median furrow
Also known as posterior median furrow or median longitudinal furrow, it's the vertical line present in the middle of the back of some individuals, specially the ones with low body fat percentage or a well-developed back musculature. This tag is better used when the back line is clearly visible, as in the examples below. For the front body equivalent, see linea alba.
Examples
post #4306850
post #4346308
post #4299162
post #5427453
External links
Basic medical key - Surface anatomy of the back and vertebral leves of clinically important structures
Vhdissector - Back surface anatomy