Figure 1. Dye distribution histograms with a 25-gauge Whitacre needle directed caudally at slow (left plot, two trials) and fast (right plot, three trials) injection rates. The abscissa is the distance along the model measured in centimeters. The injection site is zero (equivalent to the L3/L4 interspace), negative direction is caudal, and positive direction is cephalad. The ordinate is the mean fraction of the injectate.