Final answer:
Ribs 11 and 12 are known as floating ribs because they do not attach to the sternum or to another rib. They terminate within the musculature of the lateral abdominal wall.
Step-by-step explanation:
Ribs 11 and 12 are distinct in the human body due to their classification as "floating ribs." Unlike most ribs that directly connect to the sternum or breastbone, these ribs are not attached anteriorly. Instead, they are connected only to the thoracic vertebrae in the back, making them free-floating at their anterior ends.
The absence of a direct attachment to the sternum distinguishes these ribs from others in the ribcage. While ribs 1-7 are classified as true ribs (directly connected to the sternum), and ribs 8-10 are considered false ribs (indirectly connected to the sternum via cartilage), ribs 11 and 12 have no sternal attachment. Their unique structure allows for a greater degree of flexibility in the lower thoracic region, accommodating movements such as breathing and trunk rotation.