Chullin 125 - Bone with Marrow
If one touches a marrow bone of a human corpse, he receives the impurity of the dead body. Actually, that is true even if the bone does not contain marrow, but since the rules below will mention specifically marrow bones, our teacher (Tanna) chose to start with it, for symmetry. As little as a barley-corn of a human bone transmits impurity. This impurity requires the ashes of the red heifer and two immersions in the mikvah, for the person to be purified.
The bone marrow of a sacrifice that became a leftover also conveys impurity. In contrast to the impurity of the human bone, this impurity is decreed by the Sages. They wanted the priests to be alacritous in their duties and eat the sacrifices on time, not leaving them over. They also wanted to prevent the priests from taking personal revenge on the people by bringing their sacrifices with the wrong intent of eating them beyond allotted time - and intent that would ruin the sacrifice. The Sages thus decreed impurity on the hand of the priests for these violations, which would deter them, since they would not be able to continue their service for the day.
Art: David The Younger Teniers - Sausage-making