Wood is composed of a variety of longitudinal cells (those running parallel to the axis of the trunk or limb) and ray cells (those running perpendicular to that axis, from the center of the trunk to the edge).
Longitudinal cells produce the regular ring pattern (you can see them aligned vertically in the fingerboard photo). Rays (also called medullary rays, vascular rays, or pith rays) are producing the darker squiggles.
The ray cells help conduct sap and other materials radially in the trunk, between the center and the bark.
Different types of wood produce more or less prominent rays, with different characteristics. Different cuts of wood also make the rays more or less visible.