Last Updated on March 18, 2025
The thoracic cage or rib cage is the skeletal framework that encloses the contents of the thorax. The thoracic cage or rib cage along with its contents form the thorax portion of the body. Roughly speaking, this is the area of the chest.
The thoracic cage consists of 12 pairs of ribs with their costal cartilages and the sternum. The ribs are uniquely arranged to maximize the thoracic cavity volume and anchor posteriorly to the 12 thoracic vertebrae (T1–T12).
The thorax contains organs of respiration [lungs] and circulation [heart]. The functioning of both of these is very vital for life. The thoracic cage protects the heart and lungs and assists the functions of respiration and circulation by increasing and decreasing thoracic cavity volumes.
Structure of Thoracic Cage
The thoracic cage is formed by bones and cartilage [osseocartilaginous]. It is an elastic cage that is primarily designed for increasing and decreasing the intrathoracic pressure so that air is sucked into the lungs during inspiration and expelled during expiration.
The thoracic cage is formed anteriorly by the sternum, posteriorly by the 12 thoracic vertebrae, and the intervening intervertebral discs on each side. The ribs on both sides complete the cage.
Each rib articulates posteriorly with the vertebral column.
The thorax resembles a truncated cone which is narrow above and broad below. The narrow upper end is continuous with the root of the neck from which it is partly separated by the suprapleural membrane or Sibson’s fascia [a covering on the lung].
The diaphragm almost completely separates the broad lower end from the abdomen.
In the transverse section, the thorax is bean-shaped or kidney-shaped except in infants below the age of two years, where it is circular.
Bones of Thoracic Cage
Sternum
The sternum is the anterior anchor of the thoracic cage. It is an elongated and fat bone that consists of the following part
- manubrium
- body
- xiphoid process
The manubrium is the wide superior part of the sternum. It has a shallow, U-shaped upper border called the jugular or suprasternal notch that can be easily palpated at the neck base, between two medial ends of clavicle.
On either side at the superolateral margin, the manubrium has a notch for the clavicle. It is called the clavicular notch and the site of the sternoclavicular joint. The first ribs also attach to the manubrium on either site.
The body of the sternum refers to its elongated central part of the sternum. The manubrium and body join together at an angle, called the sternal angle
The second rib attaches to the sternum at the sternal angle. This rib is the first to be palpated on examination as the first rib is behind the clavicle.
The sternal angle and second rib are important landmarks for the counting of ribs.
The sternal body gives attachment to the costal cartilages of the third to seventh ribs.
The xiphoid process is the inferior tip of the sternum. It is a cartilaginous structure in early life but becomes ossified gradually with aging.
Ribs
There are a total of 12 pairs of ribs in the thoracic cage.
Each rib is a curved, flattened bone that articulates posteriorly with the thoracic vertebrae and attaches anteriorly to the sternum via their costal cartilages.
Beginning at the top, the ribs are numbered 1–12 in accordance with their articulation with the thoracic vertebrae.
Though ribs are bony structures, anteriorly, each rib ends in costal cartilage which varies in dimensions for different ribs. This costal cartilage then attaches to the sternum.
There are two main types of ribs – true ribs and false ribs depending on articulation with the sternum.
- True ribs are ribs that articulate with the sternum via individual costal cartilage. Each of these ribs articulates with a vertebra on the posterior side and sternum on the anterior side via costal cartilage These are also called vertebrosternal ribs. The first 7 ribs are true ribs
- False ribs do not articulate with the sternum. Anteriorly, these join the costal cartilage of a superior rib. Such ribs are called vertebrochondral ribs. Ribs 8-10 are false ribs. The costal cartilages of the 7th, 8th, 9th, and 10th ribs form the costal margin that can be palpated anteriorly.
- Floating ribs are false ribs that do not attach to the sternum at all, Their anterior ends are free and disappear in the muscles of the lateral abdominal wall. These are also called vertebral ribs. The 11th and 12th ribs are floating ribs.
A rib can be divided into the following parts
- Head: The head of the rib forms the posterior end of a rib and articulates with the costal facet located on the body of the same numbered thoracic vertebra and that of the next higher vertebra.
- Neck: It is a narrowed part next to the head and anatomically in a lateral position to the head.
- Tubercle: It is a small bump on the posterior surface of the rib and articulates with the facet on the transverse process of the same numbered vertebra.
- Shaft: Beyond the neck, as we move anteriorly is the shaft of the rib. The angle of the rib is the point of the greatest degree of curvature and is lateral to the tubercle.
The angles of the ribs form the most posterior limit of the thoracic cage and are in line with the medial border of the scapula.
The inferior margin of the rib has a groove for the passage of blood vessels and nerves.
The ribs of infants are horizontal. Therefore respiration is only by action of the diaphragm. In adults the thorax is oval. The ribs are oblique and their movements alternately increase and decrease the diameters of the thorax. This results in the drawing of air into the thorax (inspiration) and its expulsion (expiration). This is called thoracic respiration. Adults have both diaphragmatic and thoracic respiration.
Joints of Thoracic Cage
Manubriosternal Joint
This joint is between the manubrium and sternum and is a type of secondary cartilaginous joint. There is only slight movement possible in this joint. The joint is fused in about 10 percent people.
- Only slight movements of the body of the sternum on the manubrium during respiration.
- Fused in 10%
Costovertebral Joints
The costovertebral joint is between the head of a typical rib and two vertebrae to form two-plane synovial joints. One vertebra is of the same level as that of the rib whereas the other is the body of the next higher vertebra. The articulation on the vertebra is formed by the superior demifacet of the corresponding vertebra and the inferior demifacet of the higher vertebra. A demifacet is an incomplete facet present on the thoracic vertebra for articulation with the rib head.
Costotransverse Joints
The costotransverse joint is between the tubercle of the rib and the transverse process of the corresponding vertebra. The 11th and 12th ribs do not have articulation with the transverse process as these are floating ribs.
Costochondral Joints
The cartilaginous joints are between the sternal end of ribs and the lateral ends of the costal cartilages are called costochondral joints. Each rib has a depression shaped like a cup that the costal cartilage articulates with. There is normally no movement at these joints of the thorax.
Chondrosternal Joints
These are the joints formed between the sternum and costal cartilage of the ribs. The first chondrosternal joint is a primary cartilaginous joint. It does not permit any movement. This helps in the stability of the shoulder girdle and of the upper limb.
The 2nd to 7th costal cartilages articulate with the sternum by these synovial joints. Each joint has a single cavity except in the second joint where the cavity is divided into two parts. The joints of the thorax are held together by the capsular and radiate ligaments.
Interchondral Joints
These are synovial joints between the tips of adjacent costal cartilages of ribs 6-10 between the tips of the costal cartilages. The joint formed by the 9th and 10th costal cartilage is more like a fibrous joint than a synovial one.
Ligaments of Thoracic Cage
- Capsular ligament: Like all synovial joints costotransverse, costovertebral and interchondral joints are surrounded by capsule
- Costotransverse ligaments: There are three costotransverse ligaments
- Superior costotransverse ligament- Extends from the crest on the neck of the rib to the transverse process of the vertebra above
- Inferior costotransverse ligament- From the posterior surface of the neck to the transverse process of its own vertebra.
- Lateral costotransverse ligament- Connects the lateral nonarticular part of the tubercle to the tip of the transverse process.
- Ligament of the head of the rib: The head of the radius forms the costovertebral joint by articulating with demifacets of upper and lower vertebrae and intervening disc. The ligament of the head of the rib usually consists of 3 bands – the upper band, attaching to the upper vertebra; the middle band, connecting the head of a rib with the intervertebral disc; and the lower band, attaching to the lower vertebra.
Muscles of Thoracic Cage
Muscles of the thoracic cage are the intercostals (external, internal, and innermost), subcostals, and transversus thoracis.
All these muscles function to change the volume of the thoracic cavity during respiration.
Some other muscles not part of the thoracic cage but attached to it are the pectoralis major, minor, serratus anterior, and scalene muscles.
Intercostals
These muscles are found in the intercoastal spaces i.e. the spaces between the ribs. These are three types of muscles and are arranged in three layers.
All the intercoastal muscles are supplied by intercostal nerves [T1- T11].
- External Intercostals: These are 11 pairs of muscles that run from the lower border rib above to the upper border of the rib below. The direction of the fibers is inferoanterior. That means downward and forward. Intercostals are continuous with the external oblique of the abdomen. These act to elevate the ribs to increase the thoracic volume.
- Internal Intercostals: These muscles lie deeper to the external intercostals. The extent of the run is the same as that of external intercostals but the direction of the fibers is perpendicular. These run in the posteroinferior direction. They run from the lateral edge of the costal groove to the superior surface of the lower rib. These become continuous with the internal oblique muscle of the abdominal wall. The muscles between the bony part of the ribs decrease the thoracic volume by depressing the ribs whereas the interchondral part elevates the ribs.
- Innermost Intercostals: Deepest intercostal muscles Similar structure to the internal intercostals Originates from the medial edge of the costal groove and inserts into the superior surface of the lower rib. These are separated from the internal intercostals by the intercostal neurovascular bundle. Their action is similar to internal intercostals.
Transversus Thoracis
Transversus thoracic originates from the posterior surface of the inferior sternum and costal cartilage of the lower ribs. These insert into the internal surface of the costal cartilages of 2nd to 6th rib. It is continuous with transversus abdominis inferiorly.
It is a weak depressor of the ribs and is supplied by intercostal nerves (T2-T6).
Subcostals
These are thin muscles found on the inner surface of the posterior thoracic wall. They span 2-3 intercostal spaces. These are mainly found in the inferior portion of the thoracic wall as they are rudimentary or absent in the upper part. Subcostals extend from the inferior surface of the lower ribs, near the angle of the rib to the superior border of the 1-2 ribs below.
Their fibers are parallel to that of the innermost intercostals and action is also similar. These are supplied by intercostal nerves.
Blood Supply
Thoracic wall or thoracic cage is quite rich in the blood supply. The vessels supplying are
- Posterior intercostal arteries which are branches of the subclavian and aorta. These travel under each rib and give off collateral intercostal branches that travel just superior to each rib. These end as communication with anterior intercostal arteries.
- Internal mammary arteries are the main supplier of anterior blood supply via anterior intercostal branches
- axillary artery via its thoracodorsal, lateral thoracic, and thoracoacromial branches provides the blood supply on the lateral wall.
Clinical Significance
Understanding the anatomy of the thoracic cage is important as it is necessary to access the thoracic contents for procedures.
Intercostal nerve block
It is an anesthetic procedure for different surgical procedures and uses drugs to numb the intercostal nerve. Sometimes anesthetizing the the adjacent nerve is necessary because there is considerable overlapping
Sternal Procedures
Sternum is used in thoracic and cardiac surgery. The sternum is split in the median plane to gain access.
The sternum is a commonly used site for bone marrow aspiration
Thoracentesis
It is a procedure to remove excess fluid from the thoracic cavity for both diagnostic and treatment purposes. In the supine position, the needle is placed in the midaxillary line between the 6th and 8th ribs. In a seated upright patient, the needle is placed between the 9th and 10th rib in the mid-scapular line.
References
- Hussain A, Burns B. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): Jul 30, 2023. Anatomy, Thorax, Wall. [PubMed Link]
- Clemens MW, Evans KK, Mardini S, Arnold PG. Introduction to chest wall reconstruction: anatomy and physiology of the chest and indications for chest wall reconstruction. Semin Plast Surg. 2011 Feb;25(1):5-15. [PubMed Link]
