Lines Between Functions
Strictly speaking, lines do not exist in nature. A line is a metaphor for a division between unlike areas. (In aggregate, such as in hatching or cross-contour modelling, they can take on other uses.) Thus the artist is free to put them wherever differences exist.
Artists will sometimes emphasize lines between groups of muscles whose actions differ or oppose: between the flexors (biceps, brachialis, coracobrachialis) and extensors (triceps) on the upper arm, or between the extensors (quadriceps group and sartorius) and adductors (adductor longus and its neighbors) on the thigh. These divisions correspond to those between the masses as described above, and they may provide opportunities to reinforce the apparent action of muscles in the drawing.