Final answer:
Expressive lines in art refer to curved lines that add an organic and dynamic character to a work of art. Artists use these lines to represent the natural curves found in nature.
Step-by-step explanation:
Expressive lines in art refer to curved lines that add an organic and dynamic character to a work of art. These lines are often rounded and follow undetermined paths. Artists use expressive lines to represent the natural curves found in nature, such as the sinuous form of snakes or flowing hair.
For example, in the painting 'Las Meninas', we can see expressive lines in the aprons on the girls' dresses and in the dog's folded hind leg and coat pattern. In the sculpture 'Laocoon', expressive lines are present in the figures' flailing limbs and the sinuous form of the snakes.