Thorax Anatomy: Ribs, Sternum, and the Mechanics of Breathing

The video provides a concise tour of the thorax, focusing on the rib cage, sternum, and the joints that connect them. It explains how true, false and floating ribs differ, what makes a rib typical or atypical, and how the sternum anchors the chest wall. It also covers intercostal spaces, the superior and inferior thoracic apertures, and the key movements that drive breathing, including pump handle and bucket handle mechanics. By the end, viewers gain a clear map of the thoracic skeleton and its role in changing thoracic volume during respiration.

• Ribs are classified as true, false, and floating, with true ribs attaching directly to the sternum.
• The sternum consists of the manubrium, body, and xiphoid process with landmarks like the jugular notch and the sternal angle.
• Intercostal spaces are named for the rib forming the superior border of the space.
• Breathing involves the diaphragm descending to increase vertical dimension and rib movements that expand the AP and transverse dimensions.
• Costovertebral and costotransverse joints facilitate rib movements during respiration.

Introduction: Why the Thorax Matters

The thorax is the region between the neck and abdomen that houses the thoracic cavity and its contents, protected by a robust osteocartilaginous cage. The thoracic skeleton includes the sternum, 12 pairs of ribs with costal cartilages, and 12 thoracic vertebrae with intervertebral discs. The ribs and costal cartilages form the bulk of the thoracic cage, which can be identified numerically from 1 to 12 for ribs and 1 to 10 for costal cartilages, with ribs 11 and 12 lacking anterior cartilages. The true thoracic wall comprises the rib cage plus the muscles and fascia that cover its anterolateral aspect, while the posterior structures belong to the back. The mammary glands lie within this region as well.

"The thoracic cage forms a complete wall peripherally, but it's open superiorly and inferiorly." - Presenter

Anatomy of the Thoracic Cage

The thorax houses the thoracic cavity and its viscera, protected by the thoracic cage, which is built from the sternum, 12 ribs, costal cartilages, and the thoracic spine. The ribs are curved, flat bones with a spongy interior that contains hematopoietic marrow. They are broadly categorized by their connection to the sternum, giving rise to true ribs (ribs 1–7), false ribs (8–10), and floating ribs (11–12). This classification reflects how each rib attaches to the sternum, either directly via its own cartilage or indirectly through cartilage of the rib above. The first seven pairs attach directly to the sternum, while the 8th to 10th attach to the cartilage above, and the 11th and 12th do not connect anteriorly at all and end in the posterior abdominal musculature.

"There are three types of ribs based on their connection to the true, false and floating ribs." - Presenter

Rib Classification: True, False, and Floating

Ribs are also classified as typical or atypical. Typical ribs are the third through ninth and have standard head, neck, tubercle, and body features on the posterior view, whereas atypical ribs (notably the first, second, and the 10th to 12th) differ in their articulation and surface anatomy. The first rib is the broadest, shortest, and most sharply curved of the seven true ribs, with a nearly horizontal body and a single facet that articulates with T1. Its superior surface bears two transverse grooves for the subclavian vessels, separated by a scalene tubercle to which the anterior scalene attaches. The second rib is longer and less curved, featuring the tuberosity for serratus anterior where the muscle originates, and its head has facets for T1 and T2. The 10th through 12th ribs are atypical, with 11th and 12th ribs being particularly short and lacking a neck or tubercle, articulating with a single vertebral body at the same level as the rib, similar to rib 1 in this regard.

"The head of a typical rib is wedge shaped and has two facets superior and inferior that are separated by the crest of the head." - Presenter

Typical versus Atypical Ribs: Details

The head of a typical rib articulates with two vertebral bodies via the superior costal facet of the vertebral body of the same number and with the inferior costal facet of the vertebra above it, with the intervertebral disc between them. The tubercle of the rib articulates with the transverse process of the vertebra of the same number for ribs 1–10; ribs 11 and 12 lack such articulation. These joints are primarily synovial plane joints, which permit gliding movements that facilitate rib motion during respiration. Some atypical ribs have unique articulations or surfaces that reflect their distinct functional roles, such as the subclavian groove on the first rib for vessels and the serratus anterior origin on the second rib.

"The costa transverse joints, which are also synovial plane joints." - Presenter

The Sternum and Its Joints

The sternum is a flat, elongated bone forming the center of the anterior thoracic wall and protecting mediastinal viscera. It comprises three parts: the manubrium, the body, and the xiphoid process. The manubrium features the jugular notch (suprasternal notch) and clavicular notches. The sternoclavicular joint is a saddle-type synovial joint between the clavicles and the manubrium, with additional articulation to the first costal cartilages. The first sternocostal joint is primary cartilaginous, while the manubriosternal joint is secondary cartilaginous, and the xiphoid process is connected to the body by the xiphisternal joint, a primary cartilaginous joint. The sternal angle, or angle of Lewy, marks the junction between the manubrium and body and lies at the level of T4–T5 intervertebral disc, aligning with the second rib cartilage. It is palpable and often visible in young individuals and serves as a vital thoracic landmark.

"The sternal angle lies at the level of the T4, T5 intervertebral disc and marks the level of the second pair of costal cartilages." - Presenter

Intercostal Spaces and Thoracic Apertures

The thoracic cage is segmented into 11 intercostal spaces, each named for the rib that forms the superior border of the space. The 12th rib forms the subcostal space rather than an intercostal space. The thorax has two important openings: the superior thoracic aperture and the inferior thoracic aperture. The superior aperture is bounded by T1 posteriorly, the first pair of ribs laterally, and the superior border of the manubrium anteriorly. It allows passage for structures that serve the head, neck, and upper limbs. The inferior aperture is bounded posteriorly by T12, with the 11th and 12th ribs forming posterolateral boundaries, and the costal margins anteriorly. The diaphragm closes this opening, separating the thoracic and abdominal cavities. Structures pass through the diaphragm or posterior to it to connect the thorax and abdomen, including the esophagus, inferior vena cava, and the aorta.

"There are 11 intercostal spaces that separate the ribs and their costal cartilages from one another." - Presenter

Respiratory Mechanics: How the Thorax Expands During Breathing

Respiration relies on coordinated changes in thoracic volume. During expiration, muscles relax and intrathoracic pressure rises, expelling air. During inspiration, the vertical dimension of the thoracic cavity increases as the diaphragm contracts and moves downward into the abdominal cavity. The AP (anterior-posterior) dimension expands via pump handle movements, caused by rib 1–7 moving around the neck of the ribs, raising the anterior (sternal) ends. The transverse dimension increases through bucket handle movements, driven by the 8th–10th ribs lifting the middle and lateral portions of the lower ribs. These gliding joints (costovertebral and costotransverse) enable these movements, permitting the rib cage to expand anteriorly, superiorly, and laterally to accommodate air inflow during inspiration.

"During inspiration, the vertical dimension of the thoracic cavity increases as contraction of the diaphragm causes it to descend." - Presenter

Recap: Putting It All Together

In summary, the thoracic wall comprises the rib cage, the intercostal muscles, and the surrounding fascia, with true ribs directly attaching to the sternum and false ribs attaching indirectly via cartilage. The sternum anchors the anterior chest wall with landmarks like the jugular notch and the sternal angle, which aligns with the second pair of ribs. The intercostal spaces, borders, and joints coordinate to permit respiratory expansion. Together with the diaphragm, these structures modulate intrathoracic volume and pressure, enabling efficient breathing. Mastery of these landmarks supports clinicians in interpreting imaging, planning procedures, and understanding respiratory mechanics in health and disease.