The humerus (bone of the upper arm) connects with the trunk through the shoulder joint. This joint is of ball-and-socket type and is the only joint in the body that is capable of 360 degrees rotation. Also, the special nature of this joint allows movement in all three dimensions.
The humerus interacts with the scapula (shoulder blade) which in turn interacts with the clavicle bone. All three bones are held in place through ligaments, tendons and muscles.