I assume that a sheath is the beginning of a bud that's coming out of the leaf which produces the blooms. But yours looks too thick to be just a leaf forming. Here are a couple of pix for reviews. The first one looks like just a leaf coming out, too thin to be a bud. The other two are self explanatory.
BTW: 640 x 480 pixels seems to be a good size for pictures to be posted here. Doesn't take up the whole screen.