In this anatomy lesson, I’m going to cover the rib bones, also called costae in Latin. The ribs help protect vital organs in the thorax such as the heart and lungs, and they assist with breathing.
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Don’t be fooled their long, curved shape! Rib bones are not classified as long bones. Instead, anatomists classify the ribs as flat bones, and they are located within the axial skeleton. Together with the sternum, thoracic vertebrae, and costal cartilages, the ribs form the thoracic cage, also called the bony thorax.
The average skeleton contains 24 individual ribs, formed in 12 pairs, and they are divided into three main categories: true ribs, false ribs, and floating ribs.
True Ribs, False Ribs, and Floating Ribs
The true ribs include rib pairs 1-7, with each rib articulating posteriorly to the thoracic vertebrae and anteriorly to the sternum via costal cartilages. Because the true ribs attach to both the thoracic vertebrae and the sternum, anatomists sometimes splice those terms together and call the true ribs “the vertebrosternal ribs.”The false ribs include rib pairs 8-12. Like the true ribs, these false ribs articulate with thoracic vertebrae posteriorly. However, they do not attach directly to the sternum anteriorly, and instead, attach to the costal cartilage of the preceding (superior) rib, except for false ribs 11-12, which are the floating ribs. Because the false ribs attach to the vertebrae posteriorly and costal cartilage anteriorly, anatomists sometimes call the false ribs vertebrochondral ribs (vertebro = vertebrae; chondral = cartilage).The floating ribs (11-12) are the last two pairs of false ribs, but unlike the other false ribs, they do not attach to the cartilage of the preceding ribs on the anterior side. Instead, they attach posteriorly to the thoracic vertebrae and “float” without attaching to the costal cartilage anteriorly, so anatomists simply call them “vertebral ribs.”
The space between each rib is called the intercostal space, and there are 11 intercostal spaces in the thoracic cage, which are filled with nerves, lymph nodes, arteries, veins, and muscles. In fact, when you eat ribs at a restaurant, you’re eating the intercostal muscles of an animal.
There are three main types of intercostal muscle, which facilitate chest movement: External intercostal muscles, internal intercostal muscles, and innermost intercostal muscles.
Each intercostal space is numbered after the rib directly above it. So the first intercostal space will be the space just below the first rib; the second intercostal space is the space below the second rib, and so on.
These intercostal spaces are important landmarks for nurses and other healthcare professionals, especially when assessing lung sounds and heart sounds.
As I mentioned in my sternum anatomy video, the second pair of ribs meet at the junction between the manubrium and the body of the sternum, forming the sternal angle (or angle of Louis). The second intercostal space will be just below the second pair of ribs. From there, you can count the intercostal spaces to locate the apical pulse, which is on the left side of the chest at the fith intercostal space, down the midclavicular line.
Other intercostal spaces will allow you to assess different chest assessment sounds.
Rib Bone Anatomy and Landmarks
When examining individual rib bones, you’ll notice that some have different structures, so anatomists categorize ribs into two main types: typical and atypical.
Typical Rib Bones
Typical ribs include ribs 3-9, and they feature a shaft (or body), head, neck, groove, tubercle, and anterior extremity.
The anterior extremity is concave and porous, allowing for the attachment of the costal cartilage at the front of the thoracic cage.The shaft (body) of a typical rib is flat but long, and it curves sharply near its posterior end, forming an angle known as the costal angle (or angle of the rib). A groove called the costal groove extends down the inferior (bottom) portion of the rib’s shaft, which allows for the passage of intercostal vessels and nerves. This groove creates a sharp inferior border compared to the smoother superior border, which allows you to identify the proper orientation of the rib bone (sharper side always faces down).The tubercle is the small bumpy area where the body meets the neck. The articular part of the tubercle articulates with the transverse process of the inferior vertebra, and the non-articular portion allows for the attachment of the ligament of the tubercle.The neck connects the body (shaft) of the rib to the head.The head is on the posterior end, and it articulates with vertebrae via two facets: a smaller upper facet, which articulates with the vertebra above, and a larger facet that articulates with the vertebra of the same rib number (example: rib three articulates with thoracic vertebra 3 at the inferior facet and vertebra 2 and the superior facet). These facets are separated by a bony crest, to which the intra-articular (or interarticular) ligament attaches.
Atyipcal Rib Bones
The atypical ribs include ribs 1, 2, 10, 11, and 12.
Rib 1 is usually shorter and wider than all other ribs, and its broad, flat surface contains grooves that support the subclavian vessels. Because rib 1 articulates with first thoracic vertebra only, there is a single facet on its head (typical ribs have two facets, as mentioned above). A scalene tubercle allows for the attachment of the scalene muscles.Rib 2 is also quite curved, but it is longer than rib one and not as flat. The serratus anterior muscle originates from a roughened area near the middle of this rib, and it articulates with thoracic vertebrae 1 and 2 (T1 and T2).Rib 10 has only a single articular facet on its head.Ribs 11 and 12 also have a single articular facet on the head, but no necks or tubercles.
Free Quiz and More Anatomy Videos
Take a free ribs anatomy quiz to test your knowledge, or review our ribs anatomy video. In addition, you might want to watch our anatomy and physiology lectures on YouTube, or check our anatomy and physiology notes.