The first rib is the shortest, widest and most unusual of all ribs. Unlike other ribs, it lies in the horizontal plane and therefore is flattened superoinferiorly and positioned obliquely. This rib allows the apex of the lung to expand above the superior thoracic aperture of the thorax into the lower neck.

Due to a different plane in which the first rib is positioned, it has superior and inferior surfaces and internal and external borders. The head of the first rib articulates only with the body of the first thoracic vertebra (T1). Therefore, it has only one articular facet and no interarticular crest.

The tubercle of the first rib corresponds to the costal angle. Like other ribs, it has a facet that articulates with the transverse process of the first thoracic vertebra. The first rib does not have the costal groove.

The main distinguishing features of the first rib are found on its superior surface:

Scalene tubercle - a somewhat triangular-shaped bony prominence that serves as an attachment site for the anterior scalene muscle;
Groove for the subclavian vein - a shallow groove positioned anterior to the scalene tubercle; it holds the subclavian vein;
Groove for the subclavian artery - a shallow groove located posterior to the scalene tubercle; it lodges the subclavian artery and the inferior trunk of the brachial plexus (medial).

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Atypical ribs (first rib)