Gemara

(Mishnah): One may not stand in Reshus ha'Yachid and urinate or spit to Reshus ha'Rabim, or vice-versa;

(R. Yehudah): Even once saliva gathered in his mouth, he may not walk four Amos before spitting it out.

99a - Question: Is this even if he did not swirl the saliva [in his mouth]?!

(Beraisa - R. Meir): If one was eating pressed figs with Tamei hands, and put his hand in his mouth to remove a pebble, he was Metamei [the figs. His saliva was Machshir them];

R. Yehudah says, if he swirled the saliva, the figs are Temei'im. (The saliva is Machshir, for he will spit it out soon.) If not, they are Tehorim.

Answer #1 (R. Yochanan): The opinion(s) was (were) switched.

Answer #2 (Reish Lakish): Nothing was switched. The Mishnah discusses phlegm.

Objection (Beraisa - R. Yehudah): If phlegm was uprooted or saliva was uprooted, he may not walk four Amos before expelling it.

Conclusion: We must rely on Answer #1.

Shabbos 81b (Rav Chisda): On Shabbos, one may take rocks to clean himself after eliminating. Kevod ha'Briyos (avoiding disgrace) overrides a Lav (all mid'Rabanan laws, which are based on "Lo Sasur").

Question (Ravina - Beraisa - R. Eliezer): One may take a chip of wood to remove food between his teeth;

Chachamim permit only chips in the feeding trough [suitable for animals, but not Muktzeh]!

Answer: A person plans where he will eat (he should prepare chips before Shabbos), but he cannot always plan where he will eliminate.

92a (Mishnah): One is exempt if he was Motzi in his mouth, elbow... because he was not Motzi normally.

102a (Mishnah - R. Meir): If on Shabbos one was Motzi the food in his mouth [to Reshus ha'Rabim], he is liable for Hotza'ah.

Question: It is not normal to be Motzi in the mouth!

Answer: Since he intends to eat, he makes the mouth a proper way to be Motzi.

Rishonim

Rambam (Hilchos Shabbos 13:3): If one was eating and going from Reshus to Reshus, and intended to take the food in his mouth from Reshus to Reshus, he is liable. His intent makes his mouth like a place four [by four Tefachim], even though he was not Motzi the way people normally are Motzi.

Lechem Mishneh: In the Seifa (urinating into another Reshus), the Rambam did not mention intent for Hotza'ah, even though the Gemara said so, just like it said regarding food, for Stam we do not say that food is destined for this. Perhaps he does not intend to be Motzi. Surely, one who [spat or] urinated intended to be Motzi Reshus, for he did not want to urinate here. Therefore, the Rambam did not need to write this.

Minchas Yitzchak (5:38): When saliva was uprooted in his mouth, we cannot say that his intent makes it considered an [important] place. Even so, it is forbidden. We say that one is liable mid'Oraisa, which requires intent [to make the place important]! The Rambam says that one is liable because his intent makes his mouth like a place four [by four]. The Lechem Mishneh says that he did not need to mention this regarding urinating, for Stam, he intended to be Motzi. Spitting is just like urinating. One is liable for carrying saliva four Amos only if he intended for this.

R. Yehonason (on the Rif, Eruvin 32a DH R. Yehudah): We exempt one who was Motzi in his mouth, because he was not Motzi normally (Shabbos 92a). That refers to Kelim. It is normal to be Motzi saliva and food in the mouth. One is liable for this.

R. Chananel (99a DH Amar): R. Yochanan said that the opinions must be switched. I.e. R. Yehudah is Metamei in any case, and R. Meir is Metamei only if he swirled.

Rashi (99a DH Muchlefes): R. Yehudah switched (retracted from) his opinion in our Mishnah.

Sefer Chasidim (265): If after eating and drinking, one chews and swallows two handfuls of sesame, he will salivate the entire day like an animal. Therefore, on Shabbos, after eating one should not eat sesame, lest he go out to Reshus ha'Rabim with food in his mouth.

Poskim

Shulchan Aruch (OC 350:3): Once saliva was uprooted in his mouth and he is ready to spit it out, some forbid walking four Amos in Reshus ha'Rabim before spitting it out.

Beis Yosef (DH Lo): Once he is ready to spit it out, it is a load. The Tur understands that the first Tana does not argue. However, in Perush ha'Mishnayos, the Rambam holds that he argues, and wrote that the Halachah does not follow R. Yehudah. The Magid Mishneh says so.

Mishnah Berurah (12): Eliyahu Rabah says that this is only if the saliva already swirled around in his mouth and it is about to leave.

Mishnah Berurah (13): Even though the Mechaber wrote "some forbid", one should not veer from this, for several Rishonim hold like this.

Kaf ha'Chayim (21, citing Eliyahu Rabah): The Tur, Roke'ach, Yere'im, Ra'avan, Ran, Tzedah l'Derech and Sefer ha'Zikaron forbid.

Mishnah Berurah (14): Likewise, in such a state one may not go from Reshus to Reshus until he spits, for it is destined to leave, and it is a load.

Kaf ha'Chayim (19): Phlegm is forbidden even if it did not swirl around in his mouth.

Kaf ha'Chayim (20): On Shabbos, after eating one should not eat sesame, lest he go out to Reshus ha'Rabim with food in his mouth. However, if there is an Eruv [even] through Tzuras ha'Pesach, and he does not normally leave the Eruv, one need not be stringent.

Bi'ur Halachah (DH Roko): Perhaps the Tur and Shulchan Aruch wrote "he is ready to spit it out" to teach that he swirled it around.

Bi'ur Halachah (DH bi'Reshus): I am unsure whether this is mid'Oraisa or mid'Rabanan. They said that it is like carrying a load on Shabbos. If it is mid'Oraisa, we decree also in Karmelis. However, R. Yehudah said "even saliva." This connotes that he adds to the first Tana. Not only throwing [spitting] the saliva from Reshus to Reshus is forbidden. Rather, even before removing it from his mouth, one may not carry it four Amos, and the same applies from Reshus to Reshus. If so, R. Yehudah forbids mid'Oraisa, and also [forbids mid'Rabanan] in a Karmelis, just like in the Reisha. Also, Abaye and Rava argue in the next Mishnah about Karmelis. This connotes that all forbid even Karmelis in this Mishnah. However, one must investigate this matter, since some permit even in Reshus ha'Rabim. One may rely on them for Karmelis. Shulchan Aruch ha'Rav says so. In any case, one may be lenient only in a place that all agree that it is Karmelis, e.g. a valley or Karfef. One may not be lenient in our Reshus ha'Rabim, for some Poskim hold that it is Reshus ha'Rabim mid'Oraisa.

Kaf ha'Chayim (22): We need not protest against those who are lenient in Karmelis, since some permit in Reshus ha'Rabim.

Minchas Yitzchak (ibid.): Chachamim permit taking a chip to clean one's teeth only from what is prepared, even though Kevod ha'Briyos applies. Sometimes bits of food between his teeth are visible, and this is disgraceful. If one was invited to a meal and he could not prepare chips before Shabbos, Magen Avraham (322:4) permits moving a chip, due to Kevod ha'Briyos. Where this is not considered disgraceful, it is forbidden. He did not say that when he has nothing to clean his teeth, he must be careful not to eat food that sticks between teeth, lest he forget and go out to Reshus ha'Rabim with food between his teeth. Sefer Chasidim makes such a decree. However, perhaps there the concern is for Hotza'ah mid'Oraisa, for it makes him salivate (Eruvin 98b). The Tur and Shulchan Aruch rule like this. The Bi'ur Halachah inferred that R. Yehudah forbids mid'Oraisa. We can say that Sefer Chasidim holds like this, therefore he forbids lest one forget. All agree that Hotza'ah of food between his teeth is only mid'Rabanan, like Minchas Shabbos (Kuntres Acharon 11) says, therefore we do not decree.

Minchas Yitzchak: This is not called Hotza'ah k'Le'acher Yad (unskillfully). It is normal to be Motzi between his teeth. R. Yehonason says that it is normal to be Motzi saliva and food in the mouth. The same applies to between his teeth. If he intended, this is Melachah she'Tzerichah l'Gufah. If he was Motzi a Shi'ur, he is Chayav Chatas. Without intent, it is Melachah she'Einah Tzerichah l'Gufah, and it is Chetzi Shi'ur, so there is no concern mid'Oraisa. In any case, it is proper to clean the teeth before going outside. Perhaps are not careful about this because sometimes it is impossible, e.g. when one goes to a Seudas Mitzvah or is a guest in another's home. Chachamim did not decree in a case when one cannot be careful. In any case it is proper to be careful when possible.

Tzitz Eliezer (13:34:8): (A doctor prescribed pills for a patient with a heart condition. When he feels weak, he puts one under his tongue.) One may not go out with pills in his mouth, even for a Mitzvah such as going to the Beis ha'Keneses. He should leave them in the Beis ha'Keneses before Shabbos. If he cannot, it is better to pray at home than to take them outside the Eruv. This is Derech Hotza'ah, like for saliva and food.