Reb Boruch, firstly I note that this is not Shabbos 118, it is Shabbos 108.

1) Your question on Rashi, Shabbos 108a, DH Ochlei, reminds me of a question you asked about a year ago on Rashi at the very beginning of the 3rd chapter of Shabbos: Why does Rashi there make no mention of the fact that the source of the idea of Ma'achal ben Derosai is on Shabbos 20a? You also asked on Rashi, Shabbos 80b, why does he not refer us to the source in Chagigah about Ma'aseh Merkavah?

So this Rashi on 108a is just another proof that he does not always refer us to all the sources. Rashi wrote his Perush in a very concise way. He wants to help us understand the Gemara but once we know that Shmuel was a doctor it is quite possible for us to continue and understand the Gemara even if we do not know exactly where it says that he was a doctor.

(By the way, possibly the most specific source that Shmuel was a doctor is Bava Metzia, beginning of 86a, where we learn that he healed Rebbi Yehudah ha'Nasi.)

2)

a) We could answer the question about Rebbi Meir with the Gemara in Shabbos 116a, that to make Shalom between husband and wife one is allowed even to erase the name of Hash-m, so we see that sometimes one is allowed to do an Isur for the sake of Shalom Bayis.

b) However, we do not need to come to this, because the Midrash says that Rebbi Meir asked them to do a "Lachash" (chant) on his eye. See Shulchan Aruch, Yoreh Deah 179:11, that one is permitted to do a Lachash on Shabbos. (See also Shulchan Aruch, Orach Chaim 301:27 and 306:7.)

Kol Tuv,

Dovid Bloom

I should explain more, b'Siyata d'Shmaya, the answer I gave above in 2b.

1) The obvious question that can be asked on my answer is: Why indeed is the Lachash that Rebbi Meir requested permitted? Since he clearly knew that as part of the process of giving the Lachash she was going to spit in his eye -- since, after all, that was the very reason why he was trying to make her give him the Lachash -- why is this not prohibited, as the Gemara you cited prohibits puting the saliva on the eye? To put the question in another way: How can it be that the Shulchan Aruch which I cited above permits Lachashim, since the Etz Yosef on the Midrash you cited writes that they had to spit on the eye when they did the Lachash? Every Lachash should be prohibited because it automatically involves spitting!

2) The answer is that one must make a distinction between a "Refu'ah Tiv'is" -- a natural cure -- and a "Refu'ah Segulis" -- a cure that works through a Segulah. The scientific reason for why the latter works is not understood, and indeed it does not work in a medical way, but nevertheless we see that sometimes it does actually work.

3) The Halachah is that whilst a Refu'ah Tiv'is is prohibited on Shabbos (unless the patient is classified as a sick person even though his life is not in danger, see Mishnah Berurah OC 328:121 that such a patient is allowed to take medicines on Shabbos), a Refu'ah Segulis is permitted. The logic behind this distinction is that the Gemara in Shabbos (53b) states that the reason why one is not allowed to take medicines on Shabbos is that he might grind up herbs, etc., in order to make the medicines. According to this reasoning, only the sort of healing which involves medicines is prohibited, because it is only with this sort of process that one could come to prepare medicines in a forbidden way. In contrast, a Refu'ah Segulis is permitted on Shabbos (if it does not involve any actual Melachah), because one will not come to prepare medicines as a result, since there is no need for medicines when doing a Refu'ah Segulis.

4) The spitting done by the woman in the Midrash is different from the spitting done in Shabbos 108b. In Shabbos 108b, the saliva is a medical treatment which can be explained scientifically. It has to be saliva from the early morning before eating, because only that way is it strong enough to be effective. In contrast, any kind of spitting is sufficient in the Midrash's account, because it is a Segulah. There is no worry that if one does this sort of spitting that one may come to prepare medicines, because this spit is not comparable to medicines.

Kol Tuv,

Dovid Bloom

Here is a different approach how to answer this question in a more basic way.

The point is that once Rebbi Meir told her to do the Lachash, it automatically follows that there is no prohibiton involved in doing so. I will try now to explain what I mean, b'Siyata di'Shmaya.

1) The Gemara in Eruvin (67b, 3 lines from the bottom) states (according to Rashi's explanation) that if a Chacham rules that a certain thing is permitted, and the issue involved is a d'Oraisa prohibition, then if there is someone present who thinks that he has a challenge on this ruling he should pose his question to the Chacham before the ruling is actually carried out. However, if the issue involves only a d'Rabanan prohibition, then even if one of the onlookers genuinely believes that he has a Kushya on the ruling, one should still first do what the Posek ruled, and only afterwards ask the question that one has on the Psak to see if the ruling was correct.

2) Therefore, in the story involving Rebbi Meir, since the prohibition against healing on Shabbos is only mid'Rabanan (a Gezeirah that if one would be allowed to practice medicine on Shabbos one might come to prepare the medicines on Shabbos itself, which could entail an Isur d'Oraisa), it follows that even if there was somebody present in the Beis Midrash of Rebbi Meir who believed that what Rebbi Meir had told the woman to do was forbidden, nevertheless he should keep quiet until the woman has spit on Rebbi Meir.

3) We have explained why the woman was allowed to spit even if she thought that it was Asur to do so, and also why any onlookers were not obliged to intervene. They could rely on the assumption that Rebbi Meir knew what he was doing. However, the question now is: what was Rebbi Meir's Heter to make the woman spit in his eye? It would appear that he was telling her to transgress an Isur d'Rabanan!

4) The answer to this question is based on the Shulchan Aruch, Orach Chaim 328:37. The Shulchan Aruch there rules that if someone is totally healthy, but for some reason he wants to eat or drink, on Shabbos, food or liquid which is usually only consumed by sick people, this is permitted. He is not doing any medical act on Shabbos; he is merely eating or drinking these substances because he is hungry or thirsty, and therefore this is permitted.

5) This is what Rebbi Meir was doing. Even if spitting in the eye would be considered an act of healing which is prohibited for a person with a pain on Shabbos (i.e., even if the spitter had not tasted anything from the time he arose in the morning; see Rashi to Shabbos 108b, DH Rok), in Rebbi Meir's case it would still be permitted. This is because Rebbi Meir was a perfectly healthy person who was inducing the spitting for an entirely different reason: in order to make Shalom Bayis between this woman and her husband.

6) However, you may argue that a problem of Mar'is ha'Ayin is involved in the woman spitting in Rebbi Meir's eye. Possibly we should be concerned that since the onlookers do not know the story about the stubborn husband, they will think that she is healing Rebbi Meir on Shabbos! To this we may answer with the Shach in Yoreh Deah 87:7. The discussion there concerns making almond milk and placing pieces of meat in the "milk." In reality, it is not milk, but it looks as if one is eating meat together with milk. Generally speaking, this is forbidden because of Mar'is ha'Ayin. However, the Shach writes that sometimes the prohibition of Mar'is ha'Ayin is suspended because of various factors -- for example, if it was necessary to eat the meat with almond milk for medical reasons.

I assert that if the problem of Mar'is ha'Ayin is suspended for medical reasons, it certainly may be pushed aside because of Shalom Bayis. This is why Rebbi Meir did not hesitate to ask the wife to do something which might appear to onlookers to be an act of healing on Shabbos; he knew this act could make peace between the married couple.

7) I think we have seen, b'Siyata di'Shmaya, how one can remove the various objections to Rebbi Meir asking the wife to do something which might appear to be healing on Shabbos.

Kol Tuv,

Dovid Bloom