Just to add to Marks answer there, as he said, a Shirasaya (white scabbard/saya) is utilised to protect the blade itself, and is not a practical mounting for use. The tsuka is way too wide, for example, and there is no grip, as well as other aspects. Typically while the blade was placed in the shirasaya, though, the "regular", or "dress mountings", the koshirae, would be on display... with a wooden or bamboo "blade" in it to hold it all together. What is funniest is when people buy shirasaya mounted "swords" (they rarely are when bought like that, unless antiques) as display pieces, as that is exactly the opposite of their use!
For the record, though, the type of weapon seen in Zatoichi (and Blind Fury with Rutger Hauer, which really was just a Holywood take on Zatoichi in the first place) it not a shirasaya, it is a shikomi zue (literally: prepared stick, as in a stick with a hidden weapon 'prepared' inside it, such as a sword or knife blade, or chain, or spear tip).
