Zavim, Chapter 5, Mishnah 9. Before us stands one of the most surprising categories of impurity in the Torah. Normally tumah reaches a person from outside himself: he lays a hand on something tamei, he lifts it, he steps into a space where impurity is present. Our mishnah describes the reverse direction, a tumah that begins inside the body. Someone consumes flesh of the neveilah of a kosher bird in the measure of a kezayis (the volume of an olive), and the swallowing itself converts him into a generator of impurity.
The critical location is what the mishnah calls the beis habliah, the place of swallowing. This is the area at the back of the throat where the esophagus meets the oral cavity. Once the flesh of the bird carcass arrives at that point, the person becomes tamei, and not with a light form of tumah: he takes on the standing of an av hatumah, with all that such a status carries with it.
The Text of the Mishnah
The mishnah's opening words are "Ha'ochel mi'nivlas of tahor vehi beveis habliah": one who consumes the carcass of a kosher bird while that piece of meat is situated at the swallowing point. The term tahor is a description of the bird, indicating a permitted species; the meat itself, of course, is neveilah. So long as it remains lodged at that spot, the halachah declares him "metamei shenayim u'poseil echad", transmitting two grades of impurity and rendering a third unfit.
What does that phrase mean in practice? Since he holds the standing of an av hatumah, he is metamei begadim: the clothing he is wearing becomes impure, and so does anything else he touches. Whatever he touches becomes a rishon l'tumah. That rishon, in turn, can render a food item a sheni l'tumah. And that sheni, when it meets terumah, invalidates it. Two full levels of tumah plus one level of pesul: that is exactly what the mishnah means by metamei shenayim u'poseil echad.
His Head in the Oven
A vivid scenario follows. If this man "hichnis rosho l'avir hatannur", placing his head within the hollow interior of an oven, the verdict is "tahor vehatannur tahor": the oven is entirely unaffected. Two separate difficulties present themselves in such a picture, and this single ruling settles both of them.
First, why does the man himself, an av hatumah who is metamei begadim, not contaminate the oven? Because an oven is a kli cheres, an earthenware vessel, and such a vessel does not contract tumah from a person who touches it or from the airspace around a human being. A human av hatumah simply does not transmit impurity to earthenware in this fashion.
Second, what about the neveilah itself? After all, the flesh of the bird carcass is inside his throat, and his head is inside the oven, so the neveilah is technically within the oven's airspace. Nevertheless, the oven stays pure. The neveilah of a kosher bird has a very narrow reach: it defiles only a person, and only the person who eats it. It is not metamei keilim at all. Vessels are outside its jurisdiction entirely, so the oven contracts nothing from it.
After It Leaves the Beis Habliah
The mishnah continues: "Hekiya o vala", if he brought it back up or swallowed it down. Either way, the flesh is no longer resting in the place of swallowing. Perhaps it went all the way down into his stomach, or perhaps he regurgitated it after it had already passed that point. In both cases his standing drops one rung: he is now "metamei echad u'poseil echad", a rishon l'tumah. He no longer defiles the clothes on his back. He can convey one level of tumah, rendering a food item impure as a sheni, and that food item will then invalidate terumah that it touches.
Flesh That Never Reached the Throat
Finally the mishnah addresses the flesh that is "besoch piv", still inside his mouth: tahor. He is completely pure. The kezayis of neveilah is in his mouth, but it has not yet arrived at the beis habliah, and it is only there that this tumah takes effect. Should he now spit the piece out, nothing whatsoever has changed in his status; he was tahor before and he is tahor after.
It emerges that the difference between the two cases is entirely a matter of where the flesh has been. If it reached the back of the throat and then came back up into the mouth, he is a rishon l'tumah. If it never traveled past the front of the mouth, he never became tamei in the first place.
The logic is beautifully simple. The Torah made this tumah dependent on eating, and eating means swallowing, not merely holding food in one's mouth. Until the flesh reaches the beis habliah, no act of eating has taken place, and the person's taharah remains fully intact.