A detailed study hands under a light source and technique practice of contour lines with expression lines.
P.S: Expression Lines:
are those contour lines that vary in lightness/darkness, thickness or type based on highlights, shadows and other details. Shining a light source at a good angle can really start to make nerves, bumps, bones, and other details visible which are not normally noticeable under normal lighting.
See simple study of Hands & Feet:[link]