A crushing instrument for temporarily closing the lumen of the intestine prior to resection: Jenny learned that a splanchnotribe was a surgical device designed to crush the intestine and therefore obliterate its lumen (channel within a tube, such as a blood vessel, or to the cavity within a hollow organ like the intestine.

Lumen is a luminous term because it is Latin for "light", including the light that comes through a window. When a hollow organ is cut across, you can see light through the space that has been opened. So the word lumen came to mean this space.