The shadow is a vital organ. Although seemingly independent from the majority of biological functions, it is responsible for the most important immaterial process after consciousness- maintaining the borders of the physical body. While the rest of the organs sleep, it serves as a sort of geometric mold, pressurizing the self into its recognizable shape.
The fact that the shadow is capable of this is a testament to its surprising complexity. Despite seeming to only be a two-dimensional cross section of the body at any given time, it contains a blueprint of the complete, three-dimensional body within its origami-like folds. Once thoroughly unfurled and dissected, it resembles a pop-up book of the entire inner self.
For a time, it was speculated that severing and consuming the shadows of animals would be a more ethical practice than butchering them for meat. Removal seemed to be harmless under short-term observation, and caused no apparent pain to its subjects. However, most creatures could only actually survive for a matter of months without; eventually, those tested crumbled to dust in a sudden burst of entropy.
As for humans, in the event that their shadows are severed, it is possible for them to survive by receiving an umbral transplant; however, the new organ slowly reshapes the entirety of its recipient’s being over the course of several months. Eventually, the transplantee becomes indistinguishable in body and mind from the donor whose shadow they inherited, right down to the wrinkles on their brain.