Since the loaves that accompany the korban toda – the thanksgiving offering – are dependent on it, the Mishna (78b) teaches that under certain circumstances if the sacrifice is deemed invalid the loaves do not become sanctified. The Gemara identifies the author of this Mishna as Rabbi Meir who teaches the general principle that if the condition that makes the sacrifice invalid existed before it was slaughtered, e.g. the animal was a tereifa – it had a terminal condition – then the loaves do not become sanctified. If, however, the condition was created after the animal was slaughtered, e.g. the kohen had an inappropriate thought about where or when the sacrifice would be eaten, then the loaves do become sanctified.
What if the sacrifice was found to have a mum – a blemish? According to Rabbi Meir this question is the subject of disagreement between Rabbi Eliezer, who says that the loaves become sanctified, and Rabbi Yehoshua who rules that they do not.
The Gemara searches for an explanation for the position of Rabbi Eliezer, according to Rabbi Meir. If an animal was a tereifa we recognize that the sacrifice is invalid and the loaves do not become sanctified; why should the case of a blemish be different?
The Gemara answers that this follows the opinion of Rabbi Akiva who permits certain types of mumim to be brought on the altar, or at least that the sacrifice is not removed from the altar if it had already been brought to it. The case discussed is when the blemish was dukin she-ba-ayin. Since dukin she-ba-ayin is a relatively minor condition – in fact, it is not considered a blemish if it is found in a sacrifice brought from fowl – an animal with such a blemish is permitted, if it found its way to the altar.
Dukin she-ba-ayin is some kind of an eye condition; Rashi explains that it is a cataract on the eye. Another possible explanation presented by Rashi is that it is a blemish on the eyelid.