Yosef Karo

“ If the marrow decomposed and it can be poured like water or melted wax to the extent that the [spinal] cord, when one holds it in his hand, the part that extends out from his hand bends over and is not able to stand by itself, the animal is invalid. And so too, even though the inside didn't soften, but rather it became thick and heavy, if when one stands it up it is not able to stand because of its heaviness, it is an invalid animal. ”

“ And if most of the skin was not severed, even if all of the marrow inside was severed, it is valid.
2 נסדק העור לארכו -- כשרה. וכן אם נשברה השדרה ולא נפסק החוט או שנתמעך המוח שבתוך החוט ונתנדנד, הואיל ועורו קיים הרי זו, מותרת.
If the skin split lengthwise, it is valid. And so too, if the spine broke but the [spinal] cord was not severed, or if the marrow that is inside the [spinal] cord was crushed and it wobbles, since the skin is still intact it is permitted. ”