Keilim, Chapter 19, Mishnah 7. Our Mishnah turns to a wooden chest or trunk of the kind used for storage, and asks which forms of tumah such a chest can receive. The chest under discussion is built with its opening at the top, and its cover is fastened above on hinges, so that one lifts the lid in order to place things inside or take them out.
The Mishnah begins: "Teivah shepitchah milma'alah, temei'ah tumei met", a chest whose opening is on top is susceptible to corpse tumah. If it comes into contact with a corpse, or is otherwise exposed to that tumah, it becomes tamei like any other wooden vessel that serves as a receptacle.
Why Not Midras
What such a chest cannot receive is tum'at midras, the tumah transmitted by the pressure of a zav, for example when a zav sits down upon it. Midras applies to an object whose purpose includes being sat or leaned upon. Here, the moment someone settles his weight on the lid, the chest is no longer available for what it was made to do: the cover cannot be lifted and nothing can be put in or taken out. An object that is blocked from its normal use precisely while the person is resting upon it is not a vessel designated for sitting, and therefore the pressure of the zav does not render it tamei with midras.
When the Chest Breaks
The Mishnah now presents two ways in which such a chest can be damaged. "Nifchetah milma'alah": where the chest is broken at its top, so that the lid is gone, it remains liable to tumah from a corpse, "temei'ah tumei met". Losing the cover has not taken away the chest's purpose, since the body of the chest goes on holding whatever is placed inside it, and whatever still works as a receptacle retains the status of a vessel.
"Nifchetah milematah", if it broke below, that is, the floor of the chest gave way, "tehorah", it is tahor. With no bottom there is nothing left to contain what is placed inside, the chest can no longer store anything, and a wooden receptacle that cannot receive is no longer subject to tumah at all.
The Built-In Compartments
"Megurot sheba, temei'ot": the small storage boxes fitted inside the chest, which stayed whole even after the top or the base was ruined, are themselves capable of contracting tumah. Considered by itself, each such compartment goes on doing a container's work and holds whatever is put in it, and that independent function is sufficient for it to count as a vessel in its own right.
"Ve'einam chibbur lah": they do not form one unit together with the chest. The compartments are indeed fixed in place, yet halachah regards them as distinct objects. Should one of those compartments contract tumah, the chest itself remains tahor; and should the chest become tamei, the compartments are unaffected by it. Each is judged according to the service it performs, and each stands on its own.