The human posterior commissure

AbstractMidline cross‐sections of the posterior commissures of five adult males were used for a determination of their cross‐sectional area and fiber‐density. Due to non‐uniform fiber‐size distribution a stratified sample involving from 12–20 areas was collected from each case. A mean cross‐sectional area of 3,587 mm2 with a standard variation of ±0.820 mm2 was measured for the five cases. Fiber density was found to vary in the different strata between 86,400 to 432,000 ± 10% per mm2 on axon counts taken from sections stained by Häggqvist's method. Counting myelin sheaths on sections stained by Weigert's method fiber density varied from 40,000 to 158,400 fibers per mm2 ± 30%. Using these values for calculating a total fiber number estimate, based on the axon count, fiber number varies from 504,566 to 887,849 fibers. Comparing this total fiber estimate with a similar estimate taken from the anterior commissure, the figure appears low. This difference is due to the presence of a great number of large diameter fibers in the posterior commissures, not found in either the anterior commissure or the corpus callosum. Attaching a probability level of t0.05 to this sample of five cases, a h