Which movement is characterized as a downward motion of body structures?

Depression is defined as a movement that involves lowering or dropping a body part in comparison to its original position. This downward motion can be seen in various contexts, such as when the shoulders drop or when the jaw moves downward as in opening the mouth. In the case of the shoulder girdle, for instance, the action of bringing the shoulders down from a shrugged position is a classic example of depression.

In comparison, elevation refers to the upward movement of structures, flexion involves bending a joint that decreases the angle between body parts, and extension entails straightening a joint to increase the angle. Thus, depression specifically highlights the action of moving structures downward, making it the correct choice in this context.