1) The Gemara 21b, and also here 22a two lines later, states that there is a "Lo Ta'aseh" prohibition on the blood of the heart. This refers to the blood that is in the heart at the time of Shechitah. Kares applies only to the blood with which the Neshamah leaves.

Therefore, if the Mishnah in Chulin (109a) would have said that if he did not tear the heart "Ein Chayavim," I might have thought that this means he is not liable for Kares but he does transgress a Lo Ta'aseh. Instead, the Mishnah says "Eino Over" to teach us that he is not liable for Malkus either, because the Mishnah is discussing the heart of a bird, which does not possess a k'Zayis, as Rebbi Zeira said.

2) However, we find that Rashi on the Mishnah in Chulin 109a (DH ha'Lev Kor'o) writes that when the Mishnah states "Ein Over Alav" this means that he is not liable for Kares, because the Gemara in Kerisus 22a states that the Mishnah refers to the heart of a bird, which does not contain a k'Zayis of blood.

What Rashi writes seems to be the opposite of my answer above, since Rashi implies that there is no Kares for the blood of the heart of the bird, but there is a Lav for this, while I argued that there is not a Lav either.

3) I found that the Rosh Yosef on Chulin 109a, written by the author of the Pri Megadim, writes that the words of Rashi are inaccurate for the very reason that I mentioned -- the Halachah is that there is not a Lav on the blood of the bird's heart either. (The Rosh Yosef continues and writes a lengthy suggestion for understanding the words of Rashi as we have them.)

The Nimukei ha'Griv, printed in the back of the standard Maseches Chulin, also first writes that Rashi's words seem to be inaccurate. (See also Chidushei Mahari Shapira to Maseches Chulin.) I believe that the Rosh Yosef and Nimukei ha'Griv support my understanding of the simple meaning of "Eino Over."

3) However the Rambam, Hilchos Ma'acholos Asuros 6:6, also seems to be similar to Rashi in Chulin 109a. The Rambam writes that if one ate the heart of the bird without tearing it open, he is not liable for Kares because it does not contain a k'Zayis of blood. It is interesting to note that even though the Mishnah states "Eino Over," that does not prevent the Rambam from explaining that this means he is not Chayav Kares.

The Lechem Misnneh writes that even though the Rambam writes that he is not liable for Kares, in fact he does not get Malkus either. This seems to fit with my original answer.

In summary, the simple reading of Rashi in Chulin 109a and of the Rambam does not seem to fit well with my original answer, but it may be possible to learn Rashi and the Rambam in such a way that does not conflict with my answer.

4) After looking into your question a bit more, I would like to summarize this question which has proven to be not so simple.

a) First, I should point out that the assumption of the original question -- that "Eino Over" is typically used to describe a Lav and not Kares -- is not necessary the assumption subscribed to by Rashi and the Rambam. We see this from the fact that the Mishnah in Chulin 109a states "Eino Over Alav," but Rashi immediately adds that this means that he does not transgress Kares. There is possibly an implication in Rashi that this is not the usual meaning of "Eino Over," but it is not unacceptable to Rashi that "Eino Over" could refer to Kares.

In addition, the Rambam (cited above) explains "Eino Over" to mean that he is not Chayav Kares. We do find sometimes that the Rambam slightly changes the words of the Gemara, and in this case it is not unacceptable to him to say that "Eino Over" refers to Kares.

b) There is an explanation in the Acharonim for why Rashi and the Rambam do not write that by eating the blood of the heart that was not torn one does not transgress a Lav. This is given by the Lechem Mishneh in Hilchos Ma'achalos Asuros 6:6 in the name of Rabeinu Levi ben Chaviv. There are two kinds of blood in the heart:

1. The first kind is the blood in the heart itself. For this kind of blood one is liable for Kares but, in practice, one would not receive Kares because there is never a k'Zayis of blood in the heart of a bird.

2. The second kind is blood which enters the heart from outside, at the time of Shechitah. For this blood one is not liable for Kares, regardless of the quantity of the blood consumed.

Although there is never a k'Zayis of type (a) blood, it is possible sometimes that when type (b) blood combines with type (a), there is a k'Zayis in total. This means that sometimes one could be liable for a Lav if (a) and (b) combine together to make a k'Zayis. This is why Rashi and the Rambam write that one is not Chayav Kares for eating the blood of the bird's heart, which implies that one could sometimes transgress a Lav -- and that is when there was a k'Zayis together with type (b).

See the Rosh Yosef, on Rashi to Chulin 109a, who elaborates in a similar fashion to explain why Rashi does not write that one does not transgess a Lav with the blood of the bird's heart.

c) My colleague, Rav Yechezkel Frenkel shlit'a, explained why the Mishnah states "Eino Over." This phrase is meant to imply that there is a prohibition -- which could be mid'Oraisa -- on the blood of the bird's heart if not torn open. This is similar to the Mishnah in Yoma 73b which states that it is prohibited to eat and drink, etc., on Yom Kippur. The Gemara asks, why did the Mishnah say only that it is prohibited? It should have said a greater Chidush, that such actions are Chavav Kares! The Gemara answers that according to Rebbi Yochanan, the Mishnah states "Asur" in order to teach that even "Chatzi Shi'ur" is forbidden mid'Oraisa. In terms of our Sugya in Kerisus, Chatzi Shi'ur would mean anything less than a k'Zayis. Even though there is no Kares for less than a k'Zayis, according to Rebbi Yochanan it is a Torah prohibition. This is why the Mishnah states "Eino Over"; he is not liable for Kares but he has violated a Torah prohibition by consuming even a small amount of the heart's blood.

Kol Tuv,

Dovid Bloom