Thorax, Bones of

Definition

The skeleton of the thorax or chest is a cage that encloses and protects the main organs of respiration and circulation. It has a conical shape, being narrower at the top and broader at the bottom, and longer behind than in front. It consists of the sternum and the ribs.

Description

The bones of the thorax include the sternum, commonly called the breastbone, and the ribs.

The sternum is a narrow, elongated, flattened bone that forms the center of the front of the chest. It consists of three parts: an upper section called the manubrium, a middle section called the body, and a lower section called the xiphoid process that projects down. The junction of the manubrium and body is called the sternal angle. In early life, the xiphoid process is not a bone, but a piece of cartilage. Cartilage is a type of connective tissue containing collagen, a protein substance that forms tough and elastic fibers. It is a softer and more flexible material than bone. As the child grows, the xiphoid process slowly hardens into bone and by adulthood, it has become fused to the body of the sternum. The sides of both manubrium and body are notched so as to attach to seven costal cartilages. These are strips of strong cartilage that prolong into ribs and provide elasticity to the thorax. The upper section of the sternum supports the clavicles (shoulder blades). It contains a notch called the clavicular notch that allows it to articulate with the clavicle. The average length of the adult sternum is about 6.7 in (17 cm), and it is usually somewhat longer in the male than in the female.

The ribs are flexible, long bones that look like arches, and they form a large part of the thoracic skeleton. There are 12 ribs on each side and they are located one below the other in such a way that spaces called inter-costal spaces occur between them. The first seven (1-7) are called the true ribs or the vertebro-sternal ribs. They connect in the back to the vertebral column, and in front to the sternum, through the costal cartilages. The following three ribs (8-10) are called the false ribs or the vertebro-chondral ribs. These ribs have their costal cartilages attached to the cartilage of the true rib above. The last two ribs (11-12) are only attached to the vertebral column and are thus called the floating or vertebral ribs.

All ribs have many structural features in common:

• Head. The head of the rib is the flat surface that connects with the vertebrae in the vertebral column.
• Neck. The neck of the rib is a flattened section that has a length of about 1 in (2.5 cm). It is located between the head and the tubercle. Its inferior surface is flat and smooth and its superior surface is rough for the attachment of ligaments.
• Tubercle. The tubercle is a bony eminence, or growth, that comes right after the neck of the rib. It has two sections, one that serves as a point of attachment to the vertebrae and another that attaches to ligaments.
• Body. The body—or shaft—is the longest part of a typical rib.
• Angle. The angle is the point at which the body of the rib starts to curve, just after the tubercle.
• Costal groove. The costal groove is located on the inner surface of the body of a rib. It provides a seat and protection for the intercostal nerve bundle.

Ribs present some degree of variability. For example, they vary in their angle, the upper ribs being less oblique than the lower. Characteristic features of some special ribs include:

• Rib 1. The first rib is the most curved of all ribs; it is also the broadest, shortest and widest rib. The head is small, rounded, and only has a single bony projection for articulation with the first thoracic vertebra.
• Rib 2. The second rib is much longer than the first, and its body is not flat like that of the other ribs. It has a rough section near its angle for attachment of a large back muscle and also attaches to the sternal angle of the sternum.
• Rib 10. This rib only has one point of attachment to the vertebrae.

The upper opening of the thorax is broader from side to side than from front to back. It is formed by the first thoracic vertebra in the back, the upper section of the sternum in front, and the first rib on either side. It slopes downward and forward, so that the front part of the open ing is on a lower level than the back part. The lower opening of the thorax is formed by the twelfth thoracic vertebra in the back, by the eleventh and twelfth ribs on the sides, and by the costal cartilages of the tenth, ninth, eighth, and seventh ribs in the front. The lower opening is closed by the diaphragm, the thin muscle located below the lungs and heart, that forms the floor of the thorax.

Function

The major function of the thorax bones is to form the thoracic cavity that encloses and protects the most important organs of the circulatory and respiratory systems, the heart and lungs. The rib cage has a very special function—it allows breathing to take place, which occurs as a result of the rib cage moving up and down as air is inhaled and exhaled.

Role in human health

Besides its role in protecting major organs and in breathing, the thorax also provides a structural frame for the attachment of the trunk muscles, which are needed for movement. Thus, it also plays a role in body locomotion.

Common diseases and disorders

Injuries to the bony structures of the thorax are very serious because of the relationship of the thorax to the spine as a whole and because of the importance of the major respiratory and circulatory organs that the thoracic cavity contains. For example, broken ribs can cause disease by mechanical interference with internal organs, irritation of surrounding soft tissues, straining ligaments, impinging nerves, or blocking blood vessels. Likewise, the sternum is a very strong bone and requires great force to fracture. But the main danger in this type of injury is not so much the fracture itself, but the risk that the broken bone may be driven into the heart, which lies just behind it. Some thoracic diseases and disorders include:

• Asphyxiating thoracic dystrophy. Also known as Jeune's syndrome, this is a form of dwarfism characterized by an abnormally long and narrow thorax with a reduced thoracic cage capacity that results in the lungs not having enough room for respiration to occur.
• Chondrosarcoma. Chondrosarcoma is a cancer that can arise in the costal cartilage of the ribs.
• Costochondritis. Also called Tietze's syndrome, it is an inflammation of the costochondral or costosternal joints that causes localized pain and tenderness. Any of the seven rib junctions may be affected, and more than one site is affected in 90% of cases.
• Luxation of ribs. A luxation is a sprain of a rib. It is the result of twisting a rib about its head in such a way that the rib departs from its normal conformation.
• Pleurisy. Pleurisy is an inflammation of the membrane that covers the inside of the thorax.
• Thorax hematoma. This is bruising due to the breaking of blood vessels that results in a localized accumulation of blood.

KEY TERMS

Cartilage—Connective tissue containing collagen, the protein substance that forms tough and flexible fibers. Cartilage is more flexible and compressible than bone and often serves as a bone precursor, becoming mineralized as the body ages.

Costal cartilages—Cartilage which prolongs the ribs forward and connects each rib to the sternum.

Diaphragm—The thin muscle located below the lungs and heart that separates the chest from the abdomen.

False ribs—The three ribs, 8-10, that attach to the costal cartilage of the seventh true rib.

Floating ribs—The two last ribs, 11-12, that are not attached to the sternum. Also called the vertebral ribs.

Manubrium—The upper section of the sternum, it articulates with the shoulder blades and connects to the first seven ribs.

Ribs—The long, elastic bones resembling arches that are part of the thoracic skeleton. There are 12 ribs on either side of the thorax.

Sternum—One of the bones of the thorax, located in front of the chest. It has three sections: the manubrium, the body, and the xiphoid process.

Thorax—The bones that surround and form the chest cavity. The thorax includes the sternum and the ribs.

True ribs—The first seven ribs, 1-7, directly attached to the sternum.

Vertebra—Flat bones that make up the vertebral column. The spine has 33 vertebrae.

BOOKS

Putz, R., et al, eds. Sobotta Atlas of Human Anatomy: Thorax, Abdomen, Pelvis, Lower Limb. 12th English ed. Philadelphia: Lippincott, Williams & Wilkins, 1997.

Roussos, C. The Thorax. New York: Marcel Dekker Inc., 1995.

Simon, Seymour. Bones: Our Skeletal System. New York: Morrow (Harper-Collins), 1998.

OTHER

"The Thorax." Bartleby.com edition of Gray's Anatomy of the Human Body. <http://www.bartleby.com/107/26.html>.

Monique Laberge, PhD