The ornamental design for a cannula, as shown and described.
FIG. 1 is a perspective view of a cannula showing our new design.
FIG. 2 is a front view thereof, the rear view being a mirror image thereof.
FIG. 3 is a right side view thereof, the left side view being a mirror image thereof.
FIG. 4 is a top view thereof.
FIG. 5 is a bottom view thereof.
FIG. 6 is a perspective view of a second embodiment of a cannula showing our new design.
FIG. 7 is a front view thereof, the rear view being a mirror image thereof.
FIG. 8 is a right side view thereof, the left side view being a mirror image thereof.
FIG. 9 is a top view thereof; and,
FIG. 10 is a bottom view thereof.
The broken line showing of parts of the drawings is included for the purpose of illustrating use and environment and forms no part of the claimed design.