MEDICAL HYPERPIGMENTATION - BLASCHKO LINES
Lines of Blaschko are lines of normal cell development in the skin. These lines are invisible under normal conditions. They become apparent when some diseases of the skin manifest themselves according to these patterns. They follow a "V" shape over the back, "S" shaped whirls over the chest, and sides, and wavy shapes on the head.
The lines are believed to trace the migration of embryonic cells. They do not correspond to nervous, muscular, or lymphatic systems. The lines can be observed in other animals such as cats, dogs and rabbits.