We are now in Tractate Bechoros, chapter 6, Mishnah 11 - the final Mishnah in the chapter dealing with permanent blemishes, those blemishes that permit the slaughtering of a bechor.
"Znav ha'egel she'eino magia l'arkov" - the tail of the calf that does not reach the arkov:
The tail of the calf, which is a young cow, does not reach down to its arkov. We translate the word arkov as knee, but we must look at the skeleton of a cow to understand the exact meaning of this.
A human leg consists of one segment above the knee and one segment below it, while the joint below that - the ankle - is a completely different limb, connecting the leg to the foot. By contrast, a cow's leg has three segments:
The upper segment: similar to the upper thigh bone.
The middle segment: the shinbone (tibia), at the top of which is a knee joint called the arkov.
The lower segment: an additional bone containing another joint, which is also a sort of knee.
For this reason, the upper joint is called the arkov with the letter ayin, and the lower joint is called the arkuvah with the letter aleph - even though the names sound very similar.
The Mishnah thus establishes that if the tail of the newborn calf is not long enough to reach the leg joint called the arkov, it is a blemish - a tail that is too short. The Mishnah continues: "Amru chachamim, kol zman sheha'agalim... vechol yom shemegadlin hen nimtachot" - the Sages said, as long as the calves... and every day that they grow they stretch - it is the nature of calves that their tail begins at least at this length, and as they grow, the tail stretches and lengthens. But if on the first day its tail is so short that it does not reach the arkov, it has a blemish.
Which arkov is meant?
The Mishnah asks which joint is being referred to: the upper one between the thigh and the shin, or the lower one below it? Rabbi Yochanan ben Nuri says: "L'arkov sheba'emtza hayarech" - to the arkov in the middle of the thigh - meaning the upper joint, the arkov with the letter ayin. The tail must reach down to there, and not to the lower joint, the arkuvah with the letter aleph.
Conclusion of the chapter:
The Mishnah concludes: "Al elu mumin shochatin es habechor upesulei hamukdashin nifdin aleihen" - on account of these blemishes we slaughter the bechor, and disqualified consecrated animals are redeemed on their account. This is a repetition of what was stated at the beginning of the chapter, and looking back teaches that this entire list before us is a list of the blemishes for which it is permitted to slaughter a bechor, and because of which disqualified consecrated animals (offerings such as an olah, shelamim, and the like) must be redeemed before they are offered. This repetition also separates this list from what follows it, as other blemishes will appear before us that do not permit slaughtering the animal immediately. But why does the Mishnah repeat itself?
The Bartenura answers: The repetition is meant to confirm that everything listed in the chapter, in the eleven Mishnayos before us, are indeed permanent blemishes. This includes the three blemishes of Rabbi Ila, about which we deliberated when one beis din did not accept them and another beis din did accept them. The halachah follows Rabbi Ila, and therefore even if an animal is found with one of these three - which are also included in the list - it is permitted to slaughter the bechor outside the Beis Hamikdash and eat it.
In summary: In this Mishnah we learned about the blemish of a calf's tail that does not reach the arkuv, we discussed the anatomy of the animal's leg and the distinction between the upper 'arkuv' and the lower 'arkuvah', and we saw the opinion of Rabbi Yochanan ben Nuri that this refers to the joint in the middle of the thigh. We also discussed the conclusion of the chapter - "Al elu mumin shochatin es habechor ufesulei hamukdashin nifdin aleihen" - on account of these blemishes we slaughter the bechor, and disqualified consecrated animals are redeemed on their account - and the Bartenura's explanation that this repetition comes to confirm that all the blemishes enumerated in the chapter are permanent blemishes.