What is the definition of "midline" in anatomy?

The term "midline" in anatomy refers to an imaginary line that divides the body into left and right sections. This line is crucial for understanding anatomical positions and movements, as it provides a reference point for describing locations of structures in relation to one another.

For instance, when considering bilateral symmetry in the human body, many structures are mirrored on either side of the midline, making it essential for anatomical terminology such as medial and lateral, which describe positions relative to the midline. Understanding this concept helps in assessing conditions, planning treatment, and evaluating movement patterns in both healthy and injured individuals.

The other options present concepts that, while important in anatomy, do not accurately describe the midline. An external reference line dividing the body into upper and lower parts refers to the transverse plane, while center of mass discusses the distribution of mass in the body rather than division. Lastly, the line marking the border of motion in a range of motion test is related to movement assessment, which is distinct from the anatomical definition of midline.