The term spina bifida refers, strictly speaking, to a developmental defect of the spinal column in which the arches of one or more of the spinal vertebrae have failed to fuse together so that the spine is bifid. The term cranium bifidum refers to a defect in the fusion of the bones of the skull, usually at the back of the head, which allows soft tissues to herniate. The large muscles on the suface of the body which move the limbs and keep them in place normally work in pairs, by a process called reciprocal innervation. In spina bifida myelomeningocele it is frequently damage to the latter which plays the main part in urinary incontinence. The state of each child's urinary tract will be kept under regular surveillance by means of a number of investigations, the most important of which is the intravenous pyelogram (IVP). Ejaculation is much less often achieved by paraplegic men than is erection.