Thorax is a term that is used to describe the bones of the rib cage, the fascia and muscles that attach to the rib cage, the visceral organs within the rib cage, and even the skin that covers the rib cage. The rib cage, also called the thoracic cage or the bony thorax, consists of the thoracic vertebrae, the ribs, and the sternum (Fig. 5–1). The rib cage provides a foundation for many of the muscle attachments of the upper extremities, head and vertebral column, and pelvis. The rib cage also provides protection for the heart, lungs, and viscera. Therefore, there needs to be a certain amount of inherent stability to the thorax. The structure of the rib cage significantly increases the stability of the thoracic spine during flexion/extension, lateral bending, and rotation.1,2 One of the important functions of the chest wall is its role in ventilation. The process of ventilation, including inhalation/exhalation(inspiration/expiration), depends on the mobility of the rib cage and the ability of the muscles of ventilation to move it.3
Anterior view of the thorax including its component parts: the sternum, 12 pairs of ribs (R1–R12), the costocartilages, and the thoracic vertebrae.
Structure and Function of the Thorax
Structure of the Rib Cage
The rib cage forms a closed chain that involves many joints and muscles. The anterior border of the rib cage is the sternum, the lateral borders are the ribs, and the posterior border is formed by the thoracic vertebrae. The superior border of the rib cage is formed by the manubrium of the sternum, by the superior borders of the first costal cartilages, and by the first ribs and their contiguous first thoracic vertebra. The inferior border of the rib cage is formed by the xiphoid process, the shared costal cartilages of ribs 7 through 10, the inferior portions of the 11th and 12th ribs, and the 12th thoracic vertebra (see Fig. 5–1).
The sternum is an osseous protective plate for the heart and is composed of the manubrium, body of the sternum, and xiphoid process (Fig. 5–2). The manubrium and the body of the sternum form a posteriorly concave angle of approximately 160°. The xiphoid process often angles posteriorly from the distal end of the body of the sternum and therefore may be difficult to palpate.
The sternum. A. The sternum is composed of the manubrium, the body of the sternum, and the xiphoid process. The costal notches for the chondrosternal joints are also evident in this anterior view. B. Viewed laterally, the dorsal (posterior) concavity of the manubrium and sternum is evident.