Human rib cage

The human rib cage is a part of the human skeleton within the thoracic area. A typical human ribcage consists of 24 ribs, 12 on each side of the thoracic cavity. (A small proportion of people have one pair more or fewer.) Ribs are attached behind the vertebral column.

Types of ribs

The first seven pairs of ribs are connected to the sternum in front and are known as true ribs (costae verae, I-VII). The eighth, ninth, and tenth are attached in front to the cartilaginous portion of the next rib above and are known as false ribs (costae spuriae, VIII-X). The lower two, that is the eleventh and twelfth, are not attached in front and are called floating ribs (costae fluitantes, XI-XII). The spaces between the ribs are known as intercostal spaces; they contain the intercostal muscles, nerves, and arteries. The rib cage allows for breathing due to its elasticity. In some humans, the rib remnant of the 7th neck vertebra on one or both sides is replaced by a free extra rib called a cervical rib, which can cause trouble for the nerves going to the arm.

Rib anatomy

Typical ribs

The third through ninth ribs are "typical ribs" since they share the same structure. They each have a head that has two facets separated by a crest. One head articulates with the rib's corresponding vertebra and one head articulates with the vertebra superior (above) to it. They have a neck that connects the head with the shaft. The neck meets the shaft at a tubercle. The shaft is thin, flat, and curved. The curve is most prominent at the costal angle. The concave (inside) surface has a groove to protect the intercostal nerve and vessels.

Atypical ribs

The atypical ribs are the 1st, 2nd, and 10th to 12th.

The first rib has a shaft that is wide and nearly horizontal, and has the sharpest curve of the seven true ribs. Its head has a single facet to articulate with the first thoracic vertebra (T1). It also has two grooves for the subclavian vessels, which are separated by the scalene tubercle.

The second rib is thinner, less curved, and longer than the first rib. It has two facets to articulate with T2 and T1, and a tubercle for muscles to attach to.

The 10th to 12th ribs have only one facet on their head, and the 11th and 12th ribs are short with no necks or tubercles.

Rib fractures and associated injuries

The first rib is rarely fractured because of its protected position behind the clavicle (collarbone). However, if it is broken serious damage can occur to the brachial plexus of nerves and the subclavian vessels.

The middle ribs are the ones most commonly fractured. Fractures usually occur from direct blows or from indirect crushing injuries. The weakest part of a rib is just anterior to its angle, but a fracture can occur anywhere.

A lower rib fracture has the complication of potentially injuring the diaphragm, which could result in a diaphragmatic hernia.

Rib fractures are painful because the ribs have to move for inspiration and expiration of air. Rib pain may also be associated with metastasis of cancer, especially from the breast or prostate.

Bifid rib, bifurcated rib

A Bifid rib or bifurcated rib is a congenital abnormality occurring in about 1% of the population. The sternal end of the rib is cleaved into two. It is usually unilateral. Effects of this neuroskeletal anomaly can include respiratory difficulties, neurological difficulties, limitations, and limited energy from the stress of needing to compensate for the neurophysiological difficulties.

References

Clinically Oriented Anatomy, 4th ed. Keith L. Moor and Robert F. Dalley. pp. 62-64

Thorax | Skeletal system

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