Varix means abnormal enlargement of veins. Varices occur when excessive pressure against the normal blood flux creates enlargement of the vein and thus insufficient functioning of its valves (venous insufficiency).
Varices are more common in the veins of the inferior limbs since the fluid column above these vessels is higher. This is the reason why people that spend much time standing (e.g., surgeons) are more susceptible to varices.
In general varices are not the apparent superficial veins that appear in the leg of varix patients. These apparent vessels are the consequences of internal varices (venous insufficiency) in the deep internal veins of the legs and they appear because the blood flux is diverted to superficial veins. (Popularly however apparent superficial veins are called varices.)
