Keilim, Chapter 19, Mishnah 8. Much of this chapter has been occupied with chests and the various parts attached to them, asking when damage to one section leaves the rest of the vessel able to receive tumah, and when two attached parts are treated as a single unit. Our mishnah sets the chests aside for a moment and applies the very same questions to two other leather items, each one built out of a large container together with smaller compartments joined to it.
The shepherd's pouch and the pocket inside it
The first is a leather pouch of the sort shepherds would carry. Its main body was a roomy container for holding whatever the shepherd needed, and sewn into it were small pockets in which he would keep smaller items separately.
Here is the ruling: "Haturmal sheniftachah", describing such a satchel whose body has burst apart. A container that has split in this way holds nothing at all, and where there is no capacity to contain there is no susceptibility to tumah. The large body of the satchel, then, can no longer become tamei.
What of the small compartment? "Kis shebetocho tamei": the little pocket stitched inside suffered no tear, it still functions perfectly well as a container, and it therefore remains an item that can receive tumah. The mishnah then supplies an essential clarification in the words "ve'eino chibur lo". Physically the two are sewn to one another, yet in halachah each stands entirely on its own. Should tumah reach the ruined satchel, the pocket stays tahor; and should the pocket itself contract tumah, nothing passes from it back to the satchel.
How a leather bottle was made
How a leather bottle was made
The mishnah moves on to a chemet, the leather bottle in which water and other liquids were carried. The hide had to come off the animal in one piece. The skinner would cut a ring around the tail end and then draw the skin forward over the body until he reached the throat, severing it there a second time. What he was left with was a sleeve of hide standing open at both ends, and each end would be bound shut tightly so that nothing inside would leak away.
There was one more feature. The sacs of hide that had originally covered parts of the animal's anatomy now hung on the sides of the finished bottle as small pouches. Their openings faced inward, into the body of the bottle, so that when the bottle was filled the liquid flowed into these side pouches as well, and they held liquid together with the bottle itself.
When the side pouches tear open
When the side pouches tear open
Now the ruling: "Hachemet shebeitzim shelah mekabelot imah". This is precisely the bottle we have been picturing, the one whose side sacs draw in liquid together with the main body. "Veniftachu": the undersides of those sacs have split. "Tehorot": they are tahor and no longer subject to tumah. And the mishnah supplies the reason, "she'einan mekabelot kedarkan", for they have lost the ability to take in liquid in the fashion proper to them.
The reason is clear from how they functioned. Their normal way of filling was for liquid to enter them from inside the bottle as the bottle was filled. Now that their bottoms are torn, any liquid that enters simply drains straight out, so they no longer serve as containers at all.
One might have argued that they still hold liquid in some fashion: if a person were to tilt the bottle and hold it at an angle, liquid could remain in them. To this the mishnah answers that holding liquid in an unusual manner is not considered holding at all. Since the pouches can no longer receive in their ordinary way, they remain tahor and are not susceptible to tumah.
In both halves of the mishnah, then, we see the same principle at work from two directions: susceptibility to tumah depends on a compartment's own ability to function as a container in its normal way, and small compartments attached to a larger vessel are judged in their own right rather than as part of the whole.