Anatomically, we distinguish 12 pairs of ribs, seven of which constitute the true (vertebrosternal) ribs (costae vere). They are connected directly by cartilage to the sternum. The next three pairs of ribs (8–10), called false (vertebrochondral) ribs (costae spuriae), fuse with the cartilage of the ribs connected above to form the costal arch. The last two pairs of ribs (11 and 12) are called floating (vertebral, free) ribs (costae fluctuantes) because they connect neither to the other ribs nor the sternum. Although the anatomical ribs have the same elements, due to their location in the chest, their geometry changes. The upper ribs (1 and 2) of the thorax are short and the shafts are horizontal. The ribs on the lower thorax are long and vertical. The ribs are made of a thin layer of dense bone and spongy bone substance rich in red bone marrow.