Final answer:
Ribs 1-7 are called true ribs because their costal cartilages attach directly to the sternum. Ribs 8-12 are false ribs as their costal cartilages attach indirectly or not at all to the sternum. The last two false ribs, 11-12, are floating ribs that do not attach to the sternum.
Step-by-step explanation:
Ribs 1-7 are classified as true ribs (vertebrosternal ribs). The costal cartilage from each of these ribs attaches directly to the sternum. Ribs 8-12 are called false ribs (vertebrochondral ribs). The costal cartilages from these ribs do not attach directly to the sternum. For ribs 8-10, the costal cartilages are attached to the cartilage of the next higher rib.
Thus, the cartilage of rib 10 attaches to the cartilage of rib 9, rib 9 then attaches to rib 8, and rib 8 is attached to rib 7. The last two false ribs (11-12) are also called floating ribs (vertebral ribs). These are short ribs that do not attach to the sternum at all. Instead, their small costal cartilages terminate within the musculature of the lateral abdominal wall.