Chullin 118 - The Hide And The Gravy
Any amount of food can become ritually impure, but to transmit impurity further, it needs to have a volume of at least an egg's volume. This impurity will be only secondary, since intrinsically food does not possess impurity.
In contrast, if an animal died by itself, or even if it was killed, but not by the method of shechitah, then its meat is intrinsically impure, and it can contaminate other foods. To be significant, the amount of dead animal's meat must measure at least an olive volume.
The following parts of the animal combine with its meat in order to reach the egg's volume, a pre-requisite for the food to transmit impurity: the hide, the gravy, the sediment, the shreds, the bones, the ligaments, the horns, and the hooves. The hide adds up, because it serves as a protector for the meat. The gravy is the juice that exudes from the meat. It is not eaten by itself, but it is eaten with the meat. The same refers to the other parts. However, since they are not really food, they do not add up to the olive's volume, required for the dead animal's meat impurity.
Art: Pieter Aertsen - Butcher's Stall