蛇目砂

The janomenosuna is The snake eye of sand: it's the area of carefully swept sand around the shobudawara, which is the ring surrounding the fighting area on a Japanese sumo dohyo.

Since the shobudawara marks the area inside which the sumotori (sumo wrestlers) must stay while they fight, it is very important for the five shimpan (judges) and the gyoji (referee) to be able to tell if one sumotori has touched the outside of the ring. Even if it's only the tip of a toe it still counts, and will lose the wrestler his bout. Thus the janomenosuna is swept, repeatedly, before each bout in order to make any little mark made by an errant toe or heel - or hand - stand out.

During the period of 1897 - 1931 the janome was surrounded by another ring of tawara, the almost-buried bales of straw that make up the shobudawara. I'm guessing the practice was discontinued because the extra ring made building the dohyo unnecessarily intricate. A traditional dohyo had/has no tawara around the janome, and sumo is all about tradition.