Human ribs areflat bones that form part of therib cage to help protect internal organs. Humans usually have 24 ribs, in 12 pairs.[2] 1 in 500 people have an extra rib known as acervical rib. People may have a cervical rib on the right, left or both sides.[3] All are attached at the back to thethoracic vertebrae and are numbered from 1 to 12 according to the vertebrae to which they attach. The first rib is attached to thoracic vertebra 1 (T1). At the front of the body, most of the ribs are joined bycostal cartilage to thesternum. Ribs connect to vertebrae at thecostovertebral joints.[4]
The parts of a rib includes the head, neck, body (orshaft), tubercle, and angle.
Thehead of the rib lies next to a vertebra. The ribs connect to the vertebrae with two costovertebral joints, one on the head and one on the neck. The head of the rib has a superior and an inferior articulating region, separated by a crest. These articulate with thesuperior andinferior costal facets on the connecting vertebrae.[5] The crest gives attachment to theintra-articulate ligament that joins the rib to the vertebra of the same number, at theintervertebral disc. Anotherligament, theradiate ligament joins the head of the rib to both the body of the upper vertebra and to the body of the lower vertebra. The smaller middle part of the ligament connects to the intervertebral disc. Thisplane joint is known as thearticulation of the head of the rib.
The other costovertebral joint is that between the tubercle on the neck and the transverse process of the corresponding thoracic vertebra, known as thecostotransverse joint. Thesuperior costotransverse ligament attaches from the non-articular facet of the tubercle to the transverse process of the vertebra.
Theneck of the rib is a flattened part that extends laterally from the head. The neck is about 3 cm long. Its anterior surface is flat and smooth, whilst its posterior is perforated by numerous foramina and its surface rough, to give attachment to the ligament of the neck. Its upper border presents a rough crest (crista colli costae) for the attachment of the anteriorcostotransverse ligament; its lower border is rounded.
Atubercle of rib on the posterior surface of the neck of the rib, has two facets (surfaces) one articulating and one non-articulating. The articular facet, is small and oval and is the lower and more medial of the two, and connects to thetransverse costal facet on the thoracic vertebra of the same rib number.[5] The transverse costal facet is on the end of the transverse process of the lower of the two vertebrae to which the head is connected. The non-articular portion is a rough elevation and affords attachment to the ligament of the tubercle. The tubercle is much more prominent in the upper ribs than in the lower ribs.
The first seven sets of ribs, known as "true ribs", are attached to thesternum by thecostal cartilages. The first rib is unique and easier to distinguish than other ribs. It is a short, flat, C-shaped bone, and attaches to the manubrium.[6] The vertebral attachment can be found just below theneck at the first thoracic vertebra, and the majority of this bone can be found above the level of theclavicle. Ribs 2 through 7 then become longer and less curved as they progress downwards.[7] The following five sets are known as "false ribs", three of these sharing a common cartilaginous connection to the sternum, while the last two (eleventh and twelfth ribs) are termedfloating ribs.[2] They are attached to thevertebrae only, and not to the sternum or cartilage coming off of the sternum.
In general, human ribs increase in length from ribs 1 through 7 and decrease in length again through rib 12. Along with this change in size, the ribs become progressively oblique (slanted) from ribs 1 through 9, then less slanted through rib 12.[7]
The rib cage is separated from the lower abdomen by thethoracic diaphragm which controls breathing. When the diaphragm contracts, the thoracic cavity is expanded, reducing intra-thoracic pressure and drawing air into the lungs. This happens through one of two actions (or a mix of the two): when the lower ribs the diaphragm connects to are stabilized by muscles and the central tendon is mobile, when the muscle contracts the central tendon is drawn down, compressing the cavity underneath and expanding the thoracic cavity downward. When the central tendon is stabilized and the lower ribs are mobile, a contraction of the diaphragm elevates the ribs, which works in conjunction with other muscles to expand the thoracic indent upward.
During the fourth week (fertilization age)costal processes have formed on the vertebral bodies. These processes are small, lateral protrusions ofmesenchyme that develop in association with the vertebral arches. During the fifth week the costal processes on the thoracic vertebrae become longer to form the ribs. In the sixth week, the costovertebral joints begin to develop and separate the ribs from the vertebrae. The first seven pairs of ribs, the true ribs join at the front to thesternal bars. By thefetal stage the sternal bars have completely fused.[8]
The ribs begin as cartilage that laterossifies – a process calledendochondral ossification. Primary ossification centers are located near the angle of each rib, and ossification continues in the direction away from the head and neck. Duringadolescence secondary ossification centers are formed in the tubercles and heads of the ribs.[8]
Skeleton of a dog showing the location of the ribsRib cage of thebig brown bat (Eptesicus fuscus)
Injawed fish, there are often two sets of ribs attached to the vertebral column. One set, thedorsal ribs, are found in the dividing septum between the upper and lower parts of the main muscle segments, projecting roughly sideways from the vertebral column. The second set, theventral ribs arise from the vertebral column just below the dorsal ribs, and enclose the lower body, often joining at the tips. Not all species possess both types of rib, with the dorsal ribs being most commonly absent.Sharks, for example, have no ventral ribs, and only very short dorsal ribs. In someteleosts, there may be additional rib-like bones within the muscle mass.[9]
Tetrapods, however, only ever have a single set of ribs which are probablyhomologous with the dorsal ribs of fishes. In the earlierchoanates, every vertebra bore a pair of ribs, although those on thethoracic vertebrae are typically the longest. Thesacral ribs were stout and short, since they formed part of thepelvis, connecting the backbone to thehip bones.[9]
In most true tetrapods, many of these early ribs have been lost, and in livingamphibians andreptiles, there is great variation in rib structure and number. For example,turtles have only eight pairs of ribs, which are developed into a bony or cartilaginouscarapace andplastron, while snakes have numerous ribs running along the full length of their trunk.Frogs typically have no ribs, aside from a sacral pair, which form part of the pelvis.[9]
In birds, ribs are present as distinct bones only on the thoracic region, although small fused ribs are present on thecervical vertebrae. The thoracic ribs of birds possess a wide projection to the rear; thisuncinate process is an attachment for the shoulder muscles.[9] Usually dogs have 26 ribs.Mammals usually also only have distinct ribs on the thoracic vertebra, although fixed cervical ribs are also present inmonotremes. Intherian mammals, the cervical and lumbar ribs are found only as tiny remnants fused to the vertebrae, where they are referred to astransverse processes. In general, the structure and number of the true ribs in humans is similar to that in other mammals. Unlike reptiles,caudal ribs are never found in mammals.[9]
^Moore, Keith L.; Dalley, Arthur F.; Agur, Anne M. R. (2018).Clinically Oriented Anatomy (8th ed.). Philadelphia: Wolters Kluwer. pp. 293–297.ISBN9781496347213.
^abNetter, Frank (2014).Atlas of human anatomy (Sixth ed.). Saunders. pp. 183–184.ISBN9781455704187.
^abSaladin, K. S. (2010). Anatomy and Physiology: The Unity of Form and Function (5th ed.). New York, NY: McGraw-Hill.
^abcLarsen, William (2001).Human embryology (3rd ed.). Churchill Livingstone. pp. 80–85.ISBN0443065837.
^abcdeRomer, Alfred Sherwood; Parsons, Thomas S. (1977).The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 170–173.ISBN0-03-910284-X.
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