Beginners often look for veins. Professionals look for tendons. Specifically, the Extensor digitorum tendons on the back of the hand become visible only when the fingers are slightly spread or extended. The verified PDF uses form-annihilation (showing the model in half-shadow) to highlight these tendon ridges.
The arm is rarely a straight cylinder. It is defined by the interplay of two primary muscle groups that spiral around the humerus. arm and hand in motion by anatomy for sculptors pdf verified