How Many Ribs Do Humans Have?


How Many Ribs Do Humans Have
A human has 24 ribs. (image: OpenStax College, CC 3.0)

Humans typically have 24 ribs, 12 on each side of the rib cage. This number is the same for both men and women. However, exceptions in the number of ribs are relatively common. Supernumerary ribs are extra ribs, while agenesis of ribs refers to having fewer than 24 ribs.

The Standard Human Rib Cage

Most people have 24 ribs, arranged symmetrically as 12 pairs. These connect to the spine at the back and most also connect to the sternum in the front, forming the rib cage. This structure protects vital organs like the heart and lungs and provides a framework for muscle attachment.

Ribs fall into three groups: true ribs, false ribs, and floating ribs:

  • True Ribs (Pairs 1-7): These connect directly to the sternum via costal cartilage. They are most rigid and provide the best protection to the thoracic organs.
  • False Ribs (Pairs 8-10): These attach to the costal cartilage of the rib above rather than directly to the sternum. Their flexibility aids in breathing.
  • Floating Ribs (Pairs 11-12): The floating ribs do not connect to the sternum at all. Instead, they end in the posterior abdominal musculature. These ribs offer less protection but greater flexibility.

Man vs Woman: The Adam and Eve Story

In the Biblical story, God created Eve from one of Adam’s ribs. From this story, there is a common misconception that men have one more rib than women. So, when Flemish anatomist Vesalius noted that humans have 12 pairs of ribs in his anatomy text De Humani Corporis Fabrica in 1953, it generated a lot of controversy. Interestingly, about 0.5-1% of people have a thirteenth or cervical rib, but this is actually more common in women than in men.

Variations in Rib Numbers

Approximately 1% to 2% of the population have either extra or missing ribs. These variations result from genetic factors or developmental anomalies.

  • Cervical Rib: Cervical rib results from a mutation that results in a baby born with one or two extra ribs between the base of the neck and the collarbone. There can be one extra rib on either or both sides.
  • Lumbar Supernumerary Ribs: About 1% of humans have extra ribs below rib 12, attached to the lumbar spine. Usually the extra rib or ribs don’t cause problems.
  • Trisomy 21 (Down Syndrome): Some babies born with Down syndrome have a missing 12th rib or an extra rib.
  • Spondylocostal Dysplasia: This is a rare autosomal recessive disorder that affects the spine and ribs. It results in fused vertebrae, scoliosis, and missing or fused ribs.
  • Spondylothoracic Dysplasia: This is an autosomal recessive disorder that results in babies born with small chest cavities, fused vertebrae, and fused ribs. Usually, this causes significant breathing problems.
  • Goldenhar syndrome, Sprengel’s deformity, and Klippel-Feil syndrome are additional conditions that sometimes affect the number of ribs.

Health Implications of Rib Number Variations

Having an extra rib or fewer ribs doesn’t necessarily lead to health problems. Actually, most people are unaware they have a different number of ribs. However, sometimes an extra cervical rib causes Thoracic Outlet Syndrome, which produces pain or numbness in the arm. In rare cases, having fewer ribs compromises the protection of internal organs.

Detection and Treatment of Rib Abnormalities

A physical examination indicates cervical ribs and sometimes missing ribs in adults. An ultrasound often picks up a rib abnormality before a baby’s birth. Imaging tests like X-rays or CT scans also reveal the number of ribs.

Prosthetic ribs are an option for replacing a missing rib. If an extra rib causes a problem, surgical removal is an option. A brace or sometimes surgery addresses scoliosis.

References

  • Larsen, William (2001). Human Embryology (3rd ed.). Churchill Livingstone. ISBN 0443065837.
  • Moore, Keith L.; Dalley, Arthur F.; Agur, Anne M. R. (2018). Clinically Oriented Anatomy (8th ed.). Philadelphia: Wolters Kluwer. ISBN 9781496347213.
  • Oner, Zulal; Oner, Serkan; Sahin, Necati Emre; Cay, Mahmut (2023). “Evaluation of congenital rib anomalies with multi-detector computed tomography in the Turkish population”. Folia Morphologica. doi:10.5603/FM.a2023.0006
  • Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. ISBN 0-03-910284-X.
  • Sly, Peter D.; Collins, Rachel A. (2008) “Chapter 7: Applied Clinical Respiratory Physiology”. Pediatric Respiratory Medicine (2nd ed.), Philadelphia: Mosby. ISBN 978-0-323-04048-8. doi:10.1016/b978-032304048-8.50011-6