# Lecture 14: Thoracic Wall ## Unit 1.3: Human Gross Anatomy I - Musculoskeletal System --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Describe the bony components of the thoracic cage (ribs, sternum, thoracic vertebrae) 2. Classify ribs and describe their typical structure 3. Identify the muscles of the thoracic wall and their functions in respiration 4. Describe the intercostal neurovascular bundles 5. Explain the mechanics of respiration 6. Describe the anatomy of the diaphragm --- ## Lecture Content ### I. Overview of the Thoracic Cage The thoracic cage serves multiple essential functions. It protects the thoracic organs including the heart and lungs, as well as the upper abdominal organs such as the liver and spleen. The cage provides attachment points for the muscles of respiration, the muscles of the upper limb, and the muscles of the back. The articulated structure enables the respiratory mechanics necessary for ventilation while providing structural support for the trunk. The thoracic cage comprises three main bony components. The twelve thoracic vertebrae form the posterior wall, providing rigid support and protection for the spinal cord. Twelve pairs of ribs with their costal cartilages curve anteriorly from the vertebral column to form the lateral walls. The sternum completes the cage anteriorly, connecting the costal cartilages of the upper seven ribs to create a complete protective enclosure. Panel A: Anterior oblique view of thoracic cage with twelve thoracic vertebrae forming the posterior arch (gray-blue) and twelve rib pairs curving anterolaterally. Panel B: Costal cartilages (lighter cream) connecting ribs to sternum with true, false, and floating rib classifications indicated. Panel C: Sternum anteriorly (light blue) with manubrium, body, and xiphoid process identified. Panel D: Transparent overlay indicating protected thoracic and upper abdominal organs (heart, lungs, liver, spleen) within the barrel-shaped structure. --- ### II. The Sternum The sternum consists of three parts arranged from superior to inferior: the manubrium, the body, and the xiphoid process. The manubrium has a quadrangular shape and forms the superior portion of the sternum. Its superior border features the jugular notch, also called the suprasternal notch, which is palpable at the base of the neck at the level of T2 to T3. Lateral to the jugular notch, the clavicular notches articulate with the medial ends of the clavicles forming the sternoclavicular joints. The costal notch on the lateral margin of the manubrium receives the first costal cartilage through a synchondrosis, a primary cartilaginous joint that permits minimal movement. The body, also called the gladiolus, represents the longest part of the sternum. Its lateral margins bear costal notches for the attachment of ribs two through seven. Transverse ridges on the anterior surface indicate the fusion of the four sternal segments from which the body develops. The xiphoid process forms the smallest and most inferior part of the sternum. It remains cartilaginous until approximately age forty when it typically ossifies. The xiphoid varies considerably in shape and may be bifid or perforated. It provides attachment for the linea alba and the rectus abdominis muscle and lies at the level of T10. The sternal angle, also known as the angle of Louis, marks the junction between the manubrium and body. This landmark lies at the level of the T4/T5 intervertebral disc and serves as a crucial clinical reference point. The second rib attaches at this level, making it the key landmark for counting ribs. Additional structures at this level include the tracheal bifurcation and the beginning and ending of the aortic arch. The sternal angle also marks the boundary of the superior mediastinum. Panel A: Manubrium with jugular notch at superior border, clavicular notches laterally for sternoclavicular joints, and first costal notch at inferior-lateral corner. Panel B: Body (gladiolus) as longest section with costal notches 2-7 on lateral margins and transverse ridges from fused sternal segments visible on anterior surface. Panel C: Xiphoid process as small inferior projection (cartilaginous until approximately age forty) with linea alba and rectus abdominis attachments. Panel D: Sternal angle clearly marked at manubriosternal junction with horizontal reference line indicating T4/T5 level, second rib attachment, and associated landmarks (tracheal bifurcation, aortic arch). --- ### III. The Ribs The human thorax contains twelve pairs of ribs, classified as flat bones despite their curved shape. Each rib articulates posteriorly with the thoracic vertebrae. The anterior attachment varies, forming the basis for rib classification. True ribs, comprising ribs one through seven, attach directly to the sternum through their own individual costal cartilages. These are also called vertebrosternal ribs. False ribs include ribs eight through ten, which attach to the cartilage of the rib immediately above rather than directly to the sternum. These are termed vertebrochondral ribs. Floating ribs, ribs eleven and twelve, have no anterior attachment whatsoever and are called vertebral ribs. A typical rib, represented by ribs three through nine, demonstrates the characteristic features of rib anatomy. The head bears two articular facets that articulate with the bodies of two adjacent vertebrae. The neck connects the head to the tubercle. The tubercle itself has two parts: an articular part that articulates with the transverse process of the corresponding vertebra, and a non-articular part for ligament attachment. At the angle, the rib turns anterolaterally, marking the point of greatest curvature. The shaft or body forms the main curved portion of the rib. The costal groove runs along the inferior internal surface of the shaft and provides protection for the intercostal neurovascular bundle. The costovertebral joints include two distinct articulations. The joint of the head of the rib is a synovial plane joint where the two facets on the rib head articulate with the superior costal facet of the same-numbered vertebra and the inferior costal facet of the vertebra above. An intra-articular ligament divides this joint into two compartments. The costotransverse joint, another synovial plane joint, forms where the tubercle of the rib articulates with the transverse process of the corresponding vertebra. Only ribs one through ten possess costotransverse joints, as ribs eleven and twelve lack the necessary articular surface on their tubercles. Panel A: Head of typical rib (rib 5) with two articular facets (superior and inferior) in orange, and neck as narrow connecting segment to tubercle with articular and non-articular portions. Panel B: Angle marked with dotted line at point of greatest curvature and body/shaft forming long curved segment of the rib. Panel C: Costal groove on inferior internal surface shown in cross-section inset with neurovascular bundle (VAN) in position between internal and innermost intercostal muscles. Panel D: Separate views demonstrating articulation of rib head with two vertebral bodies and tubercle with transverse process at costovertebral joints. --- #### Atypical Ribs Several ribs deviate from the typical pattern and require separate consideration. The first rib is the shortest, broadest, and most sharply curved of all ribs. Its head bears only a single articular facet because it articulates solely with T1. The superior surface features the scalene tubercle for attachment of the scalenus anterior muscle. Grooves for the subclavian vein anteriorly and subclavian artery posteriorly cross the superior surface. Notably, the first rib lacks a costal groove. The second rib is longer than the first and has two articular facets on its head. A tuberosity on its outer surface provides attachment for the serratus anterior muscle. Ribs ten through twelve possess only a single articular facet on their heads, as each articulates with only one vertebra. Ribs eleven and twelve lack both a tubercle and a neck, and as floating ribs, they have no costotransverse articulation or anterior attachment. Panel A: First rib from superior view showing short, flat, broad shape with single facet on head and scalene tubercle labeled. Panel B: First rib grooves for subclavian vein anteriorly and subclavian artery posteriorly marked with vessel silhouettes, no costal groove noted. Panel C: Second rib with serratus anterior tuberosity highlighted on external surface and two articular facets on head. Panel D: Ribs 11-12 showing single head facet, absent tubercle and neck, and free-floating anterior ends as vertebral (floating) ribs. --- ### IV. Muscles of the Thoracic Wall #### Intercostal Muscles The intercostal muscles occupy the intercostal spaces in three layers. The external intercostals form the most superficial layer. They extend from the tubercles of the ribs posteriorly to the costochondral junctions anteriorly, where the muscle fibers are replaced by the external intercostal membrane. The fibers run downward and forward, in the direction of hands placed in pockets. These muscles elevate the ribs during inspiration and receive innervation from the intercostal nerves. The internal intercostals lie deep to the external layer. They extend from the sternum anteriorly to the angles of the ribs posteriorly, where the internal intercostal membrane continues the layer. The fibers run downward and backward, perpendicular to the external intercostals. These muscles depress the ribs during forced expiration and are also innervated by the intercostal nerves. The innermost intercostals form the deepest layer, separated from the internal intercostals by the intercostal neurovascular bundle. Their fibers parallel those of the internal intercostals. This layer is inconsistent and incomplete across the thoracic wall. Additional muscles of the thoracic wall include the transversus thoracis on the internal surface of the anterior chest wall, running from the posterior lower sternum to costal cartilages two through six and depressing the ribs. The subcostalis muscles span two to three intercostal spaces posteriorly and assist with rib depression. The levatores costarum extend from the transverse processes of C7 through T11 to the ribs below and elevate the ribs. The serratus posterior superior originates from the spinous processes of C7 through T3 and inserts on ribs two through five, acting to elevate the ribs. The serratus posterior inferior spans from T11 through L2 to ribs nine through twelve and depresses the ribs. Panel A: External intercostals as superficial layer with fibers angled anteroinferiorly like "hands in pockets," extending from rib tubercles to costochondral junctions. Panel B: Internal intercostals as middle layer with fibers angled posteroinferiorly perpendicular to external, extending from sternum to rib angles. Panel C: Innermost intercostals as deep incomplete layer with fibers paralleling internal intercostals, intercostal neurovascular bundle positioned between internal and innermost layers. Panel D: Membrane continuations shown at anterior (external intercostal membrane) and posterior (internal intercostal membrane) limits with fiber direction arrows for each layer. --- ### V. The Intercostal Space Each intercostal space contains three muscle layers and the intercostal neurovascular bundle. The bundle consists of the intercostal vein, artery, and nerve, arranged in that order from superior to inferior, easily remembered by the mnemonic VAN. The neurovascular bundle runs in the costal groove along the inferior margin of the superior rib, protected by the groove and positioned between the internal and innermost intercostal muscles. This placement within the costal groove leaves only the superior portion of each intercostal space relatively free of major vessels and nerves. Collateral branches of smaller caliber run near the superior border of the lower rib. This anatomical arrangement has critical clinical importance. Procedures that penetrate the intercostal space, such as thoracentesis and chest tube insertion, must enter immediately above the rib to avoid the neurovascular bundle running in the costal groove above. The safe zone for needle insertion lies along the superior border of the lower rib defining each intercostal space. Panel A: Cross-section through intercostal space showing all three muscle layers (external, internal, innermost) with their relative positions. Panel B: Neurovascular bundle within costal groove of upper rib with vein (blue, superior), artery (red, middle), and nerve (yellow, inferior) in VAN arrangement. Panel C: Collateral branches of smaller caliber near superior border of lower rib. Panel D: Highlighted safe zone for needle insertion above lower rib with directional arrow indicating correct approach for thoracentesis and chest tube placement. --- ### VI. Intercostal Nerves and Vessels #### Intercostal Nerves The intercostal nerves represent the ventral rami of spinal nerves T1 through T11. The twelfth thoracic nerve runs below the twelfth rib and is called the subcostal nerve. Each intercostal nerve exits its intervertebral foramen and initially passes between the parietal pleura and the internal intercostal membrane posteriorly. The nerve then enters the costal groove and travels between the internal and innermost intercostal muscle layers. At approximately the midaxillary line, the nerve gives off a lateral cutaneous branch that pierces the overlying muscles to supply skin. The nerve continues anteriorly and terminates as an anterior cutaneous branch near the sternum. Motor fibers supply the intercostal muscles while sensory fibers supply the skin of the thoracic and abdominal walls. Importantly, the lower intercostal nerves, T7 through T12, continue beyond the costal margin to innervate the abdominal wall. Key dermatomal landmarks include T4 at the nipple level and T10 at the umbilicus. #### Intercostal Arteries The posterior intercostal arteries have different origins depending on their level. The first and second intercostal spaces receive posterior intercostal arteries from the superior intercostal artery, a branch of the costocervical trunk. Intercostal spaces three through eleven receive their posterior intercostal arteries directly from the thoracic aorta. The subcostal artery, also from the thoracic aorta, supplies the region below the twelfth rib. The anterior intercostal arteries arise from different sources as well. The upper six intercostal spaces receive anterior intercostal arteries from the internal thoracic artery. Intercostal spaces seven through nine receive branches from the musculophrenic artery. The lowest intercostal spaces, ten and eleven, lack anterior intercostal arteries. The anterior and posterior intercostal arteries anastomose within each intercostal space, ensuring continuous blood supply. #### Intercostal Veins The intercostal veins follow the arteries. Posterior intercostal veins drain into the azygos and hemiazygos venous system on the right and left sides respectively. Anterior intercostal veins drain into the internal thoracic veins. Panel A: Posterior intercostal arteries arising from aorta (spaces 3-11) and superior intercostal artery (spaces 1-2) with branches labeled. Panel B: Anterior intercostal arteries from internal thoracic artery (spaces 1-6) and musculophrenic artery (spaces 7-9) with anastomotic connections between anterior and posterior systems. Panel C: Intercostal nerve course from intervertebral foramen through costal groove with lateral cutaneous branch at midaxillary line and anterior cutaneous branch near sternum marked. Panel D: Venous drainage pattern to azygos system posteriorly and internal thoracic veins anteriorly with dermatomal landmarks (T4 nipple, T10 umbilicus). --- ### VII. The Diaphragm The diaphragm is a dome-shaped musculotendinous structure that separates the thoracic and abdominal cavities. It serves as the primary muscle of respiration. The peripheral muscular portion has three origins. The sternal origin arises from the posterior surface of the xiphoid process. The costal origin arises from the internal surfaces of the lower six ribs and their costal cartilages, interdigitating with the transversus abdominis muscle. The lumbar origin involves the right and left crura arising from the lumbar vertebral bodies, plus the median and medial arcuate ligaments and the lateral arcuate ligaments. All muscle fibers converge on the central tendon, a trefoil or clover-leaf shaped aponeurosis with no bony attachment. The right dome of the diaphragm rises higher than the left due to the presence of the liver below. The right crus is larger than the left, originating from the bodies of vertebrae L1 through L3. It encircles the esophageal hiatus. The left crus is smaller, arising from L1 and L2. The arcuate ligaments represent thickenings of fascia that span muscle origins. The median arcuate ligament bridges from the right crus to the left crus, arching over the aorta. The medial arcuate ligament extends from the body of L1 or L2 to the transverse process of L1, arching over the psoas major muscle. The lateral arcuate ligament spans from the transverse process of L1 to the twelfth rib, arching over the quadratus lumborum muscle. Panel A: Inferior view of diaphragm showing central tendon (trefoil-shaped, white-cream) and muscular portions arising from sternal, costal, and lumbar origins each color-coded. Panel B: Right crus (larger, extending to L3) and left crus (smaller, to L2) with median arcuate ligament bridging crura and aorta passing beneath. Panel C: Medial arcuate ligament arching over psoas major and lateral arcuate ligament arching over quadratus lumborum with muscle outlines. Panel D: Transparent overlay showing liver pushing right dome higher than left, with phrenic nerve innervation (C3, C4, C5) indicated. --- #### Diaphragmatic Openings Three major openings pass through or behind the diaphragm, each at a specific vertebral level. The caval opening lies at the T8 level within the central tendon. It transmits the inferior vena cava and branches of the right phrenic nerve. Because this opening is within the tendon rather than muscle, the opening actually dilates during inspiration when the diaphragm contracts and descends, facilitating venous return. The esophageal hiatus lies at the T10 level within the muscular right crus. It transmits the esophagus, the vagal trunks arranged as anterior and posterior vagal nerves, and esophageal vessels. The muscular surround provides a sphincter-like action, though this is not a true anatomical sphincter. The aortic hiatus lies at the T12 level, actually behind rather than through the diaphragm, between the two crura and the vertebral column. It transmits the aorta, the azygos vein, and the thoracic duct. A useful mnemonic for the vertebral levels is "I 8 10 EGGs At 12," representing IVC at T8, Esophagus at T10, and Aorta at T12. Other structures traverse the diaphragm through additional passages. The greater and lesser splanchnic nerves pass through the crura. The sympathetic trunks pass behind the medial arcuate ligament. The subcostal vessels and nerve pass behind the lateral arcuate ligament. The diaphragm receives its motor innervation entirely from the phrenic nerve, derived from cervical roots C3, C4, and C5. The mnemonic "C3, 4, 5 keeps the diaphragm alive" emphasizes this important relationship. Sensory innervation divides between the phrenic nerve for the central portion and the intercostal nerves T6 through T12 for the peripheral muscular portion. When the diaphragm contracts, it flattens and descends, increasing the vertical dimension of the thoracic cavity and generating the negative intrathoracic pressure that draws air into the lungs during inspiration. Relaxation allows the dome to rise, decreasing thoracic volume and producing passive expiration. The diaphragm also increases intra-abdominal pressure during activities such as defecation, urination, and parturition. Panel A: Caval opening at T8 in central tendon transmitting IVC (blue) and phrenic nerve branches, dilating during inspiration. Panel B: Esophageal hiatus at T10 in right crus transmitting esophagus, vagal trunks (anterior and posterior), and esophageal vessels with sphincter-like muscular action. Panel C: Aortic hiatus at T12 behind diaphragm between crura transmitting aorta (red), azygos vein, and thoracic duct. Panel D: Additional passages showing splanchnic nerves through crura, sympathetic trunks behind medial arcuate ligament, and subcostal neurovascular bundle behind lateral arcuate ligament with vertebral level reference bar. --- ### VIII. Mechanics of Respiration Inspiration involves active muscular contraction that increases thoracic volume and decreases intrathoracic pressure, drawing air into the lungs. The diaphragm contracts and descends, flattening from its domed resting position and increasing the vertical dimension of the thorax. Simultaneously, the external intercostal muscles contract and elevate the ribs through two characteristic movements. The pump handle movement primarily affects the upper ribs, one through six. As these ribs elevate, the sternum moves anteriorly and superiorly, increasing the anteroposterior diameter of the thorax. The bucket handle movement primarily affects the lower ribs, seven through ten. As these ribs elevate, their lateral aspects rise, increasing the transverse diameter of the thorax. Accessory muscles of inspiration become active during forced or labored breathing. These include the scalene muscles and sternocleidomastoid in the neck, and pectoralis minor and serratus anterior when the upper limb is fixed. Quiet expiration is passive, requiring no muscular effort. Elastic recoil of the lung tissue and relaxation of the inspiratory muscles allows the thoracic cavity to decrease in size and air to flow out. Forced expiration requires active muscular contraction. The internal intercostal muscles contract to depress the ribs. The abdominal wall muscles contract to increase intra-abdominal pressure, forcing the relaxed diaphragm superiorly and decreasing thoracic volume. Panel A: Pump handle movement showing sternum swinging anterosuperiorly (ribs 1-6) with curved arrow indicating motion and resulting anteroposterior diameter increase marked. Panel B: Bucket handle movement showing lateral rib elevation (ribs 7-10) with bilateral curved arrows and resulting transverse diameter increase marked. Panel C: Diaphragm descent during inspiration with before and after silhouettes showing flattening and vertical dimension increase of thorax. Panel D: Volume change summary with airflow direction arrows showing negative intrathoracic pressure during inspiration and passive recoil during expiration. --- ### IX. Clinical Correlations Rib fractures commonly result from thoracic trauma. The middle ribs, four through nine, are most frequently fractured because the upper ribs are protected by the shoulder girdle and the lower ribs are more mobile. Flail chest represents a severe injury where multiple sequential rib fractures create a free-floating segment of chest wall that moves paradoxically during respiration, moving inward during inspiration and outward during expiration, compromising ventilation. Intercostal nerve blocks provide analgesia for rib fractures and thoracic surgical procedures. Local anesthetic is injected at the angle of the rib where the nerve lies within the costal groove behind the posterior intercostal membrane. Multiple levels typically require injection because of the overlapping innervation pattern. Thoracentesis and chest tube placement require knowledge of intercostal anatomy to avoid the neurovascular bundle. The needle or tube must be inserted immediately above the lower rib defining the intercostal space. The typical approach uses the fourth through sixth intercostal space at the midaxillary line for chest tubes. The safe triangle for chest tube insertion is bounded anteriorly by the lateral border of pectoralis major, posteriorly by the anterior border of latissimus dorsi, and inferiorly by a horizontal line at the level of the fifth rib. Phrenic nerve injury causes paralysis of the ipsilateral hemidiaphragm. The affected dome rises on imaging because it no longer actively contracts. The sniff test, performed under fluoroscopy, demonstrates paradoxical movement where the paralyzed hemidiaphragm rises during sniffing when the intact side descends. Causes include trauma, surgical injury during thoracic or cardiac surgery, and compression from tumors, particularly lung cancer invading the mediastinum. Diaphragmatic hernias allow abdominal contents to enter the thoracic cavity. Congenital hernias include the Bochdalek hernia through a posterolateral defect and the Morgagni hernia through a retrosternal defect. Acquired hernias result from trauma or hiatal herniation where the stomach passes through an enlarged esophageal hiatus. Herniated abdominal contents in the thorax compromise pulmonary expansion and can cause significant respiratory distress. Panel A: Rib fracture showing fracture pattern in ribs 4-9 and flail chest segment with paradoxical motion arrows during inspiration and expiration. Panel B: Proper chest tube insertion site above rib with safe triangle boundaries outlined (pectoralis major, latissimus dorsi, fifth rib line). Panel C: Elevated hemidiaphragm on chest X-ray from phrenic nerve palsy with sniff test demonstration showing paradoxical upward movement. Panel D: Bochdalek diaphragmatic hernia with bowel loops visible in left hemithorax and compressed lung, congenital posterolateral defect indicated. --- ## Summary - The thoracic cage consists of twelve thoracic vertebrae posteriorly, twelve pairs of ribs with costal cartilages laterally, and the sternum anteriorly - Ribs one through seven are true ribs with direct sternal attachment; ribs eight through ten are false ribs attaching to the rib above; ribs eleven and twelve are floating ribs with no anterior attachment - Typical ribs possess a head, neck, tubercle, angle, body, and costal groove - The intercostal muscles include the external layer for inspiration, the internal layer for forced expiration, and the innermost layer - The neurovascular bundle follows the VAN order and runs in the costal groove; procedures must enter above the lower rib to avoid injury - The diaphragm is the primary muscle of respiration, innervated by the phrenic nerve from C3, C4, and C5 - The three major diaphragmatic openings transmit the IVC at T8, the esophagus at T10, and the aorta at T12 - Inspiration occurs when the diaphragm descends and ribs elevate, increasing thoracic volume and creating negative intrathoracic pressure --- ## Key Terms | Term | Definition | |------|------------| | Sternal angle | Junction of manubrium and body at T4/T5 level; landmark for the second rib | | Costal groove | Inferior surface of rib protecting the intercostal neurovascular bundle | | Phrenic nerve | Derived from C3, C4, C5; provides motor and central sensory innervation to diaphragm | | Pump handle | Movement where sternum elevates anterosuperiorly, increasing anteroposterior diameter | | Bucket handle | Movement where lateral ribs elevate, increasing transverse thoracic diameter | | Flail chest | Free-floating chest wall segment from multiple rib fractures causing paradoxical motion | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*