Spina bifida is a congenital disorder in which there is a defective closure of the vertebral arch.

http://en.wikipedia.org/wiki/Human_vertebral_column

Clinical significance
Disease

Spina bifida is a congenital disorder in which there is a defective closure of the vertebral arch. Sometimes the spinal meninges and also the spinal cord can protrude through this, and this is called Spina bifida cystica. Where the condition does not involve this protrusion it is known as Spina bifida occulta. Sometimes all of the vertebral arches may remain incomplete. Another, though rare, congenital disease is Klippel-Feil syndrome which is the fusion of any two of the cervical vertebrae.