All of the following muscles are attached to the rib cage except:

The trapezius muscle is the correct answer, as it is not directly attached to the rib cage. The trapezius is a large, superficial muscle that extends from the back of the skull and the cervical and thoracic vertebrae down to the spine of the scapula. Its primary functions are to stabilize and move the shoulder blade, and it plays a major role in neck and upper back movement.

In contrast, the other muscles mentioned have connections to the rib cage. The serratus anterior is attached to the ribs and plays a critical role in scapular movement and holding the scapula against the rib cage. The intercostal muscles, located between the ribs, are responsible for facilitating the expansion and contraction of the rib cage during breathing. The internal obliques are part of the abdominal muscle group, which also has attachments related to the lower ribs, helping with trunk rotation and lateral flexion, as well as contributing to core stability.

Thus, the trapezius stands out as the muscle that does not have direct attachments to the rib cage, making it the correct choice in the context of the question.