1. An anatomical study of the bony structure of the pars interarticularis of the fourth and fifth lumbar vertebrae has been made in specimens from seven cadavers aged seventeen to sixty-seven.

2. Layers of cortical bone have been described antero-laterally and postero-medially which are thickest in the narrowest region of the pars.

3. In one specimen from a seventeen-year-old male, a healing fracture was found in the antero-lateral layer of cortical bone in the right neural arch of the fourth lumbar vertebra.

4. The stresses to which the pars is subject consist primarily of shear forces applied to the articular processes. The significance of these stresses to the etiology of spondylolysis is discussed.