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During routine autopsies in 11 cadavers, the intact dural sac, including its nerve roots, was circumferentially constricted by a clamp while the pressure under the clamp among the nerve roots was recorded simultaneously. The first sign of a pressure increase—the critical size—occurred at a cross-sectional area of the cauda equina of 77 ± 13 mm2. To achieve a pressure increase of 50 mm Hg, the cross-sectional area of the cauda equina had to be further constricted at an average of 19 ± 8%. The corresponding constriction needed to achieve a pressure of 100 mm Hg was 26 ± 8%. The results indicated that constriction of the cauda equina to a size less than 75 mm2 probably will affect the normal function of the nerve roots of the cauda.