(C) Thoracic cage (Figure 20.14, A& B). (1) The bony thoracic or chest cage is formed of twelve thoracic vertebrae, twelve pairs of ribs, costal cartilage and the sternum (breast bone). (2) The first seven pairs of ribs are attached to the sternum by costal cartilages and are called true ribs. The next three pairs are attached to the cartilage of the ribs just above them, and are called false ribs. (3) The last two pairs (eleventh and twelfth) are free in front; hence called floating ribs. (4) Some individuals have an extra rib called a gorilla rib because it represents the normal condition in gorillas and chimpanzees. (5) Thoracic cage protects the heart and lungs. (6) The sternum is a long flat bone and consists of the manubrium, body and xyphoid process. Its upper end articulates with the clavicle, its margins with costal cartilages of seven pairs of ribs.

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Figure 20.14 Thoracic cage of a human