Bechoros 6:4-5

Bechoros 6:4

More blemishes that permit slaughtering a firstborn animal: if its nose or its lip is pierced, defective, or slit; if its outer gums are defective or decayed, or if its inner gums have been removed. Rabbi Chaninah ben Antigonos says that we don’t examine an animal from its twin teeth inward, inclusive.

Bechoros 6:5

If the sheath of the animal’s genital is defective, that’s a blemish; the same is true of female genitals in the case of sacrifices. (Only male animals have firstborn status.) It’s a blemish if the animal’s tail is mutilated at the bone but not if it’s defective in a joint, if the top of the tail has exposed bone, or if there’s a finger-width of flesh between segments of the tail.