As already indicated, there are various degrees of this defect. It is important to bear in mind that we may have a more or less complete absence of the bone and cutaneous structures which cover in the nervous system, so that the imperfect remains of the latter are exposed. In this case we speak of the lesion as open, and, if the condition is present in the head we use the term Cranioschisis, and if it be in the spinal canal, Rhachischisis. On the other hand, the cutaneous coverings may be virtually complete, although the bones are usually defective. In this case the lesion is closed. There is commonly in these forms a tumour-like protrusion of the nervous structures or their membranes, a condition expressed by such terms as Encephalocele, Syringocele, etc. Besides these, there are less regular defects, some of which owe their • origin to adhesion or constriction of the amnion.