Ribs (Costae) are twelve pairs of thoracic cage bones, each consisting of a posterior bony part and an anterior cartilage. They form a protective trunk framework, attaching posteriorly to the vertebral column and anteriorly toward the sternum.
Number12 pairs of bones transitioning anteriorly into cartilage.
ClassificationDivided into true (I–VII), false (VIII–X), and floating (XI–XII) ribs.
AtypicalPairs I, II, X, XI, and XII have specific morphological features.
Classification of the Ribs
Each rib pair includes a bony part and a costal cartilage (cartilago costalis) directed anteriorly. Based on how the anterior end attaches to the sternum, ribs are divided into three groups:
True ribs (Costae verae) — pairs I through VII. Their cartilaginous ends articulate directly with the sternum.
False ribs (Costae spuriae) — pairs VIII, IX, and X. They do not reach the sternum directly; instead, their cartilages join the cartilage of the rib immediately superior.
Floating ribs (Costae fluctuantes) — pairs XI and XII. Their anterior ends remain free and have no attachment to the sternum.
Anatomy of a Typical Rib
The bony part of a typical rib (os costale) features an external convex surface and an internal concave surface. It is anatomically divided into three sections: the head, neck, and body.
Head of the rib (Caput costae) faces the vertebral column. It bears an articular surface (facies articularis capitis costae) for articulation with vertebral bodies. In pairs II–X, this surface is divided into two facets by the crest of the head (crista capitis costae).
Neck of the rib (Collum costae) is a narrowed segment connecting the head to the body. At the junction with the body lies the tubercle of the rib (tuberculum costae), which features its own articular facet (facies articularis tuberculi costae) for articulation with the transverse process of the vertebra.
Body of the rib (Corpus costae) is the longest part. Where the body bends sharply, it forms the angle of the rib (angulus costae). Along the inferior margin of the internal surface runs the costal groove (sulcus costae), which terminates near the crest of the rib (crista costae).
Determining the Side of a Rib
To determine whether a typical rib belongs to the right or left side of the thorax, it must be properly oriented in space using three main landmarks:
The head must point posteriorly, and the body anteriorly.
The smooth convex surface faces outward (laterally), while the concave surface faces medially (inward).
The costal groove must always be positioned strictly along the inferior margin.
Features of Atypical Ribs
Certain ribs exhibit pronounced structural differences related to their anatomical position.
First rib (Costa prima). Small and positioned horizontally. It has superior and inferior surfaces. The inferior surface is completely smooth (no costal groove). On the medial, concave margin of the superior surface is the scalene tubercle (tuberculum m. scaleni anterioris). Anterior to it runs the subclavian vein groove (sulcus venae subclaviae), and posterior to it lies the subclavian artery groove (sulcus arteriae subclaviae). The angle of the rib coincides with the tubercle, and the crest on the head is absent.
Second rib (Costa secunda). A transitional type. Its surfaces are oriented obliquely (a convex anterolateral surface and a concave posteromedial surface); the costal groove is absent. The tuberosity for the serratus anterior muscle (tuberositas m. serrati anterioris) stands out on the outer surface of the body. The head possesses a crest, and the angle is shifted distally onto the bone body away from the tubercle.
Tenth rib. Distinguished by having only a single undivided articular surface on its head.
Eleventh and twelfth ribs (Costae fluctuantes). Feature the simplest structure: the rib tubercle, angle, and crest on the head are absent. A groove may be faintly visible on the 11th rib, but is completely absent on the 12th.
Mnemonic
How to remember the groove order on the 1st rib: "Vein is always anterior." The scalene tubercle divides the superior surface such that the subclavian vein groove lies anterior to it, and the artery groove lies posterior.
Frequently asked questions
What structures run within the costal groove of a typical rib?
The intercostal neurovascular bundle runs within the costal groove (sulcus costae). The elements of the bundle are arranged in the groove from superior to inferior in the following order:
Intercostal vein (v. intercostalis) — occupies the highest position.
Intercostal artery (a. intercostalis) — lies in the middle.
Intercostal nerve (n. intercostalis) — occupies the lowest position.
Which muscles attach to the first rib?
The first rib features a key landmark associated with the anterior scalene muscle:
Scalene tubercle (tuberculum m. scaleni anterioris) — located on the superior surface along the inner (medial) concave margin, approximately at the midpoint of the rib body.
This tubercle divides the superior surface into two grooves:
Subclavian vein groove (sulcus venae subclaviae) — anterior to the tubercle.
Subclavian artery groove (sulcus arteriae subclaviae) — posterior to the tubercle.
With which parts of a thoracic vertebra does a typical rib articulate?
A typical rib articulates with the bodies and transverse processes of thoracic vertebrae. The articulation occurs at two sites:
Vertebral body — the rib head articulates here, forming a joint with the superior and inferior costal facets (fovea costalis superior and fovea costalis inferior) of two adjacent vertebrae.
Vertebral transverse process — the rib tubercle articulates with the costal facet of the transverse process (fovea costalis processus transversi).
Which ligaments reinforce the costovertebral joints?
The costovertebral joints are reinforced by several ligaments that anchor the head and tubercle of the rib. The joint of the rib head (articulatio capitis costae) is reinforced by:
Intra-articular ligament of the head of the rib (lig. capitis costae intraarticulare).
Radiate ligament of the head of the rib (lig. capitis costae radiatum).
The costotransverse joint (articulatio costotransversaria) is reinforced by:
Costotransverse ligaments (ligg. costotransversaria superius, laterale et proprium).
What is the difference between true and false ribs?
True ribs (I–VII) connect directly to the sternum via their own cartilages. False ribs (VIII–X) do not reach the sternum; their cartilages fuse with the cartilage of the ribs immediately above.
Which ribs lack the crest of the head?
The crest of the head is absent on the 1st, 11th, and 12th rib pairs. All other typical ribs possess it, dividing the articular surface of the head into two facets.
Which ribs lack a costal groove on the inferior margin?
The costal groove (sulcus costae) is completely absent on the 1st and 2nd pairs, as well as the 12th rib. On the 11th rib, it may be extremely faint.
How can you quickly identify the 1st rib among the others?
It is the shortest, widest, and lies in a horizontal plane. It has superior and inferior surfaces, with the scalene tubercle and two vascular grooves located on its superior surface.
Go deeper
Landmarks on the superior surface of the 1st rib
Articular surfaces of the head and tubercle
Formation of the rib angle in typical and atypical bones
Tuberosity for the serratus anterior and the transitional nature of the 2nd rib