Final answer:
The arm can clear the thoracic cage without problems because of the ribcage's flexibility and the shoulder girdle's range of motion, which allow for a wide range of movement while protecting vital organs.
Step-by-step explanation:
The question pertains to why there is generally no problem in the arm clearing the widest dimension of the thoracic cage. This is due to the anatomical structure and flexibility of the thoracic cage, as well as the range of motion of the shoulder girdle. The thoracic cage is composed of 12 pairs of ribs attached to the sternum and the thoracic vertebrae, encasing and protecting vital organs such as the heart and lungs. The shoulder girdle is supported by this structure, allowing the arms a wide range of motion without interference from the ribcage's dimensions. The ribs are flexible due to the costal cartilages, and the articulation of the shoulder allows the arm to move freely in clearance of the thorax. Additionally, the structure of the ribs, consisting of true ribs (1-7), false ribs (8-12), and floating ribs (11-12), provide protection while enabling significant flexibility for various movements, including the lifting and rotation of the arms.