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Rib

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Rib

This article concerns the skeletal component. For other applications, consult Rib (disambiguation).

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Collection of single ribs in the Faculty of Education of Charles University

Animation of all ribs, including the false ones in humans

Details

In the skeletal structure of vertebrates, ribs, or Latin costae, are those elongated, curved bones that form the rib cage. They are a fundamental part of the axial skeleton. [1] For the majority of tetrapods, these ribs encircle the thoracic cavity. Their primary function is to facilitate breathing by allowing the lungs to expand, which they do by expanding the thoracic cavity itself. Beyond respiration, they act as a crucial protective shield for the lungs, the heart, and other vital organs housed within the thorax. In certain species, most notably snakes, ribs extend their protective and supportive role to encompass nearly the entire body.

Human anatomy

Rib details

In humans, ribs are classified as flat bones, forming an integral component of the rib cage and providing essential protection for the internal organs. Typically, a human possesses 24 ribs, arranged in 12 pairs. [2] However, approximately 1 in 500 individuals exhibit an anatomical variation known as a cervical rib, which is an extra rib. This anomaly can manifest on the right side, the left side, or bilaterally. [3] All ribs are anchored posteriorly to the thoracic vertebrae, and their numbering, from 1 to 12, corresponds directly to the specific vertebrae they attach to, with the first rib connecting to thoracic vertebra 1 (T1). Anteriorly, the majority of these ribs are linked to the sternum via costal cartilage. The intricate connection between the ribs and the vertebrae occurs at the costovertebral joints. [4]

The structural components of a rib include the head, neck, body (or shaft), tubercle, and angle.

The head of the rib is situated adjacent to a vertebra. Ribs engage with the vertebrae through two distinct costovertebral joints: one associated with the head and another with the neck. The rib's head features both a superior and an inferior articulating region, separated by a crest. These regions articulate with the superior and inferior costal facets found on the articulating vertebrae. [5] The crest serves as an attachment point for the intra-articulate ligament, which connects the rib to the vertebra of the same numerical designation, precisely at the intervertebral disc. Furthermore, another ligament, the radiate ligament, establishes a connection between the rib's head and both the body of the vertebra situated above it and the body of the vertebra below it. The middle segment of this ligament, which is smaller, attaches to the intervertebral disc. This type of joint, classified as a plane joint, is recognized as the articulation of the head of the rib.

The second costovertebral joint involves the tubercle on the rib's neck and the transverse process of the corresponding thoracic vertebra. This is known as the costotransverse joint. The superior costotransverse ligament extends from the non-articular facet of the tubercle to the transverse process of the vertebra.

The neck of the rib is a flattened segment that extends laterally from the head. It measures approximately 3 cm in length. Its anterior surface is smooth and flat, while its posterior surface is perforated with numerous small openings (foramina) and possesses a rough texture, providing an anchor for the ligament of the neck. The superior border of the neck features a rough crest, known as the crista colli costae, which serves for the attachment of the anterior costotransverse ligament; its inferior border is rounded.

On the posterior surface of the rib's neck, there is a tubercle. This structure has two facets: one that articulates and one that does not. The articulating facet is small and oval-shaped. It is located inferiorly and more medially of the two facets and connects to the transverse costal facet on the thoracic vertebra corresponding to the rib's number. [5] The transverse costal facet is situated at the extremity of the transverse process of the lower of the two vertebrae to which the rib's head is attached. The non-articular portion of the tubercle is a rough elevation that provides attachment for the ligament of the tubercle. The prominence of the tubercle is notably more pronounced in the upper ribs compared to the lower ones.

Rib cage

• Main article: Rib cage

X-ray image of a human chest, with ribs clearly labeled.

The initial seven pairs of ribs are designated as "true ribs". These are directly anchored to the sternum via their respective costal cartilages. The first rib exhibits unique characteristics, making it distinct and easier to identify than others. It is a short, flat bone with a C-shaped curvature, attaching to the manubrium, the uppermost part of the sternum. [6] Its vertebral attachment point is located just inferior to the neck at the level of the first thoracic vertebra. A significant portion of this bone resides superior to the clavicle. From the second to the seventh rib, the bones generally increase in length and exhibit a decrease in curvature as they descend. [7] The subsequent five pairs are classified as "false ribs". Of these, three pairs share a common cartilaginous connection to the sternum. The final two pairs, the eleventh and twelfth ribs, are termed floating ribs. [2] These "floating" ribs attach solely to the vertebrae and lack any connection to the sternum or the cartilage extending from it.

Generally speaking, human ribs progressively increase in length from the first to the seventh rib, and then decrease in length again down to the twelfth rib. Concurrently with this change in size, the ribs become increasingly oblique (slanted) from the first to the ninth rib, and then become progressively less slanted through the twelfth rib. [7]

The rib cage is demarcated from the abdominal cavity below by the thoracic diaphragm, a muscle crucial for controlling breathing. During diaphragm contraction, the thoracic cavity expands, which in turn reduces the intra-thoracic pressure. This pressure differential draws air into the lungs. This expansion occurs through one of two primary mechanisms, or a combination of both: If the lower ribs, to which the diaphragm is attached, are stabilized by surrounding muscles and the central tendon is mobile, then a contraction of the diaphragm pulls the central tendon downward. This action compresses the abdominal cavity and expands the thoracic cavity vertically. Conversely, if the central tendon is stabilized and the lower ribs are mobile, a contraction of the diaphragm elevates the ribs. This action works in concert with other respiratory muscles to expand the thoracic cavity upward.

Development

In the early stages of human embryogenesis, somites begin to form. These structures rapidly subdivide into three distinct mesodermal components: the myotome, the dermatome, and the sclerotome. The development of both the vertebrae and the ribs originates from the sclerotomes. [8]

By the fourth week of fertilization age, rudimentary costal processes begin to emerge from the vertebral bodies. These processes are small, lateral extensions of mesenchyme that develop in close association with the vertebral arches. During the fifth week, these costal processes on the thoracic vertebrae elongate to form the actual ribs. By the sixth week, the initial development and separation of the costovertebral joints occur, effectively distinguishing the ribs from the vertebrae. The first seven pairs of ribs, the true ribs, establish their anterior connection to the sternal bars. As development progresses into the fetal stage, these sternal bars undergo complete fusion. [8]

Ribs initially appear as cartilage and subsequently undergo ossification, a process known as endochondral ossification. Primary centers of ossification are established near the angle of each rib, with ossification proceeding away from the head and neck. During adolescence, secondary ossification centers form within the tubercles and heads of the ribs. [8]

Other animals

Skeleton of a dog, clearly indicating the position of the ribs.

Rib cage of the big brown bat (Eptesicus fuscus).

In jawed fish, it is common to find two sets of ribs emanating from the vertebral column. One set, the dorsal ribs, are situated within the dividing septum that separates the upper and lower portions of the main muscle segments. These project roughly laterally from the vertebral column. The second set, the ventral ribs, originate from the vertebral column just below the dorsal ribs and serve to enclose the lower body, often meeting at their distal ends. It is worth noting that not all species possess both types of ribs; dorsal ribs are frequently absent. For instance, sharks lack ventral ribs entirely and possess only very rudimentary dorsal ribs. In some teleosts, additional rib-like bones may be present within the muscle mass. [9]

Tetrapods, however, characteristically have only a single set of ribs, which are likely homologous with the dorsal ribs observed in fishes. In earlier choanates, virtually every vertebra was associated with a pair of ribs, though those attached to the thoracic vertebrae tend to be the longest. The sacral ribs were notably stout and short, as they contributed to the formation of the pelvis, thereby linking the backbone to the hip bones. [9]

In the majority of extant tetrapods, many of these ancestral ribs have been lost. In living amphibians and reptiles, there is considerable diversity in rib structure and number. For example, turtles possess only eight pairs of ribs, which are highly modified to form the bony or cartilaginous carapace and plastron. In contrast, snakes exhibit numerous ribs that extend along the entire length of their trunk. Frogs, with few exceptions, lack ribs altogether, save for a single sacral pair that becomes integrated into the pelvis. [9]

Birds possess distinct ribs only in the thoracic region. However, small, fused ribs are also present on the cervical vertebrae. The thoracic ribs of birds are characterized by a broad projection extending posteriorly; this feature, known as an uncinate process, serves as an attachment site for the shoulder muscles. [9] Dogs typically have 26 ribs. Mammals, as a general rule, also exhibit distinct ribs exclusively on the thoracic vertebrae. However, fixed cervical ribs are present in monotremes. In therian mammals, cervical and lumbar ribs are observed only as minute remnants fused to the vertebrae, where they are referred to as transverse processes. Broadly speaking, the structural characteristics and number of true ribs in humans are quite similar to those found in other mammals. Crucially, unlike reptiles, caudal ribs are never found in mammals. [9]

Ribs as food

• Main article: Ribs (food)

Ribs as food are a widely consumed part of the anatomy of numerous animals. Ribs represent the less meaty sections of a meat chop and are frequently prepared as part of a larger cut; a portion comprising five or more ribs is commonly referred to as a rack, as in a rack of lamb. Short ribs are cuts of beef ribs, served either individually or in multiples as a plate. A rib steak from beef is a highly favored cut, incorporated into various cuisines. Pork ribs, including spare ribs, are particularly popular in both European and Asian cuisine.

Animated images

• Thoracic cage with spine

See also