The series of 1520 human skeletons at my disposal discloses separate neural arch in seventy-nine, or more than five per cent. In vertebral columns with twenty-five presacrals, the incidence is almost twice as great as in columns with the regular twenty-four, but the actual percentage in both series is an accident of the sampling rather than a criterion of the population at large.

Study of the defect, whether unilateral or bilateral, substantiates its essential origin in a skeletal anomaly and assigns a very minor rôle to trauma. Embryologically the defect falls into the category of anomalous ossifications, of which there are many other examples scattered over the skeleton.

It is an anomaly peculiar to the lumbar region, and may be found in any lumbar vertebra and on either or both sides, but is usually confined to the last presacral. Thus spondylolisthesis may be a sign of the anomaly. Trauma plays a secondary rô1e in this condition, finding the fibrous tissue a point of less resistance to strain than bone structure.

Separate neural arch in its origin resembles spina bifida, defective articular processes, and separate bony centers for transverse, mammillary and accessory processes which are peculiarly associated with the lumbar column; it is found more frequently when some other anomaly—such as a twenty-fifth presacral—is present. It is the ultimate result of imperfect ossification, whether this be caused by failure in the ossific process itself or by a defect of the pre-existing cartilage.