Rib Classification
Each pair of ribs includes a bony part and costal cartilage (cartilago costalis), which is directed forward. Depending on how the anterior end attaches to the sternum, three groups are distinguished:
- 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; instead, their cartilages connect to the cartilage of the rib above.
- Floating ribs (Costae fluctuantes) — pairs XI and XII. Their anterior ends remain free and have no connection to the sternum.
Anatomy of the Bony Part of a Typical Rib
The bony part of a typical rib (os costale) has an outer convex and an inner concave surface. Anatomically, it is divided into three sections: the head, neck, and body.
- Head of the rib (Caput costae) faces the spine. It features an articular surface (facies articularis capitis costae) for articulation with vertebral bodies. In pairs II–X, this surface is divided in two by the crest of the rib head (crista capitis costae).
- Neck of the rib (Collum costae) is a narrowed region connecting the head to the body. At the border with the body lies the rib tubercle (tuberculum costae), which has its own articular surface (facies articularis tuberculi costae) for articulation with the transverse process of the vertebra.
- Body of the rib (Corpus costae) is the longest part. At the point where the body makes a sharp bend, the angle of the rib (angulus costae) is formed. Along the inferior margin on the internal surface runs the costal groove (sulcus costae), which ends at the crest of the rib (crista costae).
How to Determine the Side of a Rib
To determine whether a typical rib belongs to the right or left side of the thorax, it must be correctly oriented in space. Three main landmarks are used for this:
- The head must be directed backward, and the body forward.
- The smooth convex surface faces outward, while the concave surface faces medially (inward).
- The costal groove must always be located strictly along the inferior margin.
Features of Atypical Ribs
Certain ribs have pronounced structural differences related to their position.
- I rib (Costa prima). Small, lies horizontally. It has superior and inferior surfaces. The inferior surface is completely smooth (no groove). On the medial concave margin of the superior surface is the scalene tubercle (tuberculum m. scaleni anterioris). Anterior to it runs the groove for the subclavian vein (sulcus venae subclaviae), and posterior to it is the groove for the subclavian artery (sulcus arteriae subclaviae). The angle of the rib coincides with the tubercle, and the crest on the head is absent.
- Second rib (Costa secunda). Transitional type. The surfaces are oriented obliquely (convex superior-lateral and concave inferior-medial); the groove is absent. On the outer surface of the body, the tuberosity for the serratus anterior muscle (tuberositas m. serrati anterioris) stands out. The head has a crest, and the angle is displaced distally from the tubercle onto the body of the bone.
- Tenth rib. Differs in that its head has only a single undivided articular surface.
- Eleventh and twelfth ribs (Costae fluctuantes). Have the simplest structure: the rib tubercle, angle, and crest on the head are absent. A groove may be faintly visible only on the 11th rib, while it is absent on the 12th.
Briefly: answers to topical questions
What runs in the costal groove of a typical rib?
In the costal groove (sulcus costae) runs the intercostal neurovascular bundle. The elements of the bundle are arranged in the groove from top to bottom in the following order:
- Intercostal vein (v. intercostalis) — occupies the highest position.
- Intercostal artery (a. intercostalis) — located in the middle.
- Intercostal nerve (n. intercostalis) — occupies the lowest position.
Which muscles attach to the first rib?
A landmark associated with the anterior scalene muscle is described on the first rib:
- Scalene tubercle (tuberculum m. scaleni anterioris) — located on the superior surface along the inner (medial) concave margin, approximately in the middle of the rib body.
This tubercle divides the superior surface into two grooves:
- Groove for the subclavian vein (sulcus venae subclaviae) — anterior to the tubercle.
- Groove for the subclavian artery (sulcus arteriae subclaviae) — posterior to the tubercle.
Which parts of the thoracic vertebra does a typical rib articulate with?
A typical rib articulates with the bodies of the thoracic vertebrae and their transverse processes. The connection occurs in two places:
- Vertebral body — the head of the rib articulates with it, forming a joint with the costal demifacets (fovea costalis superior and fovea costalis inferior) of two adjacent vertebrae.
- Transverse process of the vertebra — the tubercle of the rib connects with its costal facet (fovea costalis processus transversi).
Which ligaments reinforce the costovertebral joints?
The costovertebral joints are reinforced by several ligaments that fix the head and tubercle of the rib. The joint of the rib head (articulatio capitis costae) is strengthened by:
- Intra-articular ligament of the head of the rib (intraarticular lig. capitis costae).
- Radiate ligament of the head of the rib (lig. capitis costae radiatum).
The costotransverse joint (articulatio costotransversaria) is reinforced by:
- Costotransverse ligaments (lig. costotransversarium superius, laterale et proprium).
How to remember the order of the grooves on the 1st rib: "The vein is always anterior." The scalene tubercle divides the superior surface such that the groove for the subclavian vein lies anterior to it, and the groove for the artery lies posterior.
Class and exam questions
How do true ribs differ from false ribs?
True ribs (I–VII) connect directly to the sternum using their cartilages. False ribs (VIII–X) do not reach the sternum; their cartilages fuse with the cartilages of the overlying ribs.
Which ribs lack a crest of the head?
The crest of the head is absent on the 1st, 11th, and 12th rib pairs. All other typical ribs have it, and it divides the articular surface of the head into two parts.
Which ribs do not have a groove on the inferior border?
The costal groove (sulcus costae) is completely absent on the 1st and 2nd pairs, as well as on the 12th rib. On the 11th rib, it may be extremely weakly expressed.
How to 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, and on the superior surface are located the scalene tubercle and two grooves for vessels.
What else to study on this topic
- 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 muscle and the transitional nature of the 2nd rib
- Simplified structure of the floating ribs (XI–XII)
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