Gemara

9a (Beraisa): If one saw a bird or mouse making a hole in a watermelon, we are concerned lest there was already a hole there (from a snake, so one may not eat it).

46a: R. Chiya would discard it (the liver). R. Shimon bar Rebbi would dip it (in vinegar or another dip, and eat it).

Horayos 13b (Beraisa): Five things make one forget his learning - eating from what a mouse ate from, eating the heart of an animal...

13a (R. Eliezer): One who eats from what a mouse ate from, he forgets. Cats eat mice, so all the more so they forget!

Menachos 99b (Reish Lakish): One who forgets something that he learned transgresses a Lav - "Hishamer Lecha... Pen Tishkach Es ha'Devarim..."

Sukah 36b (Rav): If a mouse ate from an Esrog, this is not Hadar.

Question: R. Chanina would eat from it and be Yotzei with it!

Question: The Mishnah opposes R. Chanina! (It disqualifies a Chaser Esrog.)

Answer: The Mishnah refers to Yom Tov Rishon. R. Chanina holds that one is Yotzei on Yom Tov Sheni.

Question: How do we answer for Rav?

Answer: Mice are (worse. What they ate from is) repulsive.

Mishnah (Terumos 6:8): If there is a hole in a fig... gourd, or watermelon, whether it is big or small, attached or detached, if it has any moisture, it is forbidden.

Rishonim

Rashi (46a DH Matbil): R. Chiya discarded the liver, for he did not consider it to be an important food. R. Shimon bar Rebbi held that since it is a vital organ, it is healthy to eat.

Leket Yosher (2 YD Amud 6): The Terumas ha'Deshen ate a food containing fat of a ox' heart. If a cat ate from meat, he would not eat from the piece, and similarly for a loaf or meat pie. If a mouse ate from a piece of dry meat, he would not feed it to his children, even if it was a big Kikar. However, if a mouse ate from congealed fish, he would eat from different pieces.

Note: Usually, 'Kikar' means a loaf. Meleches Shlomo (Terumos 8:6) says that anything complete is called Kikar.

R. Shimshon (Terumos 8:6 DH Kol): Venom mixes with moisture and diffuses throughout the entire Pri. If a dry food was bitten, we cut off that place and the rest is permitted.

Poskim

Rema (YD 11:4): Some Shochtim are careful not to slaughter any goose in Teves or Shevat unless they eat from the heart, for there is a tradition that there is a time during these months that one who slaughters a goose will die if he does not eat from it. The custom is to eat from the heart.

Hagahos ha'SMA: This is from Tashbatz, but he mentions only Shevat. He brings this from Tzava'as R. Yehudah ha'Chasid 41. There it says that they give from the liver.

Shach (7): Hagahos Minhagim says that the custom is to give to the Shochet from the liver. I saw that they eat from the legs.

Shach (YD 72:2): Tashbatz says that even though the Gemara says only that eating the heart of a Behemah causes forgetting, one should avoid also the heart of a bird. I saw that people are careful about this.

Chavas Da'as (Chidushim 72:2): The custom is to be lenient and permit cooking the heart. However, males do not eat the heart, for it causes forgetting (Taz).

Kaf ha'Chayim (YD 72:6) Beis Lechem Yehudah says that pregnant women and males should not eat the heart, for it causes forgetting.

Kaf ha'Chayim (Sofer 157:28): The Ari Zal says that the heart is the root of the animal part of the Nefesh. Eating it ties one to the Yetzer Ra. Therefore it causes forgetting and foolishness. Since it ties one to the Yetzer Ra, women should not eat it, even though we are not concerned if they forget, since they are not commanded to learn. Since the Ari Zal said that the Nefesh, Ru'ach and Neshamah are primarily in the brain, heart and liver, one should avoid eating also brain and liver, for also they cause forgetting. Perhaps they also harm. One should also avoid eating what a mouse or cat ate from, or drink water leftover from washing, for they cause forgetting, and due to Berachah.

Pri Megadim (Sifsei Da'as 110:3): The Shach says that the heart, liver and intestines are not Chatichah ha'Re'uyah Lehiskaved (a nice portion fitting to honor someone, which is never Batel). It depends on (what is considered honorable) at the time. In any case the heart cause forgetting; surely it is not Re'uyah Lehiskaved. Shulchan Melachim says that Sefer Zechirah says that the brain even of a bird causes forgetting. Afarkasta d'Anya says that Sefer Zechirah says so only about the brain of animals, and brings from Midrash Eichah that R. Avahu ate brains of birds.

Yabi'a Omer (3 YD 8:3): I did not see people careful to avoid the heart, liver and brain. We find that R. Shimon bar Rebbi ate the liver. He held that it is healthy. He was not concerned lest it cause forgetting! Many Amora'im ate it (Chulin 111a). The Pri Megadim said that the heart is not Re'uyah Lehiskaved, but liver depends on the era. Also the Taz (101:11) says that liver is Re'uyah Lehiskaved.

Kaf ha'Chayim (Sofer YD 116:146): It is problematic to eat the heart regularly, but not on occasions. Even based on what I said that it causes Ru'ach Ra'ah, there is no concern for a small amount.

Yabi'a Omer (3 YD 8:3): The Rema holds that there is no Isur to eat from the heart. Even though he discussed a bird's heart and the Gemara discussed an animal's, Tashbatz (558) refrained also from a bird's heart. However, we can say that it is less prone to cause forgetting. The Sho'el u'Meshiv questioned the Rema; Zivchei Tzedek answered that since it is dangerous for the Shochet, we are not concerned lest he forget; this happens only once in a Yovel (49 years). I do not understand. He need not slaughter, and then there is no danger! Also, eating from any part evades the danger. He need not eat from the heart! Why does he say that it is only once in a Yovel? It is two months every year! However, since it is not a real Isur, rather, only good counsel, even regarding an animal's heart, (we may be lenient).

Mishneh Halachos (3:61): We cannot say that R. Chanina ate from what mice ate from, for if so, how was he Yotzei with it? We answered for Rav that it is Pasul even on Yom Tov Sheni! There is no proof from here that there is no problem eating the rest, for if not, one is not Yotzei with an Esrog that one may not eat. There is no Isur to eat things that cause forgetting. If one fell into a pot, it does not forbid the contents. It is merely good counsel not to eat it. Chayei Adam 2:2 proves from the Beis Yosef that anything forbidden due to Ru'ach Ra'ah is permitted b'Di'eved. The custom is to be lenient about things that Chazal considered dangerous. People rely on the Magen Avraham (179:8), who says that nature changed. The Rema (170:22, regarding eating salt after any food) disagrees. R. Yerucham (15:5) says that even nowadays we lash people who are not careful about dangers that Chazal warned about. He listed many, but he did not mention things that cause forgetting. Also the other Poskim omitted it. They hold that there is no Isur. Presumably, the concern is only for the part that the cat or mouse ate from. If it ate from wheat and we cannot tell which it ate from, it is Batel in a majority. Some say that if it ate from flour, we do not eat any of the flour. I say that even Tum'ah does not spread to all the flour, and all the more so here we are not concerned.

Mishneh Halachos (3:62): We do not find that people refrained from eating the heart, brain and liver. We say that the Torah forbade pork, and permitted the brain of a certain fish (Chulin 109b)!

Divrei Chayim (YD 2:60): If a mouse ate from a food, the entire food causes forgetting.

Chidushei Mahari'ach (on Terumos 6:8): It seems that 'Kikar' refers to dates pressed together. Since there is liquid, the venom spreads throughout all of it.

Note: The Beraisa (Chulin 9a) connotes that if not for concern that a snake bit it, one could eat from it! Those who permit women to eat things that cause forgetting could say that concern for a snake forbids even to women.

Yabi'a Omer (7): We learn from this that the entire loaf causes forgetting. It is not enough to scrape off the place from which it ate. Yosef Ometz (p. 273) says 'people cut off the bit place and eat the rest. I don't know if they have a tradition that this is what the Gemara means, or if they do so merely for cleanliness or due to disgust. One who is concerned for his Torah will not eat from the entire piece.' Terumos 8:6 forbids Peros with holes (lest a snake ate from it). Yalkut Avraham says that the same applies to a mouse, for all rodents have venom, and it spreads throughout the entire food. This implies that the concern is due to venom. This is difficult, for R. Shimshon says that if something dry was bitten, one cuts away that place and the rest is permitted. Regarding mice, the Gemara did not distinguish dry from wet! Leket Yosher explicitly says that there is no difference. Rather, it is a Segulah (it is not based on something physical). We say that one who eats from what a mouse ate from, he forgets, and all the more so something that eats mice!

Note: How does this prove that it is a Segulah? Perhaps there is something in mice that causes forgetting!

Arvei Nachal (Reish Parshas Ki Savo): Devarim Rabah (Ekev) says that if one does not tithe properly, mice eat his grain. The Magen Avraham (Shemen Sason Bereishis) explains that one who eats from what mice ate from, he forgets. This explains "I did not transgress your Mitzvos (to tithe), and I did not forget."

Divrei Malki'el (4:1): R. Chanina ate part of the Esrog eaten by mice, and was Yotzei with the rest. It was not repulsive to him. We must say that he cut off the part eaten by mice. If not, one may not eat from it, for it causes forgetting, and one transgresses Pen Tishkach. Also the Taz (OC 649:8) obligates cutting away the part that mice ate from.

Yabi'a Omer (2:8:8): Yalkut Avraham brings Mili d'Chasidusa, who learns from R. Chanina that one may eat from another part of a food that mice ate from. I.e. if it were forbidden, one could not be Yotzei with the rest, for it must be permitted to eat the Esrog. Surely, R. Chanina was Yotzei with a proper Esrog on the first day, and each day he ate part and was Yotzei with the rest the next day. In any case there is no proof, for there is no Isur to eat from what mice ate from. Perhaps one is Yotzei even when it is forbidden to eat due to danger, for in any case it is "Lachem".

Note: The Taz asked what was the question against Rav from R. Chanina. Clearly he holds that R. Chanina did not eat from what mice ate from!

Yabi'a Omer (citing Mili d'Avos 5 YD 5:3): We forbid only where the cat or mouse ate from, but the rest is permitted; this is the custom. We never heard of anyone who is stringent for the entire food. Mice eat from flour, and we bake bread of from it. However, perhaps there is different, for it changed.

Yabi'a Omer (ibid., 9): He overlooked the Acharonim who forbid the entire food. However, since it is not a real Isur, and also Yosef Ometz said that this is the custom, perhaps we can ignore this (i.e. we need not protest). Even though cats eat snakes, we need not be concerned lest there was venom in its mouth, and it left some in the food. If an animal ate poison, or was bitten by a snake, it is forbidden due to danger, even if it is not Tereifah! Here it is a remote concern, because snakes are not common among us.

Note: Even in the days of the Gemara, when exposed food was considered dangerous because snakes were common, we forbade what a cat ate from only due to forgetting, but not due to danger!

Yabi'a Omer (9): Chupas Eliyahu Rabah adds what a dog ate from. Zivchei Tzedek says that we rely on the Gemara (which did not mention dogs, so one need not refrain from this). I say that it is good to be careful also about things added from Kabalah, e.g. liver and brain. The Gemara mentioned matters that are more potent to cause forgetting, and the damage from them is clearer. It is better to give the food to women, who are not commanded about Divrei Torah.