Mishnah for Oholot 11:7
כֶּלֶב שֶׁאָכַל בְּשַׂר הַמֵּת וּמֵת הַכֶּלֶב וּמֻטָּל עַל הָאַסְקֻפָּה, רַבִּי מֵאִיר אוֹמֵר, אִם יֵשׁ בְּצַוָּארוֹ פּוֹתֵחַ טֶפַח, מֵבִיא אֶת הַטֻּמְאָה. וְאִם לָאו, אֵינוֹ מֵבִיא אֶת הַטֻּמְאָה. רַבִּי יוֹסֵי אוֹמֵר, רוֹאִין אֶת הַטֻּמְאָה, מִכְּנֶגֶד הַמַּשְׁקוֹף וְלִפְנִים, הַבַּיִת טָמֵא. מִכְּנֶגֶד הַמַּשְׁקוֹף וְלַחוּץ, הַבַּיִת טָהוֹר. רַבִּי אֱלִיעֶזֶר אוֹמֵר, פִּיו לִפְנִים, הַבַּיִת טָהוֹר. פִּיו לַחוּץ, הַבַּיִת טָמֵא, שֶׁהַטֻּמְאָה יוֹצֵאת דֶּרֶךְ שׁוּלָיו. רַבִּי יְהוּדָה בֶן בְּתֵירָא אוֹמֵר, בֵּין כָּךְ וּבֵין כָּךְ, הַבַּיִת טָמֵא. כַּמָּה תִשְׁהֶה בְמֵעָיו. שְׁלשָׁה יָמִים מֵעֵת לְעֵת. בְּעוֹפוֹת וּבְדָגִים, כְּדֵי שֶׁתִּפֹּל לָאוּר וְתִשָּׂרֵף, דִּבְרֵי רַבִּי שִׁמְעוֹן. רַבִּי יְהוּדָה בֶן בְּתֵירָא אוֹמֵר, בְּעוֹפוֹת וּבְדָגִים, מֵעֵת לְעֵת:
A dog who ate the flesh of a dead body, and the dog died and was lying on the threshold, Rabbi Meir says, if his neck has a space that is a handbreadth he spreads the impurity, but if not he does not spread the impurity. Rabbi Yosi says we check the impurity, [if it is found] from the threshold and inside, the house is impure, [if found] from the threshold and out, the house is pure. Rabbi Elazar says if his mouth faces to the inside, the house is pure, but if it faces to the outside, the house is impure, since the impurity leaves by way of his rear. Rabbi Yehuda Ben Betaira says in either case the house is impure. How long does it stay in his innards? Three days from the time [of day it was eaten] to that time [of day three days later]. By birds and fish, long enough for it to fall in the fire and be burned, says Rabbi Shimon. Rabbi Yehuda ben Betaira says by birds and fish it is from the time [of day it was eaten] to that time [of day one day later].