There are thirty-three vertebrae in the human vertebral column. The rib cage is associated with TH1−TH12. Ribs are described based on their location and connection with the sternum. All ribs are attached posteriorly to the
thoracic vertebrae and are numbered accordingly one to twelve. Ribs that articulate directly with the sternum are called
true ribs, whereas those that do not articulate directly are termed
false ribs. The
false ribs include the
floating ribs (eleven and twelve) that are not attached to the sternum at all.
Attachment The terms
true ribs and
false ribs describe rib pairs that are directly or indirectly attached to the
sternum respectively. The first seven rib pairs known as the
fixed or
vertebrosternal ribs are the
true ribs () as they connect directly to the sternum via their own individual
costal cartilages. The next five pairs (eighth to twelfth) are the
false ribs () or
vertebrochondral ribs, which do not connect directly to the sternum. The first three pairs of vertebrochondral ribs (eighth to tenth) connect indirectly to the sternum via the costal cartilages of the ribs above them, and the overall elasticity of their articulations allows the
bucket handle movements of the rib cage essential for respiratory activity. The phrase
floating rib () or
vertebral rib refers to the two lowermost (the eleventh and twelfth) rib pairs; so-called because they are attached only to the
vertebrae and not to the sternum or any of the costal cartilages. These ribs are relatively small and delicate, and include a cartilaginous tip. The spaces between the ribs are known as
intercostal spaces; they contain the instrinsic
intercostal muscles and the
neurovascular bundles containing
intercostal nerves,
arteries and
veins. The superficial surface of the rib cage is covered by the
thoracolumbar fascia, which provides external attachments for the
neck,
back,
pectoral and
abdominal muscles. File:vrParaPersp.gif|Human rib cage - CT scan (parallel projection (left) and perspective projection (right)) File:Costillas.png| |alt=true / fixed ribs false ribs false and floating ribs
Parts of rib Each rib consists of a head, neck, and a shaft. All ribs are attached posteriorly to the
thoracic vertebrae. They are numbered to match the vertebrae they attach to – one to twelve, from top (T1) to bottom. The head of the rib is the end part closest to the
vertebra with which it
articulates. It is marked by a kidney-shaped articular surface which is divided by a horizontal crest into two articulating regions. The upper region articulates with the
inferior costal facet on the vertebra above, and the larger region articulates with the
superior costal facet on the vertebra with the same number. The
transverse process of a thoracic vertebra also articulates at the
transverse costal facet with the tubercle of the rib of the same number. The crest gives attachment to the
intra-articular ligament. The neck of the rib is the 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 anterior
costotransverse ligament; its lower border is rounded. On the posterior surface at the neck, is an eminence—the tubercle that consists of an articular and a non-articular portion. The articular portion is the lower and more medial of the two and presents a small, oval surface for articulation with the
transverse costal facet 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 angle of a
rib (costal angle) may both refer to the bending part of it, and a prominent line in this area, a little in front of the tubercle. This line is directed downward and laterally; this gives attachment to a tendon of the
iliocostalis muscle. At this point, the rib is bent in two directions, and at the same time twisted on its long axis. The distance between the angle and the tubercle is progressively greater from the second to the tenth ribs. The area between the angle and the tubercle is rounded, rough, and irregular, and serves for the attachment of the
longissimus dorsi muscle.
Bones Ribs and vertebrae The
first rib (the topmost one) 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. File:Sobo 1909 24.png|First rib seen from above File:Gray123.png|Costal groove position on a central rib The
head is small and 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 costal facet on 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
anterior scalene; 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
medial scalene. The
under surface is smooth and without 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 the 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
posterior scalene. The internal surface, smooth, and concave, is directed downward and a little inward: on its posterior part there is a short costal groove between the ridge of the internal surface of the rib and the inferior border. It protects the intercostal space containing the
intercostal veins,
intercostal arteries, and
intercostal nerves. 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.
Sternum The sternum is a long,
flat bone that forms the front of the rib cage. The cartilages of the top seven ribs (the
true ribs) join with the sternum at the sternocostal joints. The costal cartilage of the second rib articulates with the sternum at the sternal angle making it easy to locate. The manubrium is the wider, superior portion of the sternum. The top of the manubrium has a shallow, U-shaped border called the jugular (suprasternal) notch. The clavicular notch is the shallow depression located on either side at the superior-lateral margins of the manubrium. This is the site of the
sternoclavicular joint, between the sternum and clavicle. The first ribs also attach to the manubrium. The
transversus thoracis muscle is innervated by one of the
intercostal nerves and superiorly attaches at the posterior surface of the lower sternum. Its inferior attachment is the internal surface of costal cartilages two through six and works to depress the ribs.
Development Expansion of the rib cage in males 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. The development of the rib cage is influenced by a combination of genetic and environmental factors, as well as specific stages of embryonic growth.
Genetic factors play a critical role, with specific genes regulating the formation of bones and cartilage to ensure the proper development and alignment of the ribs and sternum. During the
embryonic stage, the rib cage begins to form from the mesoderm, one of the three primary germ layers. Ribs develop from structures called somites, which later segment into vertebrae and ribs. Initially, the ribs are composed of
cartilage, which gradually ossifies into bone through a process known as endochondral ossification. As the embryo grows, the ribs elongate and differentiate into three types:
true ribs, which attach directly to the sternum;
false ribs, which connect to the sternum via cartilage; and
floating ribs, which do not attach to the sternum. Additionally,
environmental factors such as maternal health, nutrition, and exposure to certain substances can impact rib cage development. For instance, deficiencies in essential nutrients like calcium and vitamin D may hinder proper bone growth and development. Together, these genetic, developmental, and environmental influences ensure the formation of a functional rib cage.
Variation 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. 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 mechanically interfere with the nerves (
brachial plexus) going to the arm. In several ethnic groups, most significantly the Japanese, the tenth rib is sometimes a
floating rib, as it lacks a cartilaginous connection to the seventh rib. ==Function==