1) THE CONCERN FOR CRUSHED LIMBS AT BIRTH
QUESTION: Rav Nachman states that the "Beis ha'Rechem" poses no concern for crushed limbs. RASHI (DH Beis ha'Rechem) explains that this refers to a newborn calf. Rav Nachman is saying that we are not concerned that its limbs may have been crushed during birth as it passed through the narrow birth canal. Accordingly, the newborn calf is not considered a Tereifah and one may slaughter it right away without waiting.
The RAN notes that Rav Nachman refers to a case in which the calf was born at term. If the calf was born prematurely, then we must suspect that the calf is a Nefel, a non-viable fetus, for which we must wait eight days before slaughtering it (see Shabbos 135b).
The Ran cites "Rishonim" who explain that Rav Nachman permits the calf in a case in which the mother cow had a difficult birth. If Rav Nachman refers to a case of a normal birth, then there should be no reason for him to teach that the calf is permitted, because it is obvious that in such a case the calf is permitted; if we must be concerned for crushed limbs in every case of a normal birth, then it should follow that most births are fatal. The Ran disagrees with this logic and asserts that it is still necessary for Rav Nachman to teach that we are not concerned for crushed limbs in the case of a normal birth.
How does the Ran answer the logic of the "Rishonim" who argue with him? Why does Rav Nachman need to teach that there is no concern for crushed limbs even in a normal birth? It is obvious that there is no such concern, because there were such a concern, then this would mean that most baby animals do not survive for more than one year, which obviously is not true!
ANSWER: The RAN answers that certainly it is obvious that most baby animals are not born with crushed limbs. However, there might be a "Mi'ut d'Shechi'ach" -- a frequent or significant minority of animals whose limbs are crushed in the process of birth. (Some say that if one out of every ten animals are born as Tereifos, this is considered a "Mi'ut d'Shechi'ach"; see TESHUVOS MISHKENOS YAKOV YD 17, cited by the DARCHEI TESHUVAH 39:3 and TESHUVOS SHEVET HA'LEVI 4:81.) Accordingly, since it is possible to determine whether or not the animal is a Tereifah (that is, by waiting 24 hours before slaughtering it), we should not rely on the fact that the majority of animals are born healthy. Indeed, this is the reason why the lungs of every animal must be examined for Sirchos, even though a majority of animals do not have Sirchos. Since lungs with Sirchos are a "Mi'ut d'Shechi'ach," a frequent minority, we must examine all lungs and we may not rely on the Rov.
Therefore, Rav Nachman teaches that we are not concerned at all that an animal's limbs were crushed during birth. Such an occurrence is a "Mi'ut d'Lo Shachi'ach," an infrequent minority.
(The Ran earlier (3b of the pages of the Rif, DH Garsinan) cites proof for this principle from the Gemara later (51b) that says that an animal must be examined only when it fell from a roof. The accident creates a doubt about the animal's health. In a normal case, the animal does not need to be examined (besides the examination of the lungs). Moreover, from the fact that the Gemara here says that an animal that drags its hind legs does not need to be examined (because we assume that it merely has a cramp), it is clear that a perfectly healthy animal certainly does not need to be examined.) (D. BLOOM)
2) THE CALF THAT STOOD UP AFTER BIRTH
QUESTION: Rav Nachman (50a) rules that a newborn calf may be slaughtered immediately, and it is not necessary to wait 24 hours out of concern that its limbs were crushed during birth rendering it a Tereifah. The Gemara attempts to prove Rav Nachman's ruling from a Beraisa that states that when a firstborn animal is born on Yom Tov, even if it has a blemish it is not Muktzeh and may be slaughtered on Yom Tov. It is clear from there that there is no concern that an animal was born with crushed limbs. The Gemara refutes this proof and says that in the case of the Beraisa, the animal was "Hifris Al Gabei Karka." RASHI (DH Kegon) explains that this means that the calf shook itself in an attempt to stand, and it thrust its hooves into the ground. Rashi says that this is in accordance with the Gemara later that says that if an animal fell off of a roof but afterwards was able to stand up, its ability to stand is sufficient proof that the animal is not a Tereifah and one need not wait 24 hours before slaughtering it. If the animal that fell off of a roof shook itself and thrust its hooves into the ground (but did not succeed in standing up), then one must wait 24 hours before slaughtering it.
Why does Rashi explain that in the case of the Bechor born with a blemish, the animal attempted to stand up unsuccessfully, and this suffices to prove that the animal is not a Tereifah? Why does he not explain simply that the animal succeeded in standing, like the similar case in the Gemara later that Rashi cites? The Gemara there implies that an unsuccessful attempt to stand does not indicate that the animal is not a Tereifah!
ANSWER: The LEV ARYEH explains that when the Gemara attempts to cite proof for Rav Nachman's ruling, it understands that Rav Nachman permits the newborn calf even if there is some "Rei'usa" -- some weakness or negative condition that gives reason to assume that the animal is a Tereifah. The Gemara assumes that this is Rav Nachman's intention because, otherwise, Rav Nachman is teaching nothing new.
Accordingly, the Gemara's proof for Rav Nachman must also be a case in which the animal has a "Rei'usa" and yet the animal is considered Kosher and there is no need to wait before slaughtering it. In the case of the Bechor born on Yom Tov, the Gemara originally assumes that it is considered Kosher even if it has some "Rei'usa." The Gemara refutes this proof by saying that the animal was "Hifris Al Gabei Karka." Rashi does not explain that this means that the animal stood up successfully, because this would mean that the animal had no "Rei'usa" at all. If the Gemara is answering that the Beraisa's case is not a proof for Rav Nachman because the animal had no "Rei'usa," then the Gemara should have answered simply that the animal that was born on Yom Tov was observed to have no blemish at all. The fact that the Gemara answers that it was "Hifris Al Gabei Karka" suggests that there was some slight "Rei'usa," but a lesser one than the one that Rav Nachman permits. Rav Nachman would have permitted the animal to be slaughtered on Yom Tov even if it could not move at all. (D. BLOOM)