1. A differential analysis of factors related to the separate neural arch includes incidence in relation to age, ethnic group, and sex.
2. In a group of 4,200, there were 178 skeletons which presented neural-arch separations, establishing an over-all incidence of 4.2 per cent. On the basis of ethnic group and sex, the incidence is as follows: white males, 6.4 per cent.; Negro males, 2.8 per cent.; white females, 2.3 per cent.; and Negro females, 1.1 per cent
3. The frequency of occurrence of the separate neural arch is shown to remain almost constant regardless of age in those age periods (adult life) represented by this skeletal material.
4. In this series of columns with isthmal separations the coincident bone variations which had an incidence significantly different from determination in the normal were as follows: Spina bifida of the fifth lumbar vertebra was approximately four and one-half times as frequent as is normally expected; the open sacrum was approximately twice as frequent; articulation of the transverse process of the fifth lumbar vertebra was approximately one-third as frequent; accessory sacro-iliac articulations were approximately one-half as frequent.
5. Those coincident bone variations studied in this series which had an incidence essentially the same as that found in unselected bones were as follows: atypical number of presacral vertebrae, cervical ribs, spina bifida of vertebrae other than the fifth lumbar, spina bifida of the upper sacral segments.
6. Arthritic involvement of the lumbar vertebral articular processes increased in frequency and severity with increasing age, but was absent to gross inspection in over two-thirds of those columns showing isthmal separations.