Chapter 19: The skeleton of the thorax
The skeleton of the thorax includes the sternum, ribs and costal cartilages, and thoracic vertebrae and intervertebral discs (fig. 19-1).
The sternum, or breast bone, is a flat bone that, from superior to inferior, consists of three parts: manubrium, body, and xiphoid process (figs. 19-1 and 19-2). It is useful to remember that the manubrium and body are approximately on the level of thoracic vertebrae 3-4 and 5 to 10, respectively (see figs. 20-8 and 21-1). Its subcutaneous location and the thinness of its compacta have made the sternum a common site for puncture by a needle to obtain marrow for study.
The upper border of the manubrium is marked by the jugular notch, which is easily palpable and is usually at the level of the T3 vertebrae. On each side of this is a notch for the medial end of the clavicle. The first costal cartilage is attached to the side of the manubrium. Below, the manubrium articulates with the body of the sternum at the sternal angle, which is marked by a palpable (and sometimes visible) transverse ridge about 5 cm below the jugular notch. The sternal angle is an important bony landmark at the T4 or 5 vertebral level. It indicates not only the manubriosternal junction but also the level of the second costal cartilages; hence it is a reference point in counting ribs. Rarely, however, the sternal angle is at the level of the third costal cartilages. The manubriosternal junction (i.e., the union of the manubrium with the body of the sternum) is usually fibrocartilaginous, but it may become ossified.
The body of the sternum, about twice as long as the manubrium, is notched on each side to receive costal cartilages 2 to 7. Transverse ridges may indicate its development from several pieces. The xiphosternal joint (i.e., the union of the body of the sternum with the xiphoid process) is usually fibrocartilaginous, but it too may become ossified. It is at the apex of the infrasternal angle and is usually at the level of thoracic vertebra 10 or 11.
The xiphoid process is a small and variable piece of hyaline cartilage that contains a bony core. It lies in the epigastric fossa, or "pit of the stomach."
The sternum develops bilaterally in the embryo, and it later ossifies from a variable number of bilateral and median centers.
There are usually 12 ribs on each side of the body (fig. 19-3). They are elongated yet flattened bones that curve inferior and anterior from the thoracic vertebrae. The ribs, as well as the costal cartilages, increase in length from the first to the seventh, and their obliquity increases from the first to the ninth. Generally, ribs 1 to 7 are connected to the sternum' by their costal cartilages and are called true ribs, whereas ribs 8 to 12 are termed false ribs. Usually, ribs 8 to 10, by means of their costal cartilages, join the costal cartilage immediately above, whereas ribs 11 and 12, which are free, are known as floating ribs. A supernumerary rib may be found in either the cervical or lumbar region. In thoracic surgery, a portion of a rib can be excised, leaving its periosteum, which later allows regeneration of the bone.
Ribs 3 to 9 have certain features in common and are known as typical ribs (figs. 19-4 and 19-5). Each has a head, neck, and shaft. The head presents two articular surface: one for the corresponding vertebral body and one for the vertebra immediately superior. The junction of the neck and shaft is marked by a tubercle, which articulates with the transverse process of the corresponding vertebra. The shaft, which is curved and twisted, presents an angle posteriorly, which indicates the lateral extent of the erector spinae and is the weakest part of the rib. The curvature of the rib is such that a person lying on his back is supported by the spinous processes and the angles of the ribs. The concave, inner surface of the shaft is marked inferiorly by the costal groove, which gives attachment to the internal intercostal muscle and shelter to the intercostal vein, artery, and nerve (from superior to inferior). The ribs ossify from a primary center for the shaft and secondary centers for the head and tubercle.
The first rib (fig. 19-6) is short and forms part of the thoracic inlet. The head articulates with the T1 vertebra, and the neck lies behind the apex of the lung. The flat upper surface faces superiorly and may present a groove for the subclavian artery and the lower trunk of the brachial plexus, anterior to which is the tubercle for the scalenus anterior muscle. Further anteriorly is a shallow groove for the subclavian vein. The first rib is difficult to palpate in vivo, but the first intercostal space can be identified immediately below the clavicle. The second rib, which is much longer than the first, is curved but not twisted. It articulates with T1 and 2 vertebral bodies and presents a tuberosity for the serratus anterior muscle.
Rib 10 usually articulates with the tenth thoracic vertebrae, only. Rib 11, which articulates only with the T11 vertebrae, has an indistinct tubercle, angle, and costal groove. Rib 12, which articulates with the T12 vertebra, is small, slender, and variable in length. The differences in length have to be kept in mind in surgical approaches to the kidney.
The costal cartilages are comprised of hyaline cartilage, which later may become ossified. They fit into depressions in the anterior ends of the ribs, and the upper seven or eight articulate with the sternum. The costal cartilages impart resiliency to the chest wall. They often become partly ossified later in life.
The thoracic vertebrae are described with the back. The superior costal facet of a typical thoracic vertebra, together with the intervertebral disc and the inferior costal facet of the vertebra immediately superior, forms a socket for the head of the corresponding rib.
19-1 What is the vertebral level of the manubrium sterni?
19-2 What lies on the upper surface of the first rib?
19-3 How are ribs counted?
Figure 19-1 The bones of the thorax. Note that the upper two and a half and the lower two and a half thoracic vertebrae are visible.
Figure 19-2 The sternum and its muscular attachments.
Figure 19-3 Photograph of the ribs.
Figure 19-4 The internal aspect of the right seventh rib. Note the slope inferior and anterior and the twist of the shaft.
Figure 19-5 The right seventh rib from inferior and posterior.
Figure 19-6 The superior surface of the first rib, showing grooves for vessels as well as muscular and ligamentous attachments. The scalenus minimus (not shown) is also inserted into the first rib.