Which bone is found at the back of the rib-cage?

The scapula, commonly known as the shoulder blade, is located in the upper back and plays a critical role in the structure of the rib cage. It is positioned posteriorly, providing attachment points for muscles and connecting the arm to the trunk. This bone assists in the movement and stabilization of the upper extremities and protects the underlying thoracic organs by serving as a structural element of the back.

In the context of the rib cage, which is primarily composed of the ribs, sternum, and thoracic vertebrae, the scapula is important as it is positioned at the back, allowing for the attachment of muscles that contribute to the expansion and contraction of the rib cage during respiration. This anatomical relationship highlights its function not only as a supportive structure but also as a facilitator of upper body mobility.

The other choices, while relevant in their right contexts, do not pertain to the rib cage in this manner. The acromion is a bony process on the scapula itself and does not represent a separate bone, the sacrum is part of the pelvis located further down the spine, and the pectoral refers generally to the chest area rather than a specific bone structure in this context.