שולחן ערוך יורה דעה · סימן ל׳

English

If in a behemah (domesticated animal) or chaya (wild animal), most of the bone of the head is crushed, either most of its vault (gab) or most of its circumference (hekkef), the animal is a trefah, even though the membrane is intact.

English

If it was reduced, and there's missing from it [a piece] the size of a selah coin, it is an invalid animal. And if there punctured holes that are [big enough that there are pieces] missing, if between all of them [there's missing] the size of a selah coin, which is one third of a handbreadth, it is an invalid animal. Rema: But, less than this there is what to say it is valid even in our days since we're not worried in a domesticated animal for puncturing the membrane of its brain. And in a sea-bird, such as a goose and a duck , even if the bone is punctured any amount, it is an invalid animal. And a dry land bird that a weasel bit with its teeth [the bird's] head, or it became damaged on a tree or a stone, one places his finger on the side of the hole and sticks his finger there, or he puts in finger into its mouth and presses there, if the brain doesn't ooze out of the hole it is known that the membrane of the brain is not punctured, and it is valid, and if it comes out, it is an invalid animal. Rema: And we are not experts in checking for invalidations in animals, and therefore there's what to rule invalid if the bone of the skull has any hole at all in a bird, even for a dry land bird.