A spina bifida is a hernia of the membranes surrounding the spinal cord through a congenital opening in the posterior aspect of the spine. Very rarely there is also a defect in the bodies of the vertebræ. The sac is always filled with cerebrospinal fluid, and often contains nerves from the cauda equina, or a portion of the cord itself. The sac is usually covered by integument, but this is frequently so thin as to be translucent. The opening may be so small that it will scarcely admit the finest probe, or it may extend from the foramen magnum to the sacrum. Extensive deficiencies are, however, very rare. The tumor is usually located in the lumbo-sacral region, although it may appear at any point along the spine. It may be large or small, pedunculated or sessile.
A meningocele is a hernia of the membranes containing fluid only; a meningo-myelocele contains