Langer's lines
Langer's lines, sometimes called cleavage lines, is a term used to define the direction within the human skin along which the skin has the least flexibility. These lines correspond to the alignment of collagen fibers within the dermis. They were first given detailed attention in 1861 by Austrian anatomist Karl Langer (1819-1887). Though he cited the surgeon Baron Dupuytren as being the first to recognise the phenomenon. Langer punctured numerous holes at short distances from each other into the skin of a cadaver with a tool that had a circular-shaped tip, and noticed that the resultant punctures in the skin had a ellipsoidal shapes. From this testing he observed patterns, and was able to determine "line directions" by the longer axes of the ellipsoidal holes.1
Knowing the direction of Langer's lines within a specific area of the skin is important for surgical operations, particularly cosmetic surgery. Usually, a surgical cut is carried out in the direction of Langer's lines, and incisions made parallel to Langer's lines generally heal better and produce less scarring. Sometimes the exact direction of these lines are unknown, because in some regions of the body there are differences between different individuals. Also, directional changes of Langer's lines have been known to occur within the course of a person's lifetime.
Reference
1 Karl Langer, "Zür Anatomie und Physiologie der Haut. Über die Spältbarkeit der Cutis" (1861)
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