The rib cage, as an enclosure that comprises the ribs,

vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordate ...

and

sternum The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood vessels from injury. Sha ...

in the

thorax The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the cre ...

of most

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...

s, protects

vital organ In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a ...

s such as the

heart The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to t ...

, lungs and

great vessels Great vessels are the large vessels that bring blood to and from the heart. These are: *Superior vena cava *Inferior vena cava *Pulmonary arteries * Pulmonary veins *Aorta Transposition of the great vessels is a group of congenital heart defec ...

. The sternum, together known as the thoracic cage, is a semi-rigid bony and

cartilaginous Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck a ...

structure which surrounds the

thoracic cavity The thoracic cavity (or chest cavity) is the chamber of the body of vertebrates that is protected by the thoracic wall (rib cage and associated skin, muscle, and fascia). The central compartment of the thoracic cavity is the mediastinum. There ...

and supports the shoulder girdle to form the

core Core or cores may refer to: Science and technology * Core (anatomy), everything except the appendages * Core (manufacturing), used in casting and molding * Core (optical fiber), the signal-carrying portion of an optical fiber * Core, the centra ...

part of the human skeleton. A typical human thoracic cage consists of 12 pairs of ribs and the adjoining

costal cartilage The costal cartilages are bars of hyaline cartilage that serve to prolong the ribs forward and contribute to the elasticity of the walls of the thorax. Costal cartilage is only found at the anterior ends of the ribs, providing medial extension. ...

s, the sternum (along with the manubrium and

xiphoid process The xiphoid process , or xiphisternum or metasternum, is a small cartilaginous process (extension) of the inferior (lower) part of the sternum, which is usually ossified in the adult human. It may also be referred to as the ensiform process. Bo ...

), and the 12

thoracic vertebra In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebrae and they are intermediate in size between the cervical ...

e articulating with the ribs. Together with the

skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different de ...

and associated fascia and muscles, the thoracic cage makes up the

thoracic wall The thoracic wall or chest wall is the boundary of the thoracic cavity. Structure The bony skeletal part of the thoracic wall is the rib cage, and the rest is made up of muscle, skin, and fasciae. The chest wall has 10 layers, namely (from sup ...

and provides attachments for extrinsic skeletal muscles of the neck,

upper limb The upper limbs or upper extremities are the forelimbs of an upright-postured tetrapod vertebrate, extending from the scapulae and clavicles down to and including the digits, including all the musculatures and ligaments involved with the shou ...

upper abdomen In anatomy, the epigastrium (or epigastric region) is the upper central region of the abdomen. It is located between the costal margins and the subcostal plane. Pain may be referred to the epigastrium from damage to structures derived from the f ...

and

back The human back, also called the dorsum, is the large posterior area of the human body, rising from the top of the buttocks to the back of the neck. It is the surface of the body opposite from the chest and the abdomen. The vertebral column runs ...

. The rib cage intrinsically holds the

muscles of respiration The muscles of respiration are the muscles that contribute to inhalation and exhalation, by aiding in the expansion and contraction of the thoracic cavity. The diaphragm and, to a lesser extent, the intercostal muscles drive respiration during ...

( diaphragm,

intercostal muscles Intercostal muscles are many different groups of muscles that run between the ribs, and help form and move the chest wall. The intercostal muscles are mainly involved in the mechanical aspect of breathing by helping expand and shrink the size of ...

, etc.) that are crucial for active inhalation and forced

exhalation Exhalation (or expiration) is the flow of the breath out of an organism. In animals, it is the movement of air from the lungs out of the airways, to the external environment during breathing. This happens due to elastic properties of the lungs, ...

, and therefore has a major ventilatory function in the

respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies ...

Structure

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

e 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

respectively. The first seven rib pairs known as the ''fixed'' or vertebrosternal ribs are the ''true ribs'' ( la, costae verae) as they connect directly to the sternum; the next five pairs (eighth to twelfth) are the ''false ribs'' ( la, costae spuriae). The false ribs include both vertebrochondral ribs and vertebral ribs. There are three pairs of vertebrochondral ribs (eighth to tenth) that connect indirectly to the sternum via the costal cartilages of the ribs above them. Their elasticity allows rib cage movement for respiratory activity. The phrase ''floating rib'' or vertebral rib ( la, costae fluctuantes) refers to the two lowermost, the eleventh and twelfth rib pairs; so-called because they are attached only to the

vertebrae The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...

–and not to the sternum or cartilage of the sternum. These ribs are relatively small and delicate, and include a cartilaginous tip. The spaces between the ribs are known as

intercostal space The intercostal space (ICS) is the anatomic space between two ribs (Lat. costa). Since there are 12 ribs on each side, there are 11 intercostal spaces, each numbered for the rib superior to it. Structures in intercostal space * several kind ...

s; they contain the intercostal

muscles Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of musc ...

, and neurovascular bundles containing

nerves A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...

arteries An artery (plural arteries) () is a blood vessel in humans and most animals that takes blood away from the heart to one or more parts of the body (tissues, lungs, brain etc.). Most arteries carry oxygenated blood; the two exceptions are the pu ...

, and veins.

Parts of rib

Each rib consists of a head, neck, and a shaft. All ribs are attached posteriorly to the

thoracic vertebrae In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebrae and they are intermediate in size between the cervical ...

. 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 The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...

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 The transverse costal facet (or transverse costal fovea) is one of the costal facets, a site where a rib forms a joint with the transverse process of a thoracic vertebra In vertebrates, thoracic vertebrae compose the middle segment of the verte ...

with the tubercle of the rib of the same number. The crest gives attachment to the intra-articular ligament.http://www.teachmeanatomy.com/osteology-of-the-thorax/ 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 A costotransverse ligament is a short fibrous band that connects a rib with the transverse process of vertebra The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their ...

; 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

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

. 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

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 The scalene tubercle is a small projection that runs along the medial border of the first rib between two grooves, which travel anteriorly for the subclavian artery and posteriorly for the subclavian vein. It projects outward medially, and is the ...

, 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 The brachial plexus is a network () of nerves formed by the anterior rami of the lower four cervical nerves and first thoracic nerve ( C5, C6, C7, C8, and T1). This plexus extends from the spinal cord, through the cervicoaxillary canal in t ...

. 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

anteriorly (at the front). Posteriorly, the second rib is connected with the

by the second thoracic vertebra. The second rib is much longer than the

first rib The rib cage, as an enclosure that comprises the ribs, vertebral column and sternum in the thorax of most vertebrates, protects vital organs such as the heart, lungs and great vessels. The sternum, together known as the thoracic cage, is a semi ...

, 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.Moore, Dalley & Agur. 2009. ''Clinically Oriented Anatomy'', 6th Edition. 90 Pp. Lippincott, Williams & Wilkins, , The ninth rib has a frontal part at the same level as the first lumbar vertebra. This level is called the

transpyloric plane The transpyloric plane, also known as Addison's plane, is an imaginary horizontal plane, located halfway between the suprasternal notch of the manubrium and the upper border of the symphysis pubis at the level of the first lumbar vertebrae, L1. It ...

, since the

pylorus The pylorus ( or ), or pyloric part, connects the stomach to the duodenum. The pylorus is considered as having two parts, the ''pyloric antrum'' (opening to the body of the stomach) and the ''pyloric canal'' (opening to the duodenum). The ''pylori ...

is also at this level.Bålens ytanatomi (surface anatomy). Godfried Roomans, Mats Hjortberg and Anca Dragomir. Institution for Anatomy, Uppsala. 2008. 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. Skeleton woman back

The eleventh and twelfth ribs, 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 at their anterior ends. The eleventh has a slight angle and a shallow costal groove, whereas the twelfth does not. The twelfth rib is much shorter than the eleventh rib, and only has a one articular facet.

Sternum

The sternum is a long,

flat bone Flat bones are bones whose principal function is either extensive protection or the provision of broad surfaces for muscular attachment. These bones are expanded into broad, flat plates,''Gray's Anatomy'' (1918). (See infobox) as in the cranium ...

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

transversus thoracis The transversus thoracis muscle (), also known as triangularis sterni, lies internal to the thoracic cage, anteriorly. It is usually a thin plane of muscular and tendinous fibers, however on athletic individuals it can be a thick 'slab of meat', ...

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 Testosterone is the primary sex hormone and anabolic steroid in males. In humans, testosterone plays a key role in the development of male reproductive tissues such as testes and prostate, as well as promoting secondary sexual characteristi ...

during puberty.Testosterone causes expansion of rib cage during puberty as one of secondary sex characteristics. Thus, males generally have broad shoulders and expanded chests, allowing them to inhale more air to supply their muscles with oxygen. C7ribMark

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 (

) 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

The human rib cage is a component of the human

. 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 the contraction of

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. The third, transverse, plane is primarily expanded by the lower ribs (some say it is the 7th to 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 (meaning turn outwards or inside out) 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. The circumference of the normal adult human rib cage expands by 3 to 5 cm during inhalation.

Clinical significance

Rib fracture A rib fracture is a break in a rib bone. This typically results in chest pain that is worse with inspiration. Bruising may occur at the site of the break. When several ribs are broken in several places a flail chest results. Potential complicatio ...

s are the most common injury to the rib cage. These most frequently affect the middle ribs. When several adjacent ribs incur two or more fractures each, this can result in a flail chest which is a life-threatening condition. A dislocated rib can be painful and can be caused simply by coughing, or for example by trauma or lifting heavy weights. One or more costal cartilages can become inflamed – a condition known as

costochondritis Costochondritis, also known as chest wall pain syndrome or costosternal syndrome, is a benign inflammation of the upper costochondral (rib to cartilage) and sternocostal (cartilage to sternum) joints. 90% of patients are affected in multiple ri ...

; the resulting pain is similar to that of a heart attack. Abnormalities of the rib cage include

pectus excavatum Pectus excavatum is a structural deformity of the anterior thoracic wall in which the sternum and rib cage are shaped abnormally. This produces a caved-in or sunken appearance of the chest. It can either be present at birth or develop after pubert ...

("sunken chest") and pectus carinatum ("pigeon chest"). A bifid rib is a bifurcated rib, split towards the sternal end, and usually just affecting one of the ribs of a pair. It is a

congenital defect A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities can r ...

affecting about 1.2% of the population. It is often without symptoms though respiratory difficulties and other problems can arise. Rib removal is the surgical removal of one or more ribs for therapeutic or cosmetic reasons. Rib resection is the removal of part of a rib.

Regeneration

Since the early part of the 20th century, the ability of the human rib to regenerate itself has been appreciated. However, scientific reports demonstrating repair have been sporadic and anecdotal. Currently, this phenomenon is best taken advantage of by craniomaxillofacial surgeons, who use both cartilage and bone material from the rib for jaw, face, and ear reconstruction. The perichondrium is a fibrous sheath of vascular connective tissue surrounding the rib cartilage, containing a source of progenitor stem cells required for rib regeneration.

Society and culture

The position of ribs can be permanently altered by a form of

body modification Body modification (or body alteration) is the deliberate altering of the human anatomy or human physical appearance. In its broadest definition it includes skin tattooing, socially acceptable decoration (''e.g.'', common ear piercing in many so ...

called

tightlacing Tightlacing (also called corset training) is the practice of wearing a tightly-laced corset. It is done to achieve cosmetic modifications to the figure and posture or to experience the sensation of bodily restriction. History Corsets were first ...

, which uses a corset to compress and move the ribs. The ribs, particularly their sternal ends, are used as a way of estimating age in

forensic pathology Forensic pathology is pathology that focuses on determining the cause of death by examining a corpse. A post mortem examination is performed by a medical examiner or forensic pathologist, usually during the investigation of criminal law cases ...

due to their progressive ossification.

Biblical Story

The number of ribs as 24 (12 pairs) was noted by the

Flemish Flemish (''Vlaams'') is a Low Franconian dialect cluster of the Dutch language. It is sometimes referred to as Flemish Dutch (), Belgian Dutch ( ), or Southern Dutch (). Flemish is native to Flanders, a historical region in northern Belgium; ...

anatomist Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having it ...

Vesalius Andreas Vesalius (Latinized from Andries van Wezel) () was a 16th-century anatomist, physician, and author of one of the most influential books on human anatomy, ''De Humani Corporis Fabrica Libri Septem'' (''On the fabric of the human body'' '' ...

in his key work of anatomy ''

De humani corporis fabrica ''De Humani Corporis Fabrica Libri Septem'' (Latin, lit. "On the fabric of the human body in seven books") is a set of books on human anatomy written by Andreas Vesalius (1514–1564) and published in 1543. It was a major advance in the history ...

'' in 1543, setting off a wave of controversy, as it was traditionally assumed from the Biblical story of

Adam and Eve Adam and Eve, according to the creation myth of the Abrahamic religions, were the first man and woman. They are central to the belief that humanity is in essence a single family, with everyone descended from a single pair of original ancestors. ...

that men's ribs would number one fewer than women's. This false belief is still commonly believed today. However, thirteenth or “cervical rib” occurs in 1% of humans and this is more common in females than in males.

Other animals

In herpetology, costal grooves refer to lateral indents along the integument of

salamander Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All t ...

s. The grooves run between the

axilla The axilla (also, armpit, underarm or oxter) is the area on the human body directly under the shoulder joint. It includes the axillary space, an anatomical space within the shoulder girdle between the arm and the thoracic cage, bounded superior ...

to the

groin In human anatomy, the groin (the adjective is ''inguinal'', as in inguinal canal) is the junctional area (also known as the inguinal region) between the abdomen and the thigh on either side of the pubic bone. This is also known as the medial comp ...

. Each groove overlies the myotomal septa to mark the position of the internal rib.J. W. Petranka. 1998. Salamanders of the United States and Canada. 587 Pp. Smithsonian Institution Press, , Birds and reptiles have bony uncinate processes on their ribs that project caudally from the vertical section of each rib. These serve to attach sacral muscles and also aid in allowing greater inspiration. Crocodiles have cartilaginous uncinate processes.

Additional images

File:Thoracic Cage with Spine - Anatomy.gif, Thoracic cage with spine File:Gray115.png, Anterior surface of sternum and costal cartilages File:Ribs labeled.png, X-ray image of a human chest, with ribs labelled File:BodyParts3D Rib cage.stl, 3D model of rib cage File:Surface projections of the organs of the trunk.png, Surface projections of the trunk, including each rib, and the costal margin File:Thoracic Cage with Both Humerii.gif, Thoracic cage with both humerii

Notes

References

* ''Orientation of the intercostal muscle fibers in the human rib cage'', Subit D., Glacet A., Hamzah M., Crandall J., Computer Methods in Biomechanics and Biomedical Engineering, 2015, 18, pp. 2064–2065 * ''Clinically Oriented Anatomy'', 4th ed. Keith L. Moore and Robert F. Dalley. pp. 62–64 * ''Principles of Anatomy Physiology'', Tortora GJ and Derrickson B. 11th ED. John Wiley and Sons, 2006. *
De Humani Corporis Fabrica
': online English translation of Vesalius' books on human anatomy.

External links

{{DEFAULTSORT:Rib cage Bones of the thorax