thorax-part-1.pdf

The THORAX Emmylou Jane B. Valencia, M.D. Clibbeth U. Tupas, M.D.  GENERAL DESCRIPTION ◈ The thorax is an irregularly shaped cylinder with a narrow opening (superior thoracic aperture) superiorly and a relatively large opening (inferior thoracic aperture) inf...

The THORAX Emmylou Jane B. Valencia, M.D. Clibbeth U. Tupas, M.D.  GENERAL DESCRIPTION ◈ The thorax is an irregularly shaped cylinder with a narrow opening (superior thoracic aperture) superiorly and a relatively large opening (inferior thoracic aperture) inferiorly (Fig. 3.1). ◈ The superior thoracic aperture is open, allowing continuity with the neck; the inferior thoracic aperture is closed by the diaphragm. ◈ The musculoskeletal wall of the thorax is flexible and consists of segmentally arranged vertebrae, ribs, and muscles and the sternum.  GENERAL DESCRIPTION ◈ The thoracic cavity enclosed by the thoracic wall and the diaphragm ◈ is subdivided into three major compartments: 🞚 a left and a right pleural cavity, each surrounding a lung 🞚 mediastinum  GENERAL DESCRIPTION ◈ The mediastinum is a thick, flexible soft tissue partition oriented longitudinally in a median sagittal position. ◈ It contains the heart, esophagus, trachea, major nerves, and major systemic blood vessels.  GENERAL DESCRIPTION ◈ The pleural cavities are completely separated from each other by the mediastinum. ◈ Therefore abnormal events in one pleural cavity do not necessarily affect the other cavity. ◈ This also means that the mediastinum can be entered surgically without opening the pleural cavities. ◈ Another important feature of the pleural cavities is that they extend above the level of rib I. ◈ The apex of each lung actually extends into the root of the neck. ◈ As a consequence, abnormal events in the root of the neck can involve the adjacent pleura and lung, and events in the adjacent pleura and lung can involve the root of the neck  FUNCTIONS ◈ Breathing ◈ Protection of vital organs ◈ Conduit  Breathing ◈ One of the most important functions of the thorax is breathing. ◈ The thorax not only contains the lungs but also provides the machinery necessary—the diaphragm, thoracic wall, and ribs—for effectively moving air into and out of the lungs. ◈ Up and down movements of the diaphragm and changes in the lateral and anterior dimensions of the thoracic wall, caused by movements of the ribs, alter the volume of the thoracic cavity and are key elements in breathing  Protection of vital organs ◈ The thorax houses and protects the heart, lungs, and great vessels. ◈ Because of the upward domed shape of the diaphragm, the thoracic wall also offers protection to some important abdominal viscera. ◈ Much of the liver lies under the right dome of the diaphragm, and the stomach and spleen lie under the left. ◈ The posterior aspects of the superior poles of the kidneys lie on the diaphragm and are anterior to rib XII, on the right, and to ribs XI and XII, on the left.  Conduit ◈ The mediastinum acts as a conduit for structures that pass completely through the thorax from one body region to another and for structures that connect organs in the thorax to other body regions. ◈ The esophagus, vagus nerves, and thoracic duct pass through the mediastinum as they course between the abdomen and neck. ◈ The phrenic nerves, which originate in the neck, also pass through the mediastinum to penetrate and supply the diaphragm. ◈ Other structures such as the trachea, thoracic aorta, and superior vena cava course within the mediastinum en route to and from major visceral organs in the thorax. COMPONENT PARTS  Thoracic wall ◈ The thoracic wall consists of skeletal elements and muscles (Fig. 3.1): ◈ Posteriorly, it is made up of twelve thoracic vertebrae and their intervening intervertebral discs; ◈ Laterally, the wall is formed by ribs (twelve on each side) and three layers of flat muscles, which span the intercostal spaces between adjacent ribs, move the ribs, and provide support for the intercostal spaces; ◈ Anteriorly, the wall is made up of the sternum, which consists of the manubrium of sternum, body of sternum, and xiphoid process. ◈ The manubrium of sternum, angled posteriorly on the body of sternum at the manubriosternal joint, forms the sternal angle, which is a major surface landmark used by clinicians in performing physical examinations of the thorax. ◈ The anterior (distal) end of each rib is composed of costal cartilage, which contributes to the mobility and elasticity of the wall. ◈ All ribs articulate with thoracic vertebrae posteriorly. ◈ Most ribs (from rib II to IX) have three articulations with the vertebral column. ◈ The head of each rib articulates with the body of its own vertebra and with the body of the vertebra above (Fig. 3.2). ◈ As these ribs curve posteriorly, each also articulates with the transverse process of its vertebra. ◈ Anteriorly, the costal cartilages of ribs I to VII articulate with the sternum ◈ The costal cartilages of ribs VIII to X articulate with the inferior margins of the costal cartilages above them. ◈ Ribs XI and XII are called floating ribs because they do not articulate with other ribs, costal cartilages, or the sternum. ◈ Their costal cartilages are small, only covering their tips. ◈ The skeletal framework of the thoracic wall provides extensive attachment sites for muscles of the neck, abdomen, back, and upper limbs. ◈ A number of these muscles attach to ribs and function as accessory respiratory muscles; some of them also stabilize the position of the first and last ribs.  Superior thoracic aperture ◈ Completely surrounded by skeletal elements, the superior thoracic aperture consists of the body of vertebra TI posteriorly, the medial margin of rib I on each side, and the manubrium anteriorly. ◈ The superior margin of the manubrium is in approximately the same horizontal plane as the intervertebral disc between vertebrae TII and TIII. ◈ The first ribs slope inferiorly from their posterior articulation with vertebra TI to their anterior attachment to the manubrium. ◈ Consequently, the plane of the superior thoracic aperture is at an oblique angle, facing somewhat anteriorly. ◈ At the superior thoracic aperture, the superior aspects of the pleural cavities, which surround the lungs, lie on either side of the entrance to the mediastinum (Fig. 3.3). ◈ Structures that pass between the upper limb and thorax pass over rib I and the superior part of the pleural cavity as they enter and leave the mediastinum. ◈ Structures that pass between the neck and head and the thorax pass more vertically through the superior thoracic aperture.  Inferior thoracic aperture ◈ The inferior thoracic aperture is large and expandable. ◈ Bone, cartilage, and ligaments form its margin (Fig. 3.4A). ◈ The inferior thoracic aperture is closed by the diaphragm, and structures passing between the abdomen and thorax pierce or pass posteriorly to the diaphragm. ◈ Skeletal elements of the inferior thoracic aperture are: ◈ the body of vertebra TXII posteriorly, ◈ rib XII and the distal end of rib XI posterolaterally, ◈ the distal cartilaginous ends of ribs VII to X, which unite to form the costal margin anterolaterally, and ◈ the xiphoid process anteriorly. ◈ The joint between the costal margin and sternum lies roughly in the same horizontal plane as the intervertebral disc between vertebrae TIX and TX. ◈ In other words, the posterior margin of the inferior thoracic aperture is inferior to the anterior margin. ◈ When viewed anteriorly, the inferior thoracic aperture is tilted superiorly.  Diaphragm ◈ The musculotendinous diaphragm seals the inferior thoracic aperture (Fig. 3.4B). ◈ Generally, muscle fibers of the diaphragm arise radially, from the margins of the inferior thoracic aperture, and converge into a large central tendon. ◈ Because of the oblique angle of the inferior thoracic aperture, the posterior attachment of the diaphragm is inferior to the anterior attachment. ◈ The diaphragm is not flat; rather, it “balloons” superiorly, on both the right and left sides, to form domes. The right dome is higher than the left, reaching as far as rib V. ◈ As the diaphragm contracts, the height of the domes decreases and the volume of the thorax increases. ◈ The esophagus and inferior vena cava penetrate the diaphragm; the aorta passes posterior to the diaphragm.  Mediastinum ◈ The mediastinum is a thick midline partition that extends from the sternum anteriorly to the thoracic vertebrae posteriorly, and from the superior thoracic aperture to the inferior thoracic aperture. ◈ A horizontal plane passing through the sternal angle and the intervertebral disc between vertebrae TIV and TV separates the mediastinum into superior and inferior parts (Fig. 3.5). ◈ The inferior part is further subdivided by the pericardium, which encloses the pericardial cavity surrounding the heart. ◈ The pericardium and heart constitute the middle mediastinum. ◈ The anterior mediastinum lies between the sternum and the pericardium; ◈ the posterior mediastinum lies between the pericardium and thoracic vertebrae.  Pleural cavities ◈ The two pleural cavities are situated on either side of the mediastinum (Fig. 3.6). ◈ Each pleural cavity is completely lined by a mesothelial membrane called the pleura. ◈ During development, the lungs grow out of the mediastinum, becoming surrounded by the pleural cavities. ◈ As a result, the outer surface of each organ is covered by pleura. ◈ Each lung remains attached to the mediastinum by a root formed by the airway, pulmonary blood vessels, lymphatic tissues, and nerves.  Pleural cavities ◈ The pleura lining the walls of the cavity is the parietal pleura, whereas that reflected from the mediastinum at the roots and onto the surfaces of the lungs is the visceral pleura. ◈ Only a potential space normally exists between the visceral pleura covering lung and the parietal pleura lining the wall of the thoracic cavity. ◈ The lung does not completely fill the potential space of the pleural cavity, resulting in recesses, which do not contain lung and are important for accommodating changes in lung volume during breathing. ◈ The costodiaphragmatic recess, which is the largest and clinically most important recess, lies inferiorly between the thoracic wall and diaphragm  Breast ◈ consisting of mammary glands, superficial fascia, and overlying skin, are in the pectoral region on each side of the anterior thoracic wall (Fig. 3.9). ◈ Vessels, lymphatics, and nerves associated with the breast are as follows: ◈ Branches from the internal thoracic arteries and veins perforate the anterior chest wall on each side of the sternum to supply anterior aspects of the thoracic wall. ◈ Those branches associated mainly with the second to fourth intercostal spaces also supply the anteromedial parts of each breast. ◈ Lymphatic vessels from the medial part of the breast accompany the perforating arteries and drain into the parasternal nodes on the deep surface of the thoracic wall. ◈ Vessels and lymphatics associated with lateral parts of the breast emerge from or drain into the axillary region of the upper limb. ◈ Lateral and anterior branches of the fourth to sixth intercostal nerves carry general sensation from the skin of the breast. KEY FEATURES  Vertebral level TIV/V ◈ The horizontal plane passing through the disc that separates thoracic vertebrae TIV and TV is one of the most significant planes in the body (Fig. 3.10) because it: ◈ passes through the sternal angle anteriorly, marking the position of the anterior articulation of the costal cartilage of rib II with the sternum. ◈ The sternal angle is used to find the position of rib II as a reference for counting ribs (because of the overlying clavicle, rib I is not palpable); ◈ separates the superior mediastinum from the inferior mediastinum and marks the position of the superior limit of the pericardium; ◈ marks where the arch of the aorta begins and ends; ◈ passes through the site where the superior vena cava penetrates the pericardium to enter the heart; ◈ is the level at which the trachea bifurcates into right and left main bronchi; and ◈ marks the superior limit of the pulmonary trunk. Regional anatomy ◈ The cylindrical thorax consists of: ◈ a wall, ◈ two pleural cavities, ◈ the lungs, and ◈ the mediastinum. ◈ The thorax houses the heart and lungs, acts as a conduit for structures passing between the neck and the abdomen, and plays a principal role in breathing. ◈ In addition, the thoracic wall protects the heart and lungs and provides support for the upper limbs. ◈ Muscles anchored to the anterior thoracic wall provide some of this support, and together with their associated connective tissues, nerves, and vessels, and the overlying skin and superficial fascia, define the pectoral region. THORACIC WALL ◈ The thoracic wall is segmental in design and composed of skeletal elements and muscles. ◈ It extends between: ◈ the superior thoracic aperture, bordered by vertebra TI, rib I, and the manubrium of the sternum; and ◈ the inferior thoracic aperture, bordered by vertebra TXII, rib XII, the end of rib XI, the costal margin, and the xiphoid process of the sternum.  Skeletal framework ◈ The skeletal elements of the thoracic wall consist of the: 🞚 thoracic vertebrae, 🞚 intervertebral discs, 🞚 ribs, 🞚 sternum.  Thoracic vertebrae ◈ There are twelve thoracic vertebrae, each of which is characterized by articulations with ribs.  Typical thoracic vertebra ◈ A typical thoracic vertebra has a heart-shaped vertebral body, with roughly equal dimensions in the transverse and anteroposterior directions, and a long spinous process (Fig. 3.18). ◈ The vertebral foramen is generally circular and the laminae are broad and overlap with those of the vertebra below. ◈ The superior articular processes are flat, with their articular surfaces facing almost directly posteriorly, while the inferior articular processes project from the laminae and their articular facets face anteriorly. ◈ The transverse processes are club shaped and project posterolaterally  Articulation with ribs ◈ A typical thoracic vertebra has three sites on each side for articulation with ribs. ◈ Two demifacets (i.e., partial facets) are located on the superior and inferior aspects of the body for articulation with corresponding sites on the heads of adjacent ribs. ◈ The superior costal facet articulates with part of the head of its own rib, and the inferior costal facet articulates with part of the head of the rib below. ◈ An oval facet (transverse costal facet) at the end of the transverse process articulates with the tubercle of its own rib.  Articulation with ◈ ribs Not all vertebrae articulate with ribs in the same fashion (Fig. 3.19): ◈ The superior costal facets on the body of vertebra TI are complete and articulate with a single facet on the head of its own rib—in other words, the head of rib I does not articulate with vertebra CVII. ◈ Similarly, vertebra TX (and often TIX) articulates only with its own ribs and therefore lacks inferior demifacets on the body. ◈ Vertebrae TXI and TXII articulate only with the heads of their own ribs—they lack transverse costal facets and have only a single complete facet on each side of their bodies.  Ribs ◈ There are twelve pairs of ribs, each terminating anteriorly in a costal cartilage (Fig. 3.20). ◈ Although all ribs articulate with the vertebral column, only the costal cartilages of the upper seven ribs, known as true ribs, articulate directly with the sternum. ◈ The remaining five pairs of ribs are false ribs: ◈ The costal cartilages of ribs VIII to X articulate anteriorly with the costal cartilages of the ribs above. ◈ Ribs XI and XII have no anterior connection with other ribs or with the sternum and are often called floating ribs. ◈ A typical rib consists of a curved shaft with anterior and posterior ends (Fig. 3.21). ◈ The anterior end is continuous with its costal cartilage. ◈ The posterior end articulates with the vertebral column and is characterized by a head, neck, and tubercle.  Typical rib ◈ The head is somewhat expanded and typically presents two articular surfaces separated by a crest. ◈ The smaller superior surface articulates with the inferior costal facet on the body of the vertebra above, whereas the larger inferior facet articulates with the superior costal facet of its own vertebra. ◈ The neck is a short flat region of bone that separates the head from the tubercle. ◈ The tubercle projects posteriorly from the junction of the neck with the shaft and consists of two regions, an articular part and a nonarticular part: ◈ The articular part is medial and has an oval facet for articulation with a corresponding facet on the transverse process of the associated vertebra. ◈ The raised nonarticular part is roughened by ligament attachments.  Typical rib ◈ The shaft is generally thin and flat with internal and external surfaces. ◈ The superior margin is smooth and rounded, whereas the inferior margin is sharp. ◈ The shaft bends forward just laterally to the tubercle at a site termed the angle. ◈ It also has a gentle twist around its longitudinal axis so that the external surface of the anterior part of the shaft faces somewhat superiorly relative to the posterior part. ◈ The inferior margin of the internal surface is marked by a distinct costal groove.  Distinct features of upper and lower ribs ◈ Rib I is flat in the horizontal plane and has broad superior and inferior surfaces. ◈ From its articulation with vertebra TI, it slopes inferiorly to its attachment to the manubrium of the sternum. ◈ The head articulates only with the body of vertebra TI and therefore has only one articular surface. Like other ribs, the tubercle has a facet for articulation with the transverse process. ◈ The superior surface of the rib is characterized by a distinct tubercle, the scalene tubercle, which separates two smooth grooves that cross the rib approximately midway along the shaft. ◈ The anterior groove is caused by the subclavian vein, and the posterior groove is caused by the subclavian artery. ◈ Anterior and posterior to these grooves, the shaft is roughened by muscle and ligament attachments  Distinct features of upper and lower ribs ◈ Rib II 🞚 Rib II, like rib I, is flat but twice as long. It articulates with the vertebral column in a way typical of most ribs. ◈ Rib X 🞚 The head of rib X has a single facet for articulation with its own vertebra. ◈ Ribs XI and XII 🞚 Ribs XI and XII articulate only with the bodies of their own vertebrae and have no tubercles or necks. 🞚 Both ribs are short, have little curve, and are pointed anteriorly.  Sternum ◈ The adult sternum consists of three major elements: ◈ the broad and superiorly positioned manubrium of the sternum, ◈ the narrow and longitudinally oriented body of the sternum, ◈ and the small and inferiorly positioned xiphoid process (Fig. 3.23).  Manubrium of the sternum ◈ The manubrium of the sternum forms part of the bony framework of the neck and the thorax. ◈ The superior surface of the manubrium is expanded laterally and bears a distinct and palpable notch, the jugular notch (suprasternal notch), in the midline. ◈ On either side of this notch is a large oval fossa for articulation with the clavicle. Immediately inferior to this fossa, on each lateral surface of the manubrium, is a facet for the attachment of the first costal cartilage. ◈ At the lower end of the lateral border is a demifacet for articulation with the upper half of the anterior end of the second costal cartilage.  Body of the sternum ◈ The body of the sternum is flat. ◈ The anterior surface of the body of the sternum is often marked by transverse ridges that represent lines of fusion between the segmental elements called sternebrae, from which this part of the sternum arises embryologically. ◈ The lateral margins of the body of the sternum have articular facets for costal cartilages. ◈ Superiorly, each lateral margin has a demifacet for articulation with the inferior aspect of the second costal cartilage. ◈ Inferior to this demifacet are four facets for articulation with the costal cartilages of ribs III to VI. ◈ At the inferior end of the body of the sternum is a demifacet for articulation with the upper demifacet on the seventh costal cartilage. ◈ The inferior end of the body of the sternum is attached to the xiphoid process.  Xiphoid process ◈ The xiphoid process is the smallest part of the sternum. ◈ Its shape is variable: it may be wide, thin, pointed, bifid, curved, or perforated. ◈ It begins as a cartilaginous structure, which becomes ossified in the adult. ◈ On each side of its upper lateral margin is a demifacet for articulation with the inferior end of the seventh costal cartilage.  JOINTS ◈ Costovertebral joints ◈ A typical rib articulates with: ◈ the bodies of adjacent vertebrae, forming a joint with the head of the rib; and ◈ the transverse process of its related vertebra, forming a costotransverse joint (Fig. 3.24). ◈ Together, the costovertebral joints and related ligaments allow the necks of the ribs either to rotate around their longitudinal axes, which occurs mainly in the upper ribs, or to ascend and descend relative to the vertebral column, which occurs mainly in the lower ribs. ◈ The combined movements of all of the ribs on the vertebral column are essential for altering the volume of the thoracic cavity during breathing.  Joint with head of rib ◈ The two facets on the head of the rib articulate with the superior facet on the body of its own vertebra and with the inferior facet on the body of the vertebra above. ◈ This joint is divided into two synovial compartments by an intra-articular ligament, which attaches the crest to the adjacent intervertebral disc and separates the two articular surfaces on the head of the rib. ◈ The two synovial compartments and the intervening ligament are surrounded by a single joint capsule attached to the outer margins of the combined articular surfaces of the head and vertebral column.  Costotransverse joints ◈ are synovial joints between the tubercle of a rib and the transverse process of the related vertebra (Fig. 3.24). ◈ The capsule surrounding each joint is thin. ◈ The joint is stabilized by two strong extracapsular ligaments that span the space between the transverse process and the rib on the medial and lateral sides of the joint: ◈ The costotransverse ligament is medial to the joint and attaches the neck of the rib to the transverse process. ◈ The lateral costotransverse ligament is lateral to the joint and attaches the tip of the transverse process to the roughened nonarticular part of the tubercle of the rib. ◈ A third ligament, the superior costotransverse ligament, attaches the superior surface of the neck of the rib to the transverse process of the vertebra above. ◈ Slight gliding movements occur at the costotransverse joints.  STERNOCOSTAL JOINTS ◈ The sternocostal joints are joints between the upper seven costal cartilages and the sternum (Fig. 3.25). ◈ The joint between rib I and the manubrium is not synovial and consists of a fibrocartilaginous connection between the manubrium and the costal cartilage. ◈ The second to seventh joints are synovial and have thin capsules reinforced by surrounding sternocostal ligaments. ◈ The joint between the second costal cartilage and the sternum is divided into two compartments by an intraarticular ligament. ◈ This ligament attaches the second costal cartilage to the junction of the manubrium and the body of the sternum.  Interchondral joints ◈ Interchondral joints occur between the costal cartilages of adjacent ribs (Fig. 3.25), mainly between the costal cartilages of ribs VII to X, but may also involve the costal cartilages of ribs V and VI. ◈ Interchondral joints provide indirect anchorage to the sternum and contribute to the formation of a smooth inferior costal margin. ◈ They are usually synovial, and the thin fibrous capsules are reinforced by interchondral ligaments.  Manubriosternal and xiphisternal joints ◈ The joints between the manubrium and the body of the sternum and between the body of the sternum and the xiphoid process are usually symphyses (Fig. 3.25). ◈ Only slight angular movements occur between the manubrium and the body of the sternum during respiration. ◈ The joint between the body of the sternum and the xiphoid process often becomes ossified with age. ◈ A clinically useful feature of the manubriosternal joint is that it can be palpated easily. This is because the manubrium normally angles posteriorly on the body of the sternum, forming a raised feature referred to as the sternal angle. This elevation marks the site of articulation of rib II with the sternum. ◈ Rib I is not palpable, because it lies inferior to the clavicle and is embedded in tissues at the base of the neck. ◈ Therefore, rib II is used as a reference for counting ribs and can be felt immediately lateral to the sternal angle.  Manubriosternal and xiphisternal joints ◈ In addition, the sternal angle lies on a horizontal plane that passes through the intervertebral disc between vertebrae TIV and TV (see Fig. 3.10). ◈ This plane separates the superior mediastinum from the inferior mediastinum and marks the superior border of the pericardium. ◈ The plane also passes through the end of the ascending aorta and the beginning of the arch of the aorta, the end of the arch of the aorta and the beginning of the thoracic aorta, and the bifurcation of the trachea, and just superior to the pulmonary trunk (see Fig. 3.79 and 3.86).  INTERCOSTAL SPACES ◈ Intercostal spaces lie between adjacent ribs and are filled by intercostal muscles (Fig. 3.26). ◈ Intercostal nerves and associated major arteries and veins lie in the costal groove along the inferior margin of the superior rib and pass in the plane between the inner two layers of muscles. ◈ In each space, the vein is the most superior structure and is therefore highest in the costal groove. ◈ The artery is inferior to the vein, and the nerve is inferior to the artery and often not protected by the groove. ◈ Therefore, the nerve is the structure most at risk when objects perforate the upper aspect of an intercostal space. ◈ Small collateral branches of the major intercostal nerves and vessels are often present superior to the inferior rib below. ◈ Deep to the intercostal spaces and ribs, and separating these structures from the underlying pleura, is a layer of loose connective tissue, called endothoracic fascia, which contains variable amounts of fat. ◈ Superficial to the spaces are deep fascia, superficial fascia, and skin. Muscles associated with the upper limbs and back overlie the spaces.  MUSCLES OF THE THORACIC WALL ◈ Muscles of the thoracic wall include those that fill and support the intercostal spaces, those that pass between the sternum and the ribs, and those that cross several ribs between costal attachments (Table 3.2). ◈ The muscles of the thoracic wall, together with muscles between the vertebrae and ribs posteriorly (i.e., the levatores costarum and serratus posterior superior and serratus posterior inferior muscles) alter the position of the ribs and sternum and so change the thoracic volume during breathing. ◈ They also reinforce the thoracic wall.  Intercostal muscles ◈ The intercostal muscles are three flat muscles found in each intercostal space that pass between adjacent ribs (Fig. 3.27). ◈ Individual muscles in this group are named according to their positions: 🞚 The external intercostal muscles are the most superficial. 🞚 The internal intercostal muscles are sandwiched between the external and innermost muscles. 🞚 The innermost intercostal muscles are the deepest of the three muscles. ◈ Are innervated by the related intercostal nerves. ◈ As a group, the intercostal muscles provide structural support for the intercostal spaces during breathing. ◈ They can also move the ribs.  External intercostal muscles ◈ The eleven pairs of external intercostal muscles extend from the inferior margins (lateral edges of costal grooves) of the ribs above to the superior margins of the ribs below. ◈ When the thoracic wall is viewed from a lateral position, the muscle fibers pass obliquely anteroinferiorly (Fig. 3.27). ◈ The muscles extend around the thoracic wall from the regions of the tubercles of the ribs to the costal cartilages, where each layer continues as a thin connective tissue aponeurosis termed the external intercostal membrane. ◈ Most active in inspiration.  Internal intercostal muscles ◈ The eleven pairs of internal intercostal muscles pass between the most inferior lateral edge of the costal grooves of the ribs above, to the superior margins of the ribs below. ◈ They extend from parasternal regions, where the muscles course between adjacent costal cartilages, to the angle of the ribs posteriorly (Fig. 3.27). ◈ This layer continues medially toward the vertebral column, in each intercostal space, as the internal intercostal membrane. ◈ The muscle fibers pass in the opposite direction to those of the external intercostal muscles. ◈ When the thoracic wall is viewed from a lateral position, the muscle fibers pass obliquely posteroinferiorly. ◈ Most active during expiration.  Innermost intercostal muscles ◈ The least distinct of the intercostal muscles, and the fibers have the same orientation as the internal intercostals (Fig. 3.27). ◈ These are most evident in the lateral thoracic wall. ◈ They extend between the inner surfaces of adjacent ribs from the medial edge of the costal groove to the deep surface of the rib below. Importantly, the neurovascular bundles associated with the intercostal spaces pass around the thoracic wall in the costal grooves in a plane between the innermost and internal intercostal muscles.  Subcostales ◈ The subcostales are in the same plane as the innermost intercostals, span multiple ribs, and are more numerous in lower regions of the posterior thoracic wall (Fig. 3.28A). ◈ They extend from the internal surfaces of one rib to the internal surface of the second (next) or third rib below. ◈ Their fibers parallel the course of the internal intercostal muscles and extend from the angle of the ribs to more medial positions on the ribs below.  Transversus thoracis muscles ◈ found on the deep surface of the anterior thoracic wall (Fig. 3.28B) and in the same plane as the innermost intercostals. ◈ originate from the posterior aspect of the xiphoid process, the inferior part of the body of the sternum, and the adjacent costal cartilages of the lower true ribs. ◈ They pass superiorly and laterally to insert into the lower borders of the costal cartilages of ribs III to VI. 🞚 They most likely pull these latter elements inferiorly. ◈ They lie deep to the internal thoracic vessels and secure these vessels to the wall.  ARTERIAL SUPPLY OF THE THORACIC WALL ◈ Vessels that supply the thoracic wall consist mainly of posterior and anterior intercostal arteries, which pass around the wall between adjacent ribs in intercostal spaces (Fig. 3.29). ◈ These arteries originate from the aorta and internal thoracic arteries, which in turn arise from the subclavian arteries in the root of the neck. ◈ Together, the intercostal arteries form a basket-like pattern of vascular supply around the thoracic wall.  Posterior intercostal arteries ◈ originate from vessels associated with the posterior thoracic wall. ◈ The upper two posterior intercostal arteries on each side are derived from the supreme intercostal artery, which descends into the thorax as a branch of the costocervical trunk in the neck. The costocervical trunk is a posterior branch of the subclavian artery (Fig. 3.29). ◈ The remaining nine pairs of posterior intercostal arteries arise from the posterior surface of the thoracic aorta. Because the aorta is on the left side of the vertebral column, those posterior intercostal vessels passing to the right side of the thoracic wall cross the midline anterior to the bodies of the vertebrae and therefore are longer than the corresponding vessels on the left. ◈ In addition to having numerous branches that supply various components of the wall, the posterior intercostal arteries have branches that accompany lateral cutaneous branches of the intercostal nerves to superficial regions.  Anterior intercostal arteries ◈ originate directly or indirectly as lateral branches from the internal thoracic arteries (Fig. 3.29). ◈ Each internal thoracic artery arises as a major branch of the subclavian artery in the neck. ◈ It passes anteriorly over the cervical dome of the pleura and descends vertically through the superior thoracic aperture and along the deep aspect of the anterior thoracic wall. ◈ On each side, the internal thoracic artery lies posterior to the costal cartilages of the upper six ribs and about 1 cm lateral to the sternum.  Anterior intercostal arteries ◈ At approximately the level of the sixth intercostal space, it divides into two terminal branches: 🞚 the superior epigastric artery, which continues inferiorly into the anterior abdominal wall (Fig. 3.29); and 🞚 the musculophrenic artery, which passes along the costal margin, goes through the diaphragm, and ends near the last intercostal space.  Anterior intercostal arteries ◈ Anterior intercostal arteries that supply the upper six intercostal spaces arise as lateral branches from the internal thoracic artery, whereas those supplying the lower spaces arise from the musculophrenic artery. ◈ In each intercostal space, the anterior intercostal arteries usually have two branches: 🞚 One passes below the margin of the upper rib. 🞚 The other passes above the margin of the lower rib and meets a collateral branch of the posterior intercostal artery.  Anterior intercostal arteries ◈ The distributions of the anterior and posterior intercostal vessels overlap and can develop anastomotic connections. ◈ The anterior intercostal arteries are generally smaller than the posterior vessels. ◈ In addition to anterior intercostal arteries and a number of other branches, the internal thoracic arteries give rise to perforating branches that pass directly forward between the costal cartilages to supply structures external to the thoracic wall. 🞚 These vessels travel with the anterior cutaneous branches of the intercostal nerves.  Venous drainage of the Thoracic Wall ◈ Venous drainage from the thoracic wall generally parallels the pattern of arterial supply (Fig. 3.30). ◈ Centrally, the intercostal veins ultimately drain into the azygos system of veins or into internal thoracic veins, which connect with the brachiocephalic veins in the neck. ◈ Often the upper posterior intercostal veins on the left side come together and form the left superior intercostal vein, which empties into the left brachiocephalic vein. ◈ Similarly, the upper posterior intercostal veins on the right side may come together and form the right superior intercostal vein, which empties into the azygos vein.  LYMPHATIC DRAINAGE OF THE THORACIC WALL ◈ Lymphatic vessels of the thoracic wall drain mainly into lymph nodes associated with the internal thoracic arteries (parasternal nodes), with the heads and necks of ribs (intercostal nodes), and with the diaphragm (diaphragmatic nodes) (Fig. 3.31). ◈ Diaphragmatic nodes are posterior to the xiphoid and at sites where the phrenic nerves penetrate the diaphragm. 🞚 They also occur in regions where the diaphragm is attached to the vertebral column. ◈ Parasternal nodes drain into bronchomediastinal trunks. ◈ Intercostal nodes in the upper thorax also drain into bronchomediastinal trunks, whereas intercostal nodes in the lower thorax drain into the thoracic duct. ◈ Nodes associated with the diaphragm interconnect with parasternal, prevertebral, and juxta-esophageal nodes, brachiocephalic nodes (anterior to the brachiocephalic veins in the superior mediastinum), and lateral aortic/lumbar nodes (in the abdomen). ◈ Superficial regions of the thoracic wall drain mainly into axillary lymph nodes in the axilla or parasternal nodes.  INNERVATION OF THE THORACIC WALL ◈ Innervation of the thoracic wall is mainly by the intercostal nerves, which are the anterior rami of spinal nerves ◈ T1 to T11 and lie in the intercostal spaces between adjacent ribs. ◈ The anterior ramus of spinal nerve T12 (the subcostal nerve) is inferior to rib XII (Fig. 3.32).  INNERVATION OF THE THORACIC WALL ◈ A typical intercostal nerve passes laterally around the thoracic wall in an intercostal space. ◈ The largest of the branches is the lateral cutaneous branch, which pierces the lateral thoracic wall and divides into an anterior branch and a posterior branch that innervate the overlying skin. ◈ The intercostal nerves end as anterior cutaneous branches, which emerge either parasternally, between adjacent costal cartilages, or laterally to the midline, on the anterior abdominal wall, to supply the skin.  INNERVATION OF THE THORACIC WALL ◈ In addition to these major branches, small collateral branches can be found in the intercostal space running along the superior border of the lower rib. ◈ In the thorax, the intercostal nerves carry: 🞚 somatic motor innervation to the muscles of the thoracic wall (intercostal, subcostal, and transversus thoracis muscles), 🞚 somatic sensory innervation from the skin and parietal pleura, and 🞚 postganglionic sympathetic fibers to the periphery.  INNERVATION OF THE THORACIC WALL ◈ Sensory innervation of the skin overlying the upper thoracic wall is supplied by cutaneous branches (supraclavicular nerves), which descend from the cervical plexus in the neck. ◈ In addition to innervating the thoracic wall, intercostal nerves innervate other regions: 🞚 The anterior ramus of T1 contributes to the brachial plexus. ◈ The lateral cutaneous branch of the second intercostal nerve (the intercostobrachial nerve) contributes to cutaneous innervation of the medial surface of the upper arm. 🞚 The lower intercostal nerves supply the muscles, skin, and peritoneum of the abdominal wall.  Segmental neurovascular supply of thoracic wall ◈ The arrangement of vessels and nerves that supply the thoracic wall reflects the segmental organization of the wall. ◈ Arteries to the wall arise from two sources: ◈ the thoracic aorta, which is in the posterior mediastinum, and ◈ a pair of vessels, the internal thoracic arteries, which run along the deep aspect of the anterior thoracic wall on either side of the sternum. ◈ Posterior and anterior intercostal vessels branch segmentally from these arteries and pass laterally around the wall, mainly along the inferior margin of each rib (Fig. 3.12A). ◈ Running with these vessels are intercostal nerves (the anterior rami of thoracic spinal nerves), which innervate the wall, related parietal pleura, and associated skin. ◈ The position of these nerves and vessels relative to the ribs must be considered when passing objects, such as chest tubes, through the thoracic wall. ◈ Dermatomes of the thorax generally reflect the segmental organization of the thoracic spinal nerves (Fig. 3.12B). ◈ The exception occurs, anteriorly and superiorly, with the first thoracic dermatome, which is located mostly in the upper limb, and not on the trunk. ◈ The anterosuperior region of the trunk receives branches from the anterior ramus of C4 via supraclavicular branches of the cervical plexus. ◈ The highest thoracic dermatome on the anterior chest wall is T2, which also extends into the upper limb. In the midline, skin over the xiphoid process is innervated by T6. ◈ Dermatomes of T7 to T12 follow the contour of the ribs onto the anterior abdominal wall (Fig. 3.12C). Sympathetic system ◈ All preganglionic nerve fibers of the sympathetic system are carried out of the spinal cord in spinal nerves T1 to L2 (Fig. 3.13). ◈ This means that sympathetic fibers found anywhere in the body ultimately emerge from the spinal cord as components of these spinal nerves. ◈ Preganglionic sympathetic fibers destined for the head are carried out of the spinal cord in spinal nerve T1.  Flexible wall and inferior thoracic aperture ◈ The thoracic wall is expandable because most ribs articulate with other components of the wall by true joints that allow movement, and because of the shape and orientation of the ribs (Fig. 3.14). ◈ A rib’s posterior attachment is superior to its anterior attachment. ◈ Therefore, when a rib is elevated, it moves the anterior thoracic wall forward relative to the posterior wall, which is fixed. ◈ In addition, the middle part of each rib is inferior to its two ends, so that when this region of the rib is elevated, it expands the thoracic wall laterally. ◈ Finally, because the diaphragm is muscular, it changes the volume of the thorax in the vertical direction. ◈ Changes in the anterior, lateral, and vertical dimensions of the thoracic cavity are important for breathing.  Innervation of the diaphragm ◈ The diaphragm is innervated by two phrenic nerves that originate, one on each side, as branches of the cervical plexus in the neck (Fig. 3.15). ◈ arise from the anterior rami of cervical nerves C3, C4, and C5, with the major contribution coming from C4. ◈ The phrenic nerves pass vertically through the neck, the superior thoracic aperture, and the mediastinum to supply motor innervation to the entire diaphragm, including the crura (muscular extensions that attach the diaphragm to the upper lumbar vertebrae). ◈ In the mediastinum, the phrenic nerves pass anteriorly to the roots of the lungs.  Innervation of the diaphragm ◈ The tissues that initially give rise to the diaphragm are in an anterior position on the embryological disc before the head fold develops, which explains the cervical origin of the nerves that innervate the diaphragm. ◈ In other words, the tissue that gives rise to the diaphragm originates superior to the ultimate location of the diaphragm. ◈ Spinal cord injuries below the level of the origin of the phrenic nerve do not affect movement of the diaphragm. RELATIONSHIP TO OTHER REGIONS  Neck ◈ The superior thoracic aperture opens directly into the root of the neck (Fig. 3.7). ◈ The superior aspect of each pleural cavity extends approximately 2 to 3 cm above rib I and the costal cartilage into the neck. ◈ Between these pleural extensions, major visceral structures pass between the neck and superior mediastinum. ◈ In the midline, the trachea lies immediately anterior to the esophagus. ◈ Major blood vessels and nerves pass in and out of the thorax at the superior thoracic aperture anteriorly and laterally to these structures.  Upper limb ◈ An axillary inlet, or gateway to the upper limb, lies on each side of the superior thoracic aperture. ◈ These two axillary inlets and the superior thoracic aperture communicate superiorly with the root of the neck (Fig. 3.7). ◈ Each axillary inlet is formed by: ◈ the superior margin of the scapula posteriorly, ◈ the clavicle anteriorly, and ◈ the lateral margin of rib I medially.  Upper limb ◈ The apex of each triangular inlet is directed laterally and is formed by the medial margin of the coracoid process which extends anteriorly from the superior margin of the scapula. ◈ The base of the axillary inlet’s triangular opening is the lateral margin of rib I. ◈ Large blood vessels passing between the axillary inlet and superior thoracic aperture do so by passing over rib I. ◈ Proximal parts of the brachial plexus also pass between the neck and upper limb by passing through the axillary inlet.  Abdomen ◈ The diaphragm separates the thorax from the abdomen. ◈ Structures that pass between the thorax and abdomen either penetrate the diaphragm or pass posteriorly to it (Fig. 3.8): ◈ The inferior vena cava pierces the central tendon of the diaphragm to enter the right side of the mediastinum near vertebral level TVIII. ◈ The esophagus penetrates the muscular part of the diaphragm to leave the mediastinum and enter the abdomen just to the left of the midline at vertebral level TX. ◈ The aorta passes posteriorly to the diaphragm at the midline at vertebral level TXII. ◈ Numerous other structures that pass between the thorax and abdomen pass through or posterior to the diaphragm.  PECTORAL REGION ◈ The pectoral region is external to the anterior thoracic wall and helps anchor the upper limb to the trunk. ◈ It consists of: ◈ a superficial compartment containing skin, superficial fascia, and breasts; and ◈ a deep compartment containing muscles and associated structures. ◈ Nerves, vessels, and lymphatics in the superficial compartment emerge from the thoracic wall, the axilla, and the neck.  Breast ◈ The breasts consist of mammary glands and associated skin and connective tissues. ◈ The mammary glands are modified sweat glands in the superficial fascia anterior to the pectoral muscles and the anterior thoracic wall (Fig. 3.16). ◈ The mammary glands consist of a series of ducts and associated secretory lobules. ◈ These converge to form 15 to 20 lactiferous ducts, which open independently onto the nipple. ◈ The nipple is surrounded by a circular pigmented area of skin termed the areola.  Breast ◈ A well-developed, connective tissue stroma surrounds the ducts and lobules of the mammary gland. ◈ In certain regions, this condenses to form well-defined ligaments, the suspensory ligaments of breast, which are continuous with the dermis of the skin and support the breast. ◈ Carcinoma of the breast creates tension on these ligaments, causing pitting of the skin. ◈ In nonlactating women, the predominant component of the breasts is fat, while glandular tissue is more abundant in lactating women.  Breast ◈ The breast lies on deep fascia related to the pectoralis major muscle and other surrounding muscles. ◈ A layer of loose connective tissue (the retromammary space) separates the breast from the deep fascia and provides some degree of movement over underlying structures. ◈ The base, or attached surface, of each breast extends vertically from ribs II to VI, and transversely from the sternum to as far laterally as the midaxillary line.  Arterial supply ◈ The breast is related to the thoracic wall and to structures associated with the upper limb; therefore, vascular supply and drainage can occur by multiple routes (Fig. 3.16): ◈ laterally, vessels from the axillary artery—superior thoracic, thoraco-acromial, lateral thoracic, and subscapular arteries; ◈ medially, branches from the internal thoracic artery; ◈ the second to fourth intercostal arteries via branches that perforate the thoracic wall and overlying muscle.  Venous drainage ◈ Veins draining the breast parallel the arteries and ultimately drain into the axillary, internal thoracic, and intercostal veins  Innervation ◈ Innervation of the breast is via anterior and lateral cutaneous branches of the second to sixth intercostal nerves. ◈ The nipple is innervated by the fourth intercostal nerve.  Lymphatic drainage ◈ Lymphatic drainage of the breast is as follows: ◈ Approximately 75% is via lymphatic vessels that drain laterally and superiorly into axillary nodes (Fig. 3.16). ◈ Most of the remaining drainage is into parasternal nodes deep to the anterior thoracic wall and associated with the internal thoracic artery. ◈ Some drainage may occur via lymphatic vessels that follow the lateral branches of posterior intercostal arteries and connect with intercostal nodes situated near the heads and necks of ribs. ◈ Axillary nodes drain into the subclavian trunks, parasternal nodes drain into the bronchomediastinal trunks, and intercostal nodes drain either into the thoracic duct or into the bronchomediastinal trunks.  Breast in men ◈ The breast in men is rudimentary and consists only of small ducts, often composed of cords of cells, that normally do not extend beyond the areola. ◈ Breast cancer can occur in men.  Muscles of the pectoral region ◈ Each pectoral region contains the pectoralis major, pectoralis minor, and subclavius muscles (Fig. 3.17 and Table 3.1). ◈ All originate from the anterior thoracic wall and insert into bones of the upper limb.  Pectoralis major ◈ The pectoralis major muscle is the largest and most superficial of the pectoral region muscles. ◈ It directly underlies the breast and is separated from it by deep fascia and the loose connective tissue of the retromammary space. ◈ The pectoralis major has a broad origin that includes the anterior surfaces of the medial half of the clavicle, the sternum, and related costal cartilages. ◈ The muscle fibers converge to form a flat tendon, which inserts into the lateral lip of the intertubercular sulcus of the humerus. ◈ The pectoralis major adducts, flexes, and medially rotates the arm.  Subclavius and pectoralis minor muscles ◈ The subclavius and pectoralis minor muscles underlie the pectoralis major: ◈ The subclavius is small and passes laterally from the anterior and medial part of rib I to the inferior surface of the clavicle. ◈ The pectoralis minor passes from the anterior surfaces of ribs III to V to the coracoid process of the scapula. ◈ Both the subclavius and pectoralis minor pull the tip of the shoulder inferiorly ◈ A continuous layer of deep fascia, the clavipectoral fascia, encloses the subclavius and pectoralis minor and attaches to the clavicle above and to the floor of the axilla below. ◈ The muscles of the pectoral region form the anterior wall of the axilla, a region between the upper limb and the neck through which all major structures pass. ◈ Nerves, vessels, and lymphatics that pass between the pectoral region and the axilla pass through the clavipectoral fascia between the subclavius and pectoralis minor or pass under the inferior margins of the pectoralis major and minor. TO BE CONTINUED… DIAPHRAGM MEDIASTINUM LUNGS HEART ESOPHAGUS