Chapter 20: The thoracic wall and mediastinum
The muscles of the thoracic and abdominal walls are in general arranged in external, middle, and internal layers. In the thorax (figs. 20-1 and 20-2), these are the (1) external intercostal muscles, (2) internal intercostal muscles, and (3) innermost intercostal muscles, subcostal muscles, and transversus thoracis. The internal layer and the thoracic skeleton are separated from the costal pleura by the endothoracic fascia. The diaphragm separates the thoracic and abdominal viscera.
The external intercostal muscles are attached to the lower margins of ribs 1 to 11. Their fibers pass inferior and anterior to insert on the upper margin of the rib below. Anteriorly, at the costochondral junctions, the external intercostal muscles are replaced by the external intercostal membranes (fig. 20-2). The muscles are supplied by the corresponding intercostal nerves. They elevate the ribs and hence are considered to be muscles of inspiration. They are assisted posteriorly by the levatores costarum, which run from the transverse processes to the backs of the subjacent ribs and are supplied by primary dorsal rami.
The internal intercostal muscles are attached to the lower margins of the ribs and costal cartilages and to the floor of the costal groove. Their fibers pass inferior and posterior to insert on the upper margin of the rib and costal cartilage below. Posteriorly, at the angles of the ribs, the internal intercostal muscles are replaced by the internal intercostal membranes (fig. 20-2). The muscles are supplied by the corresponding intercostal nerves. For the most part, they are muscles of expiration.
The innermost intercostal muscles may be regarded as those parts of the internal intercostal muscles that are internal to the intercostal vessels and nerves. Their action is unknown. The subcostal muscles, which are quite variable, arise from the ribs posteriorly and are inserted into the second or third rib below. They probably elevate the ribs. The transversus thoracis (or sternocostalis) (see fig. 19-2) arises from the posterior surface of the xiphoid process and body of the sternum and is inserted posteriorly into several costal cartilages. It appears to be expiratory in function. All these muscles are supplied by the corresponding intercostal nerves.
The diaphragm, is the most important muscle of respiration. It separates the thoracic and abdominal viscera. Three of its parts (sternal, costal, and lumbar) are inserted into the central tendon, a trifoliate structure that lies immediately inferior to the heart. The sternal part consists of slips from the xiphoid process, which (in vivo) descend to the central tendon. On each side, a small gap known as the sternocostal triangle is present between the sternal and costal parts. It transmits the superior epigastric vessels and some lymphatics, and it may be the site of a diaphragmatic hernia. The costal parts, which form the right and left "domes," arise from the inner surfaces of the lower costal cartilages and ribs and interdigitate with the transversus abdominis. They are inserted into the central tendon anterolaterally. Each lumbar (or vertebral) part arises from (1) a lateral arcuate ligament over the quadratus lumborum, (2) a medial arcuate ligament over the psoas major, and (3) a crus from the upper lumbar vertebrae (see fig. 25-13B). Usually the right crus arises from the first to third (or fourth) lumbar vertebrae (L1 to 3 or 1 to 4) and the left from L.V.l to 2 or 1 to 3. The crura are united anterior to the aorta by the median arcuate ligament, a fibrous arch that forms the aortic opening. The right crus splits around the esophagus (see figs. 21-2 and 25-13B), and part of it continues into the suspensory ligament of the duodenum. The left crus is smaller and more variable.
The diaphragm has three major openings (see fig. 25-13B). The esophageal opening in the right crus transmits the esophagus and vagus nerves. The aortic opening lies posterior to the crura and transmits the aorta, the thoracic duct and greater splanchnic nerves, and occasionally the azygos vein. The foramen for the inferior vena cava, in the right half of the central tendon, transmits the vena cava, right phrenic nerve, and lymphatic vessels. Other structures that pierce or are related to the diaphragm include the splanchnic nerves, sympathetic trunk, subcostal nerves and vessels, superior epigastric and musculophrenic vessels, and azygos and hemiazygos veins.
The portion of the costal part of the diaphragm that arises from ribs 11 and 12 is often separated from the lumbar part by an interval termed the vertebrocostal trigone (see fig. 25-13B). Such a triangle is occupied by connective tissue that separates the pleura above from the suprarenal gland and upper pole of the kidney below.
Variations in the degree of development of the muscular parts are not uncommon. Congenital diaphragmatic hernia, whereby an abdominal organ may enter the thoracic cavity, may occur through the esophageal opening (hiatal hernia), through a gap in the costal part of the diaphragm (e.g., from a persistent pleuroperitoneal canal), or through the sternocostal triangle. Diaphragmatic herniae usually have peritoneal sacs.
The diaphragm, together with the adjacent pleura and peritoneum, is supplied by the phrenic nerves (see fig. 24-6), each of which is distributed to one half of the diaphragm. Although the two halves of the diaphragm usually contract synchronously, paralysis of one half does not affect the other half, because each half of the diaphragm has a separate innervation. The diaphragm is under only limited voluntary control. Hiccups are sharp, spasmodic contractions of the diaphragm. The peripheral portion of the diaphragm is supplied with sensory and vasomotor fibers from the intercostal nerves.
The diaphragm descends when it contracts. The volume of the thorax is thereby increased, whereas intrathoracic pressure is decreased; the converse holds for the abdominal cavity. The decreased intrathoracic pressure and increased abdominal pressure that accompany descent of the diaphragm facilitate the return of blood to the heart.
When a subject is in the erect position and the midphase of respiration, the summit of the domes of the diaphragm is at about the same level as the apical region of the heart (fifth intercostal space; rib 6; or thoracic vertebral body T10 or 11). The right dome of the diaphragm is commonly about 1 cm higher than the left. During quiet breathing, the diaphragm undergoes an excursion of about 1/2 cm, whereas, during deep breathing, the excursion may be as much as 10 cm.
Each of the 12 thoracic (spinal) nerves gives off a recurrent meningeal branch, emerges from an intervertebral foramen, and divides into a dorsal and a ventral ramus. These rami contain motor fibers to muscles, sensory fibers from skin and deep tissues, and postganglionic sympathetic fibers to blood vessels, sweat glands, and arrector pili muscles (fig. 20-4).
The dorsal primary rami (figs. 20-2 and 20-4) pass posteriorward and supply the bones, joints, muscles, and skin of the mid-back. The ventral primary rami run anteriorward and supply the serous membranes, muscles, and skin of the thoracic and abdominal walls. Each is connected to the sympathetic trunk by a variable number of rami communicantes (figs. 20-3 and 20-4). Although the distribution of the ventral rami issegmental, overlap of adjacent nerves is so great that section of three consecutive nerves would be necessary to produce complete anesthesia and paralysis within the middle one of the three intercostal spaces supplied.
The ventral primary rami of the first 11 thoracic (spinal) nerves are called intercostal nerves, whereas that of the twelfth is subcostal. Ventral rami 1 to 3 contribute to the upper limb as well as to the thoracic wall, and ventral rami 7 to 12 are thoraco-abdominal in their distribution. Intercostal nerves can be "blocked" posteriorly by a local anesthetic, e.g., for pain after fracture of a rib.
Intercostal nerves 4 to 6 are "typical" (figs. 20-1, 20-3 and 20-4) in that they supply only the thoracic wall and its associated muscles (intercostal, subcostal, serratus posterior superior, and transversus thoracis). Each passes inferior to the neck of the corresponding rib and enters the costal groove. In its course anteriorward, it lies first on the pleura and endothoracic fascia, then between the innermost and internal intercostal muscles, and finally on the transversus thoracis and internal thoracic vessels. At the anterior end of the intercostal space, it passes through the internal intercostal muscle, external intercostal membrane, and pectoralis major, to be distributed as the anterior cutaneous branch to the anterior chest. Each intercostal nerve gives off a collateral branch to the inferior part of the intercostal space and a lateral cutaneous branch to the side of the chest. In addition to being distributed to muscle and skin, branches are given to the parietal pleura, mammary gland, and periosteum of the ribs.
The first thoracic nerve divides into a superior part, which joins the brachial plexus, and an inferior part, which becomes the first intercostal nerve (fig. 20-5). The lateral cutaneous branches of intercostal nerves 1 to 3 contribute to the upper limb, that of the second being known as the intercostobrachial nerve.
Intercostal nerves 7 to 11 supply the abdominal as well as the thoracic wall; hence they may be termed thoraco-abdominal (see fig. 25-12). At the anterior end of the intercostal spaces, they pass between the muscles of the abdominal wall and come to lie between the rectus abdominis and the posterior wall of its sheath. Here each nerve divides into branches that supply the rectus and the overlying skin. Their lateral cutaneous branches also contribute to the abdominal wall. The thoraco-abdominal nerves give branches to thoracic and abdominal muscles and skin and sensory twigs to the pleura, diaphragm, and peritoneum.
The ventral ramus of thoracic nerve 12 is known as the subcostal nerve. It enters the abdomen posterior to the lateral arcuate ligament, crosses posterior to the kidney, penetrates the muscles of the abdominal wall, enters the rectus sheath, and becomes cutaneous (see fig. 25-12).
The thoracic wall is supplied by branches of (1) the subclavian artery (internal thoracic and highest intercostal arteries), (2) the axillary artery, and (3) the aorta (posterior intercostal and subcostal arteries).
The internal thoracic artery (previously called the internal mammary) artery (fig. 19-2) arises from the first part of the subclavian artery. It descends posterior to the sternomastoid muscle, clavicle, and subclavian and internal jugular veins. It is crossed by the phrenic nerve, and it lies on the pleura. It then descends posterior to the upper six costal cartilages, immediately lateral to the sternum, and anterior to the pleura. It gives branches to the intercostal spaces, pleura, pericardium, and breast. At the sixth intercostal space, it divides into the superior epigastric and musculophrenic arteries. The superior epigastric artery traverses the sternocostal triangle of the diaphragm, descends between the rectus abdominis and the posterior layer of its sheath, and anastomoses with the inferior epigastric artery. The musculophrenic artery, more laterally placed, supplies several intercostal spaces, pierces the diaphragm, and anastomoses with the deep circumflex iliac artery.
Posterior intercostal arteries 1 and 2 arise from the highest intercostal artery, which is a branch of the costocervical trunk of the subclavian artery. Posterior intercostal arteries 3 to 11 arise from the aorta (figs. 20-2 and 20-3). The right-sided arteries are longer because they have to cross the vertebral column. They lie posterior to the pleura, azygos venous system, and sympathetic trunk. Each artery enters the costal groove, runs forward between the vein and nerve ("V.A.N.") (between the innermost and internal intercostals muscles), and anastomoses with branches of the internal thoracic or musculophrenic arteries. A lateral cutaneous branch accompanies the corresponding nerve. The two subcostal arteries are in series with the intercostal arteries, and they enter the abdomen with the corresponding nerves.
The anastomoses between the internal thoracic, posterior intercostal, and inferior epigastric arteries provide an important collateral circulation in obstruction of the aorta, e.g., from coarctation. In such instances, the enlarged intercostal arteries in the costal grooves may erode the bone and show radiographically as notching of the ribs.
The parietal lymph nodes of the thorax are the parasternal, phrenic, and intercostal. The parasternal nodes, situated along the upper part of the internal thoracic artery, receive lymphatics from the medial part of the breast, the intercostal spaces, the costal pleura, and the diaphragm and drain into the bronchomediastinal trunk. The parasternal nodes allow the spread of carcinoma of the breast to the lungs and mediastinum and, by way of the diaphragm, even downward to the liver. The phrenic nodes are situated on the thoracic surface of the diaphragm. They receive lymphatics from the pericardium, diaphragm, and liver and drain into the parasternal nodes. Intercostal nodes are found at the vertebral end of the intercostal spaces.
The joints of the thorax occur between (1) vertebrae, (2) ribs and vertebrae, (3) ribs and costal cartilages, (4) costal cartilages, (5) costal cartilages and the sternum, and (6) the parts of the sternum.
The costovertebral joints (figs. 20-3 and 20-6), synovial in type, are those of the heads of the ribs and the costotransverse joints. The head (n) of a typical rib (ribs 2 to 9) articulates with the inferior and superior costal facets of two adjacent vertebral bodies (n-1 and n) and the intervening intervertebral disc. The heads of ribs 1 and 10 to 12 articulate with only one vertebra each; The tubercle of a typical rib forms a costotransverse joint with the costal facet on the transverse process of the corresponding vertebra. These joints are absent in ribs 11 and 12.
The costochondral joints are hyaline cartilaginous joints between the ends of the costal cartilages and ribs. The interchondral joints, generally synovial, are between costal cartilages 5 to 8 or 5 to 9.
The sternocostal (or sternochondral) joints, often considered synovial, are between costal cartilages 1 to 7 and the lateral margin of the sternum. The manubriosternal joint is fibrocartilaginous but may become ossified. The xiphosternal joint is cartilaginous but later becomes ossified.
The numerous thoracic joints are subject to continual movement, and any disorder that reduces their mobility hampers respiration.
In general, the ribs move around two axes. Movement at costovertebral joints 2 to 6 about a side-to-side axis results in raising and lowering the sternal end of the rib, the "pump-handle" movement (fig. 20-7). The downward slope of the rib ensures that, in elevation, the sternum moves upward and forward, increasing the anteroposterior diameter of the thorax. Movement at costovertebral joints 7 to 10 about an anteroposterior axis results in raising and lowering the middle of the rib, the "bucket-handle" movement (fig. 20-7). In elevation, this increases the transverse diameter of the thorax.
A movement of only a few millimeters is sufficient to increase the volume of the thoracic cage by the usual volume of air that enters and leaves the lungs during quiet breathing. In deep breathing the excursions are greater. The descent of the diaphragm increases the height of the thoracic cavity and hence increases the volume of the thorax. However, in unilateral or even bilateral paralysis of the diaphragm, there may be no significant disability. Diaphragmatic breathing is often called abdominal (as distinct from thoracic) breathing.
The diaphragm, whose descent increases the volume of the thoracic cavity, is the most important muscle of respiration. The mechanical actions of the intercostal muscles are not fully understood, but the external intercostal muscles (and intercartilaginous parts of the internal intercostal muscles) probably elevate the ribs and thus are inspiratory in function. The interosseous portions of the internal intercostal muscles probably depress the ribs and are thus expiratory in function.
In the inspiratory phase of quiet breathing, the diaphragm, "parasternal" intercostal muscles, and external intercostal muscles posteriorly are active, and, in some people, so are the scalene muscles. Expiration is mainly passive, because of the elasticity of the lungs, but the interosseous internal intercostal muscles of the lower interspaces also contribute. When breathing becomes deeper, the sternocleidomastoids and extensors of the vertebral column are active near the end of inspiration. Moreover, the external abdominal muscles anterolaterally become increasingly active during expiration. These muscles draw the ribs down and are the most important expiratory muscles. They compress the abdominal viscera and are active in coughing, straining, and vomiting. Muscular control of expiration is important in speaking and singing.
The mediastinum is defined as the interval between the two pleural sacs. It is commonly considered to comprise a superior mediastinum, above the level of the pericardium, and three lower divisions: anterior, middle, and posterior. Figure 20-8 shows the arrangement in vivo.
The anterior mediastinum. between the sternum and pericardium, contains the thymus. The middle mediastinum contains the pericardium, heart, and the main bronchi and other structures of the roots of the lungs. The posterior mediastinum, behind the pericardium, contains the esophagus and thoracic aorta. The superior mediastinum contains the thymus, great vessels related to the heart, the trachea, and the esophagus. The mediastinum contains various groups of visceral lymph nodes. The various structures in the mediastinum are surrounded and supported by loose connective tissue often infiltrated with fat.
Campbell, E. J. M., Agostoni, E., and Davis, J. N., The Respiratory Muscles. 2nd ed., Lloyd-Luke, London, 1970. A detailed account of respiratory mechanics and neural control.
20-1 What is the action of the diaphragm in respiration?
20-2 What is the vertebrocostal trigone?
20-3 Where may a congenital diaphragmatic hernia be found?
20-4 Which nerves are (a) intercostal, (b) subcostal, (c) thoraco-abdominal?
20-5 What does notching of ribs seen radiographically suggest?
20-6 What is the mediastinum?
Figure 20-1 The intercostal muscles. A shows the direction of fibers of the external and internal intercostal muscles. B shows a vertical section through an intercostal space. The white arrow represents the path of a needle in pleural aspiration, avoiding the intercostal vessels and nerve.
Figure 20-2 The nerves, arteries, and muscles of the thoracic wall. Note that the intercostal vessels pass behind the longitudinally disposed structures of the posterior mediastinum. The thickness of the intercostal muscles is exaggerated.
Figure 20-3 Intercostal vessels and nerve. A part of the sympathetic trunk is shown, including some rami communicantes.
Figure 20-4 Functional components of a thoracic nerve. For purposes of simplification, each component is shown as a single fiber. Motor fibers to skeletal muscle are shown in red, sympathetic fibers in black, and sensory fibers in blue. The branches of the ventral ramus to the pleura and peritoneum are not shown.
Figure 20-5 The ventral ramus of the first thoracic nerve, viewed from below. Note how the upper division of T1 joins C8 and forms the lower trunk of the brachial plexus, which rests on the first rib. Part of the sympathetic trunk is shown. The cervicothoracic ganglion is tightly bound to the first thoracic nerve by rami communicantes, but these lie posteriorly and are hidden.
Figure 20-6 The costovertebral joints viewed from (A) above and (B) behind.
Figure 20-7 Diagram of certain movements of the ribs. In A, when the upper ribs are elevated, the anteroposterior diameter of the thorax is increased ("pump-handle" movement). In B, the lower ribs move laterally when they are elevated, and the transverse diameter of the thorax is increased ("bucket-handle" movement).