|Human rib cage|
The human rib cage. (Source: Gray's Anatomy of the Human Body, 20th ed. 1918.)
The rib cage is an arrangement of bones in the thorax of animals, including humans. It is formed by the vertebral column, ribs, and sternum and encloses the heart and lungs. In humans, the rib cage, also known as the thoracic cage, is a bony and cartilaginous structure which surrounds the thoracic cavity and supports the pectoral girdle, forming a core portion of the human skeleton. A typical human rib cage consists of 24 ribs, the sternum (with Xiphoid process), costal cartilages, and the 12 thoracic vertebrae. It, along with the skin and associated fascia and muscles, makes up the thoracic wall and provides attachments for the muscles of the neck, thorax, upper abdomen, and back.
Ribs are described based on their location and connection with the sternum. Ribs that articulate with the sternum are called true ribs, whereas those that connect with cartilage are termed false ribs.
The terms true and false rib describe rib pairs that are directly attached to the sternum. The phrase true rib (Latin: costae verae), or fixed rib, refers to the first seven, or vertebrosternal, rib pairs. The phrase false rib (Latin: costae spuriae), or vertebrochondral ribs refers to the eighth-to-twelfth pairs of ribs. The eighth-to-tenth pairs of ribs connect to the sternum indirectly via the costal cartilages of the ribs above them. Their elasticity allows ribcage movement for respiratory activity.
The phrase floating rib (Latin: costae fluitantes) refers to the two lowermost, the eleventh and twelfth, rib pairs; so-called because they are attached to only the vertebrae--and not to the sternum or cartilage of the sternum. These ribs are relatively small and delicate, and include a cartilaginous tip.
Each rib consists of a head, neck, and a shaft. The head typically has two facets on its surface; one for articulation with the corresponding vertebra, and one for articulation with the immediately superior vertebra. All ribs are attached in the back to the thoracic vertebrae.
The first rib is the most curved and usually the shortest of all the ribs; it is broad and flat, its surfaces looking upward and downward, and its borders inward and outward. The head is small, rounded, and possesses only a single articular facet, for articulation with the body of the first thoracic vertebra. The neck is narrow and rounded. The tubercle, thick and prominent, is placed on the outer border. It bears a small facet for articulation with the Transverse Process of T1. There is no angle, but at the tubercle the rib is slightly bent, with the convexity upward, so that the head of the bone is directed downward. The upper surface of the body is marked by two shallow grooves, separated from each other by a slight ridge prolonged internally into a tubercle, the scalene tubercle, for the attachment of the Scalenus anterior; the anterior groove transmits the subclavian vein, the posterior the subclavian artery and the lowest trunk of the brachial plexus. Behind the posterior groove is a rough area for the attachment of the Scalenus medius. The under surface is smooth, and destitute of a costal groove. The outer border is convex, thick, and rounded, and at its posterior part gives attachment to the first digitation of the Serratus anterior. The inner border is concave, thin, and sharp, and marked about its center by the scalene tubercle. The anterior extremity is larger and thicker than that of any of the other ribs.
The second rib is the second uppermost rib in humans or second most frontal in animals that walk on four limbs. In humans the second rib is defined as a true rib since it connects with the sternum through the intervention of the costal cartilage anteriorly (at the front). Posteriorly, the second rib is connected with the vertebral column by the second thoracic vertebra. The second rib is much longer than the first rib, but has a very similar curvature. The non-articular portion of the tubercle is occasionally only feebly marked. The angle is slight, and situated close to the tubercle. The body is not twisted, so that both ends touch any plane surface upon which it may be laid; but there is a bend, with its convexity upward, similar to, though smaller than that found in t he first rib. The body is not flattened horizontally like that of the first rib. Its external surface is convex, and looks upward and a little outward; near the middle of it is a rough eminence for the origin of the lower part of the first and the whole of the second digitation of the serratus anterior; behind and above this is attached the scalenus posterior. The internal surface, smooth, and concave, is directed downward and a little inward: on its posterior part there is a short costal groove.
The tenth rib attaches directly to the body of vertebra T10 instead of between vertebrae like the second through ninth ribs. Due to this direct attachment, vertebra T10 has a complete costal facet on its body.
The eleventh and twelfth fibs, the floating ribs, have a single articular facet on the head, which is of rather large size. They have no necks or tubercles, and are pointed and their anterior ends. The eleventh has a slight angle and a shallow costal groove, whereas the does not. The twelfth rib is much shorter than the eleventh rib, and its head is inclined slightly downward.
In males, expansion of the ribcage is caused by the effects of testosterone during puberty. Thus, males generally have broad shoulders and expanded chests, allowing them to inhale more air to supply their muscles with oxygen.
Variations in the number of ribs occur. About 1 in 200-500 people have an additional cervical rib, and there is a female predominance. Intrathoracic supernumerary ribs are extremely rare. Bifid or bifurcated ribs, in which the sternal end of the rib is cleaved in two, is a congenital abnormality occurring in about 1.2% of the population. The rib remnant of the 7th cervical vertebra on one or both sides is occasionally replaced by a free extra rib called a cervical rib, which can cause problems in the nerves going to the arm.
The human rib cage is a component of the human respiratory system. It encloses the thoracic cavity, which contains the lungs. An inhalation is accomplished when the muscular diaphragm, at the floor of the thoracic cavity, contracts and flattens, while contraction of intercostal muscles lift the rib cage up and out.
Expansion of the thoracic cavity is driven in three planes; the vertical, the anteroposterior and the transverse. The vertical plane is extended by the help of the diaphragm contracting and the abdominal muscles relaxing to accommodate the downward pressure that is supplied to the abdominal viscera by the diaphragm contracting. A greater extension can be achieved by the diaphragm itself moving down, rather than simply the domes flattening. The second plane is the anteroposterior and this is expanded by a movement known as the 'pump handle.' The downward sloping nature of the upper ribs are as such because they enable this to occur. When the external intercostal muscles contract and lift the ribs, the upper ribs are able also to push the sternum up and out. This movement increases the anteroposterior diameter of the thoracic cavity, and hence aids breathing further. Finally, you have the transverse. In this situation, it involves mainly the lower ribs (some say it is the 7th-10th ribs in particular) with the diaphragm's central tendon acting as a fixed point. When the diaphragm contracts, the ribs are able to evert and produce what is known as the 'bucket handle' movement, facilitated by gliding at the costovertebral joints. In this way, the transverse diameter is expanded and the lungs can fill.
Breathing may be assisted by other muscles that can raise the ribs, such as sternocleidomastoid, pectoralis major and minor as well as the scalenes. While under most circumstances, individuals respire via eupnea, exercise and other forms of physiological stress can cause the body to require forced expiration, rather than the simple elastic recoil of the thoracic cage, lungs and diaphragm. In this case, muscles are recruited which can help depress the ribs and raise the diaphragm - such as the anterior abdominal wall muscles, excluding the transversus abdominis muscle. Latissimus dorsi can also aid deep, forced expiration.
Another way the thoracic cavity can expand during inhalation is called belly breathing. This also involves a contraction of the diaphragm, but the lower ribs are stabilized so that when the muscle contracts, rather than the central tendon remaining stable and lifting the ribs up, the central tendon moves down, compressing the sub-thoracic cavity and allowing the thoracic cavity and lungs room to expand downward.
These actions produce an increase in volume, and a resulting partial vacuum, or negative pressure, in the thoracic cavity, resulting in atmospheric pressure pushing air into the lungs, inflating them. An exhalation results when the diaphragm and intercostal muscles relax, and elastic recoil of the rib cage and lungs expels the air.
The circumference of the normal adult human rib cage expands by 3 to 5 cm during inhalation.
Rib removal is the surgical excision of ribs for therapeutic or cosmetic reasons.
Society and culture
The number of ribs as 24 (12 pairs) was noted by the Flemish anatomist Vesalius in his key work of anatomy De humani corporis fabrica in 1543, setting off a wave of controversy, as it was traditionally assumed[by whom?] from the Biblical story of Adam and Eve that men's ribs would number one fewer than women's[according to whom?].
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This article uses anatomical terminology; for an overview, see anatomical terminology.
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