side of the mediastinum. Each surrounds a lung and its associated bronchial tree, vessels, nerves and lymphatics ( Ch. 54 ). The walls are formed by a serous membrane, the pleura, arranged as a closed sac. Each cavity represents a potential space between the parietal and visceral pleura, two serous membranous layers that are continuous on the medial aspect of the lung root and pulmonary ligament at the pleural reflection. The outermost layer, the parietal pleura, lines the inner surface of the corresponding half of the thoracic wall, the superior surface of the ipsilateral hemidiaphragm and structures along the ipsilateral lateral side of the mediastinum. The parietal pleura covers part of the lung root, close to the hilum, then folds internally on itself to become continuous with the innermost layer, the visceral pleura, which adheres to the outer surface of the lobes of the lung, following the interlobar fissures. The two layers of pleura are in close contact throughout all phases of breathing, yet are able to slide freely over each other due to the presence of an intervening thin layer of serous fluid. The pleural cavity is maintained normally at a negative pressure by the inward elastic recoil of the lung and the outward pull of the thoracic wall. The lungs do not occupy the full extent of the pleural cavity during quiet breathing, but are free to move into recesses such as the costodiaphragmatic and costomediastinal recesses, during deep breathing.
Mediastinum
The mediastinum is a visceral compartment of the thoracic cavity, located within and adjacent to the median sagittal plane, between the right and left pleural cavities (see ( Figs 52.1 , Fig 56.1 ): the mediastinal part of the parietal pleura forms its lateral boundaries. The mediastinum extends from the sternum anteriorly to the thoracic vertebral column posteriorly, and from the superior thoracic aperture superiorly to the respiratory diaphragm inferiorly. The sternal plane (which passes horizontally through the manubriosternal joint at the level of the second costal cartilage anteriorly, intersecting the vertebral column posteriorly in a region that extends from the body of the fourth thoracic vertebra to the superior half of the body of the fifth thoracic vertebra) is used to divide the mediastinum into superior and inferior parts. The sternal plane is sometimes defined as a fixed plane that passes from the manubriosternal joint to the most inferior part of the body of the fourth thoracic vertebra. Regional mediastinal content is reviewed in Ch. 56 .
Superior mediastinum
The superior mediastinum lies between the manubrium of the sternum anteriorly and the anterior longitudinal ligament covering the bodies of the first four or five thoracic vertebrae and their intervening intervertebral discs posteriorly (depending upon the level at which the sternal plane intersects the vertebral column). It contains the lower ends of sternohyoid, sternothyroid and longus colli on each side; the aortic arch, brachiocephalic trunk and thoracic portions of the left common carotid and left subclavian arteries; the brachiocephalic veins and upper half of the superior vena cava; the supreme and superior intercostal veins; the vagus, cardiac, phrenic and left recurrent laryngeal nerves; and the trachea, oesophagus, thoracic duct, thymic remnants and lymph nodes (e.g. upper paratracheal, para-aortic, subaortic and prevascular nodes).
Inferior mediastinum
The inferior mediastinum, between the sternal plane and diaphragm, is divided into three parts: anterior, middle and posterior.
Anterior mediastinum
The anterior mediastinum is primarily located on the left side where the point of reflection between the left costal and mediastinal part of the parietal pleura diverges from the median plane. It is bounded anteriorly by the sternum, laterally by the left and right mediastinal parietal pleura and posteriorly by the fibrous pericardium. It is approximately triangular in shape, being wider inferiorly, and may be absent above the level of the fourth costal cartilage if the left and right parietal pleural layers meet. Its anterior wall is formed mainly by the left transversus thoracis and the fifth to seventh left costal cartilages and intervening intercostal muscles. The anterior mediastinum contains loose areolar tissue, lymphatic vessels from the liver, thymic remnants (or fatty replacement), parasternal lymph nodes and mediastinal branches of the internal thoracic artery.
Middle mediastinum
The middle mediastinum is the broadest part of the mediastinum. It contains the heart within the pericardial coverings and cavity; the ascending aorta; the inferior part of the superior vena cava with the azygos vein joining it; the tracheal bifurcation and right and left main bronchi; the pulmonary trunk dividing into the left and right pulmonary arteries; the right and left pulmonary veins (superior and inferior) and phrenic nerves; and hilar, subcarinal and lower paratracheal (tracheobronchial) lymph nodes.
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here