In this condition the arches of the vertebrae are usually more or less incomplete, and there is frequently a tumour projecting whose internal cavity communicates with the spinal canal. But this tumour is absent in certain forms, and the condition may be divided into those cases in which the vertebral canal is open without tumour, and those in which there is a tumour. The condition is commonest in the lumbar region and next to that in the cervical, these being the seats of sharp curvatures in the embryo.
Fig. 9. - Cyclopia. Deformed nose projects above orbit. (Glasgow Hunterian preparation).
This form is strictly comparable with anencephalus, with which it is often associated. The medullary canal has never been completed or an early rupture has occurred. Accordingly the integuments have not been carried to the middle line behind, and the arches of the vertebrae are wanting, so that the vertebral canal is exposed, covered only with a membrane. This membrane is continuous laterally with the skin. The exposed canal does not even form a gutter; it is flattened out and shows at most a slight groove, but 1s frequently convex posteriorly. The surface of the membrane represents the internal surface of the medullary canal, that is to say, the central canal of the spinal cord, and the cord itself, to the extent of the lesion of the spine, is absent or present as a mere trace, like the brain in anencephalus. In a case described by Cleland the membranous surface was continuous with a dilated central canal, thus proving that the former is really the open central canal. Although the cord is absent, the spinal nerves are present, arising from the membrane in an inner and an outer series, representing the anterior and posterior roots.
Rhachischisis is often associated with anencephalus, in which case it affects the upper part of the spine or its whole length, the condition being inconsistent with life. On the other hand, it may affect a limited area usually at the lower part of the column, being produced by a local rupture or defect of the medullary canal. In that case we may have a persistent dilatation of the central canal of the cord, and perhaps a hydrocephalus.
In this case there is a persistent dropsy with protrusion of a portion of the sac, a condition comparable with hydrencephalocele. The dropsy may be of the central canal of the cord (syringocele or myelocystocele), or merely of the meninges (meningocele), or the cord may be carried outwards with the protrusion (myelomeningocele). In the syringocele or syringomyelocele the central canal is expanded so as to form the internal lining of the sac, and the spinal cord may be atrophied or partially preserved. The nerve roots arise in front of the sac. In the meningocele and myelomeningocele the fluid is in the subarachnoid or arachnoid space, usually the latter, and, according to its seat in relation to the circumference of the cord, will be the condition of the cord itself and the' nerve roots. The nerves may lie in front of the sac, or may arise within it and course in its walls.
Spina bifida with tumour may occur in any part of the spine, but is most common in the lumbo-sacral region. (See under Affections of the Nervous System).