Gemara

(Mishnah): If the bone of the foreleg or hind leg is broken, even if it is not noticeable, it is a Mum.

Question: If a broken bone is not noticeable, it is not a Mum!

Answer (Rav Papa): It is not intrinsically noticeable, but it is evident when the animal works.

45a (Mishnah): If a Kohen's fingers or toes overlap each other, he is disqualified.

If his fingers or toes are connected until the (middle) joint, he is Kosher;

If they were connected past the joint, and they were cut apart, he is Kosher.

If he had an extra finger or toe and it was cut off, if it had a bone, he is Pasul. If not, he is Kosher.

R. Yehudah says if he has six fingers on every hand and foot, 24 in all, he is Kosher;

Chachamim disqualify.

Rebbi is Posel one who is ambidextrous;

Chachamim are Machshir.

Rebbi holds that his right hand is weaker than it should be. Chachamim hold that his right hand is fine, his left hand is extra strong.

Chagigah 16a (R. Yehudah b'Rebbi Nachmani): If one looks at the Kohanim, his eyes weaken.

This is when the Mikdash stood. The Kohanim stood on the Duchan and blessed Yisrael with Shem ha'Mefurash (Hash-m's actual name).

Megilah 24b (Mishnah): A Kohen who has Mumim on his hands may not Duchan;

R. Yehudah forbids even if his hands are dyed, because people look at him.

(Beraisa): Mumim on hands, feet and face disqualify a Kohen from Berachah.

(R. Yehoshua ben Levi, and Beraisa): One whose hands are Bohakniyos (white or freckled) may not bless.

(Beraisa): One whose hands are Akumos or Akushos (bent) may not bless.

Rishonim

Rambam (Hilchos Bi'as Makdish 6:7): Only visible Mumim disqualify Kohanim. It says "Shever Regel Oh Shever Yad." Just like these are visible, only visible Mumim disqualify.

Rambam (Hilchos Tefilah 14:7): When Kohanim bless the people, the people may not look at the Kohanim, lest this distract them. They point their face towards the Kohanim, but do not look at their faces.

Rambam (15:2): If a Kohen has Mumim on his hands, feet or face, he may not give Birkas Kohanim, for people will stare at him.

Tosfos (Chagigah 16a DH ba'Kohanim): Rashi says that we are concerned lest people stare at his hands, for this weakens the eyes. This is only in the Mikdash, yet we forbid one with Mumim on his hands even outside the Mikdash! Rather, the Yerushalmi explains that we are concerned lest they stare and be distracted.

Or Zaru'a (2, Sof Siman 411): R. Yosi said that he can look at the Kohanim and not be distracted. Such people may look at the Kohanim.

Rosh (Megilah 3:22): R. Yehoshua ben Levi says that a Kohen who (is in the Beis ha'Keneses and) does not Duchan transgressed three Mitzvos Aseh. The Yerushalmi says that when R. Ada ben Pazi's head Chadik (it seems that this means it hurts), he stood in back of a pillar (so people would not see him). R. Eliezer went outside, so he would not need to ascend to the Duchan when the Chazan calls 'Kohanim'.

Mordechai (Megilah 817): R. Yakov ha'Levi of Shapira learned from this Yerushalmi that if a Kohen will not Duchan, e.g. his head (hurts) or he is an Avel within 12 months of the passing of his parent, he should leave the Beis ha'Keneses before the Shali'ach Tzibur begins Retzei.

Poskim

Shulchan Aruch (OC 128:4): When Kohanim do not want to ascend to the Duchan, they need not leave the Beis ha'Keneses, except for when the Chazan calls 'Kohanim.' However, the custom is that they do not enter the Beis ha'Keneses until they finish Birkas Kohanim, lest people say that they are Pegumim (disqualified).

Magen Avraham (4): The Mordechai says that they should leave before they (i.e. the Shali'ach Tzibur) begin Retzei, lest people say that they are Pegumim when they see that they do not start heading to the Duchan. The Bach says that Pesulim mid'Rabanan need not leave when the Chazan says 'Kohanim', for he does not intend for Pesulim.

Mishnah Berurah (12): E.g. he does not want to Duchan because he is weak. Another reason to leave before Retzei is lest Leviyim (who wash the Kohanim's hands) tell him to ascend.

Mishnah Berurah (14): We are concerned lest people say that they are Pegumim, i.e. the son of a divorcee or a woman who did Chalitzah (so he is a Chalal, and has no Kedushas Kehunah).

Shulchan Aruch (14): Birkas Kohanim is said only with lifted hands.

Shulchan Aruch (23): When Kohanim bless the people, they may not look, lest this distract them. Rather, their eyes must be towards the ground. They point their faces towards the Kohanim, but do not look at their faces.

Magen Avraham (35): To be Mistakel, i.e. to gaze, is forbidden. A mere glance is permitted. The custom is not to look at all. Perhaps this is a commemoration of the Mikdash, in which even a glance is forbidden due to honor of the Shechinah. If one gazed (at their hands) in the Mikdash, his eyes would weaken.

Kaf ha'Chayim (140): The Zohar says that even nowadays one may not even glance elsewhere, due to honor of the Shechinah.

Rema: Also the Kohanim may not look at their own hands. Therefore, the custom is to lower the Talis over their faces and their hands are outside the Talis. In some places the custom is have their hands inside the Talis, so people will not look at their hands.

Beis Yosef (DH uvi'Sha'as): In Mitzrayim, they lower a Talis of a Mitzvah over their faces and hands. This is a good custom, lest they look at the people or people look at them and be distracted.

Gra (51): Megilah 24b supports the former custom. See Sa'if 31.

Note: The Mishnah disqualifies Mumim on the hands, but not on the face (like the Beraisa does), for it discusses a place where Kohanim cover their faces with a Talis. If the hands were inside the Talis, we would permit even Mumin on the hands! (Based on Dameshek Eliezer.)

Kaf ha'Chayim (142): In every place it is proper for people to cover their faces with a Talis or to close their eyes.

Shulchan Aruch (30): If one has Mumim on his face or hands, e.g. they are Bohakniyos or Akumos or Akushos (Rema - 'Bohakniyos' are white like Tzara'as; Rashi says that they are freckled. He says that Akumos or Akushos are bent, or bent to the side. The Ran says that they are bent back, or he cannot separate his fingers), he may not Duchan, for people look at him.

Taz (26): One who has any of these Mumim must leave the Beis ha'Keneses at the time of Birkas Kohanim, lest people say that his mother was divorced (so he has no Kedushas Kehunah). The Mordechai in Megilah says that a Kohen Avel leaves the Beis ha'Keneses, for the custom is that he does not Duchan.

Eliyahu Rabah (51): Sa'if 4 is difficult for the Taz. Also, an Avel is different, for according to the law of the Gemara, he may Duchan. Shiltei ha'Giborim (Megilah 15b) and the Levush connote that we distinguish like this.

Mishbetzos Zahav: We can distinguish. It is not evident that he is an Avel. Mumim are evident.

Bi'ur Halachah (DH Mi): Eliyahu Rabah questioned the Taz. Also the Pri Megadim rejected the Taz' proofs. Also Nehar Shalom says that since these Mumim are evident, people will not err (to think that he is a divorcee's son). This is why the Poskim did not say that he must leave the Beis ha'Keneses. The Magen Avraham (4) connotes like this. However, since not everyone knows the Halachah (that these Mumim disqualify from Birkas Kohanim), perhaps it is good that he leave, like the Taz says. The Pri Chodosh (33) says that in every case that we say that he may not ascend (to Duchan), if he ascended, he does not descend.

Kaf ha'Chayim (167): Eliyahu Rabah, Eliyahu Rabah, the Pri Megadim and Nehar Shalom rejected the Taz, but Kitzur Shulchan Aruch and the Mishnah Berurah rule like the Taz.

Magen Avraham (42): Bohakniyos means freckled with small dots.

Mishnah Berurah (108): If it is common for people in the region to have small white dots (summer sprinkles), even if the majority do not have them, he blesses.

Mishnah Berurah (109): Since he looks astonishing, people will cease to intend for the Berachah.

Kaf ha'Chayim (172): Even though one may not look at the Kohanim at the time, it is people's nature to look at unusual things, and they cannot overcome their nature (Ma'amar Mordechai).

Shulchan Aruch (31): If the local custom is for Kohanim to lower a Talis over their faces, even if one has many Mumim on his face and hands, he may bless.

Beis Yosef (DH v'Nireh): Where Kohanim lower a Talis over their faces, even if he has many Mumim on his hands and face, he must Duchan. Perhaps we do not decree due to other places, for even there letter of the law he may bless. We forbid only lest people be distracted. Therefore, where there is no concern for distraction, he may bless. However, where others do not cover their faces, he may not, for this will cause people to look at him and be distracted, and all the more so if he is alone.

Magen Avraham (45): If it is not the custom, it is forbidden even if all the Kohanim will do so, for this will distract people.

Rema: This is only if his hands were inside the Talis. If they were outside, the Talis does not help for Mumim on his hands.

Pri Chodosh (30): The Rif and Rosh omitted Mumim on the face and hands, for they already mention Zavalgan and a blind person, which are on the face. The Tur says that the Gemara mentioned only Mumim of the face, but not of the hands. He means that the Rosh mentioned only these.

Question (Taz 28): The Rema should have taught that nowadays that also the Tzibur are careful to cover their faces themselves with Taleisim, it is permitted! This is better than Kohanim covering their faces!

Answer (Mishnah Berurah 115): If people know that a Kohen has a Mum, they will look. Also, single men do not have Talesim.