Human Anatomy - Kotsan I. Ya. 2009

Anatomy of the Axial Skeleton
Classification and Structure of the Ribs

Depending on how the Ribs attach to the Sternum, they are classified into true ribs (costae verae), false ribs (costae spuriae), and floating ribs (costae fluitantes). The top seven pairs of ribs, whose cartilages attach directly to the sternum, are called true ribs. The eighth, ninth, and tenth pairs, whose cartilages attach to the Cartilage of the rib immediately above them, are called false ribs.

The eleventh and twelfth pairs are not anchored anteriorly; they lie freely within the Muscles and are referred to as floating, or oscillating, ribs.

Each rib (costa), with the exception of the eleventh and twelfth, consists of a bony part, or rib bone (os costalis), and a costal cartilage (cartilago costalis). At their junction, the periosteum of the rib bone transitions directly into the perichondrium of the hyaline costal cartilage.

A rib features a body (corpus costae) and two ends: an anterior, or sternal end (extremitas anterior), and a posterior, or vertebral end (extremitas posterior) (Fig. 26B).

The body of the rib is long, flat, and curved. It presents upper and lower margins (margo superior and margo inferior), as well as outer and inner surfaces (facies externa and facies interna). On the inner surface—predominantly of the middle ribs—along the lower margin, a shallow costal groove (sulcus costae) runs, lodging the intercostal vessels and nerve.

At its posterior end, the rib exhibits a thickening known as the HEAD of the rib (caput costae). By means of its articular surface of the rib head (facies articularis capitis costae), the head articulates with the bodies of two adjacent thoracic vertebrae. The articular surface of the head is divided by the crest of the head (crista capitis costae) into two facets of unequal size (the upper being smaller than the lower). The head is followed by a narrowed section, the neck of the rib (collum costae), along the upper margin of which runs a longitudinal crest, the crest of the neck of the rib (crista colli costae); this crest is absent in the first and last ribs. The neck is followed by the tubercle of the rib (tuberculum costae), which features the articular surface of the rib tubercle (facies articularis tuberculi costae) for articulation with the transverse process of the corresponding thoracic vertebra. At some distance from the tubercle, the body of the rib bends forward, forming the angle of the rib (angulus costae). On the first rib, this angle coincides with the tubercle; on the lower ribs, the distance between the tubercle and the costal angle increases up to the eleventh rib, whereas on the twelfth rib, the angle disappears entirely.

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Fig. 26. Sternum and ribs

A — sternum: 1 — manubrium, 2 — body, 3 — xiphiid process, 4 — costal notches, 5 — sternal angle, 6 — jugular notch, 7 — clavicular notch. B — eighth rib (internal view): 1 — articular surface of the rib head, 2 — neck of the rib, 3 — angle of the rib, 4 — body of the rib, 5 — costal groove. C — first rib (superior view): 1 — tubercle of the rib and its articular surface, 2 — neck of the rib, 3 — groove for the Subclavian Artery, 4 — groove for the subclavian vein, 5 — tubercle of the anterior scalene Muscle

The size and shape of individual ribs depend on the conformation of The thoracic cage and their position relative to it.

The length of the ribs increases from top to bottom up to the seventh rib; the seventh and eighth ribs are the longest, and from the ninth rib onward, they progressively shorten down to the twelfth rib. It should be noted that the ribs are arched and twisted around their axis in such a way that their anterior ends are directed downwards and medially, with this downward inclination increasing from superior to inferior. While the end of the first rib lies at the level of the body of the third thoracic vertebra, a line extending the axis of the twelfth rib intersects the Pubic Symphysis. Because of this arrangement, the width of the intercostal spaces increases in the anteroposterior direction (from back to front).

The first two and last two ribs possess certain distinctive features that set them significantly apart from the other ribs.

The first rib (costa prima) is the broadest, most curved, and lies almost horizontally (Fig. 26C). Consequently, unlike other ribs, it has superior and inferior surfaces (facies superior and facies inferior) rather than outer and inner surfaces, and external and internal margins (margo externus and margo internus) rather than upper and lower margins. On the superior surface of the first rib is the tubercle of the anterior scalene muscle (tuberculum musculi scaleni anterioris), which serves as the attachment site for the muscle of the same name. Anterior to this tubercle lies the groove for the subclavian vein (sulcus venae subclaviae), and posteriorly lies the groove for the subclavian artery (sulcus arteriae subclaviae), across which the subclavian artery arches and where it can be compressed if necessary. Posterior to the subclavian artery groove is a small roughened area for the attachment of the middle scalene muscle. The first rib lacks both a costal groove and a crest of the head. The neck of the first rib is long and slender, and the costal angle coincides with the costal tubercle.

The second rib (costa secunda) features a roughened area on its external surface—the tuberosity of the anterior serratus muscle (tuberositas musculi serrati anterioris)—where a slip of the corresponding muscle attaches.

The eleventh and twelfth ribs (costa XI and costa XII) have articular surfaces on their heads that are not divided by a crest, because—unlike other ribs—their heads articulate with the body of a single vertebra rather than two adjacent ones. The angle, neck, tubercle, and costal groove are poorly defined on the eleventh rib and entirely absent on the twelfth.

The sternum (sternum) is a flat bone that resembles a sword in shape. It consists of a manubrium (manubrium sterni), a body (corpus sterni), and a xiphoid process (processus xiphoideus) (Fig. 26A). The manubrium is the widest and thickest part of the sternum and features several notches: the unpaired jugular notch (incisura jugularis) and the paired clavicular and costal notches (incisura clavicularis and incisura costalis). The body of the sternum bears only costal notches for articulation with the ribs. The lower margin of the manubrium and the upper margin of the body form the sternal angle (angulus sterni). The xiphoid process terminates in a blunt end and is occasionally bifurcated or pierced by an aperture.