FIG. 16A is a plan view of another embodiment of the ridges 76 depicted in FIG. 8C. FIG. 16B is an enlarged cross-sectional view of another embodiment of the ridges 76 depicted in FIG. 8D. FIGS. 16A and 16B depict many ridges on the outer surface 160E of the dilator shaft 36E. Between adjacent ridges are splines 76F. The splines 76F form a plurality of channels 156E between the sheath body 40E and