Which term describes a line that runs directly down the center of the body?

The term that describes a line running directly down the center of the body is "midline." This anatomical concept refers to an imaginary line that divides the body into equal right and left halves, providing a reference point for understanding body symmetry and the location of structures in relation to each other. The midline is crucial in anatomy for identifying regions, understanding movement patterns, and establishing a baseline for various anatomical orientations used in clinical practice.

The other terms provided refer to specific anatomical landmarks or lines that do not represent the central division of the body. For instance, anterior axillary and mid axillary lines are used to describe points on the torso relative to axillary (armpit) structures, while posterior axis isn't a standard anatomical term recognized in the same way. Hence, the midline is the most accurate descriptor for a central line through the body.