A foramen is a natural hole or opening in a bone or other body structure, usually to allow the passage of nerves or blood vessels. The foramen magnum is the large opening in the base of the skull through which the spinal cord passes. The foramen ovale is the opening between the two atria of the fetal heart, which allows blood to flow from the right to the left side of the heart by displacing a membranous valve. The apical foramen is the small opening at the apex of a tooth. Plural: formina.