Bekhorot 39: three things to remember

Missing an internal organ disqualifies an offering, but once blood is collected, even losing most flesh still allows sprinkling.
Piercings and gum injuries only disqualify a firstborn if exposed on the outer surface, excluding concealed molars and inner gums.
A damaged genital sheath is a permanent blemish, but complete tearing is not because the tissue can fully regenerate.