The ornamental design for a cannula, as shown and described.
FIG. 1 is a front elevation of a cannula, showing our new design, the side
of the cannula opposite from that of FIG. 1 being identical with what is
shown in FIG. 1;
FIG. 2 illustrates the cannula of FIG. 1 as seen from the right of FIG. 1,
the cannula as viewed from the left of FIG. 1 being identical with what is
shown in FIG. 2;
FIG. 3 is a top plan view of the cannula of FIG. 1; and
FIG. 4 is a bottom plan view of the cannula of FIG. 1.