Skeleton of the Thorax Anatomy
The skeleton of the thorax consists of 12 thoracic vertebrae, the 12 pairs of ribs and their costal cartilages, and the sternum (Fig. 2.5). Structures in continuity between the root of the neck and the upper part of the thoracic cavity pass through the superior thoracic aperture (thoracic inlet), which is bounded by the first thoracic vertebral body, the first pair of ribs and costal cartilages and the upper border of the sternum. The inferior thoracic aperture (thoracic outlet), through which structures pass between the thoracic and abdominal cavities, is formed by the twelfth thoracic vertebral body, the twelfth and eleventh ribs and the costal margin (the fused costal cartilages of the seventh to the tenth ribs inclusive).
Although the ribs differ in size and shape, most (2–9 inclusive) have features in common and are described as typical ribs (Fig. 2.6). Each typical rib consists of a head, neck, tubercle, shaft, upper and lower borders and inner and outer surfaces. The heads of the ribs are those parts that articulate with the thoracic vertebral bodies. The lower part of the head forms a synovial joint with its own vertebral body, while the upper part articulates with the vertebra above. The intermediate part of the head lies against the intervertebral disc. The neck of the rib connects the head and the tubercle and lies in front of the transverse process. The tubercle of the rib faces posteriorly and the medial part of its surface forms a synovial joint with the articular facet on the transverse process of the corresponding vertebra. The shaft forms the remainder of the rib and ends anteriorly at a shallow depression, which receives the costal cartilage. Passing laterally from the tubercle, the shaft slopes downwards and backwards before turning forwards and outwards to form the angle. Lateral to the angle, the shaft possesses a sharp lower border, which bounds the costal groove.
The first rib is atypical. Its head possesses an articular facet solely for its own vertebral body. The shaft is short and broad and has superior and inferior surfaces. In addition, its superior surface carries a ridge that forms a projection on the inner border of the rib, the scalene tubercle, to which is attached the scalenus anterior. Two grooves lie across the shaft, one in front of the ridge (for the subclavian vein) and the other behind (for the subclavian artery and lowest trunk of the brachial plexus). The tenth, eleventh and twelfth ribs are also atypical, in that each head pos- sesses a single facet and the rib is usually devoid of a tubercle or an angle.
All ribs possess costal cartilages, and those of the upper seven pairs (true ribs) articulate with the sides of the sternum. Pairs 8–12 (false ribs) fall short of the sternum. These articulate with the cartilage immediately above, while 11 and 12 (floating ribs) are pointed and end freely in the muscle of the abdominal wall.
The sternum is a flat bone and consists of the manubrium, the body (Fig. 2.7) and the xiphoid process. The manubrium articulates with the medial end of each clavicle at the sternoclavicular joint and with the first costal cartilage. Its upper margin includes the jugular notch, which forms part of the superior thoracic aperture. A palpable secondary cartilaginous joint (the manubriosternal joint) unites the manubrium and body and forms a useful guide to the second costal cartilage, which abuts the sternum at the lateral margin of the joint. The lateral margins of the body of the sternum are indented by the medial ends of the second to the seventh costal cartilages. The xiphoid process lies in the subcostal angle and projects downwards and backwards from the body of the sternum.