Fig. 6. Box plots of the volume of distributions (milliliters). The top line of the box represents the 75th percentile, the bottom line represents the 25th percentile, and the “middle” line represents the 50th percentile or the median. The opioids are arranged along the x -axis in the order of increasing lipophilicity. Different y -axis scales are used for the three volumes. Vepifatis the sum of Vepiand Vfat, which allows a fair comparison of epidural distribution volume between morphine and the other opioids, because Vfatfor morphine was considered negligible.