Chullin, Chapter 4, Mishnah 6. This mishnah turns to a question of tereifah: when a limb of an animal has been cut off or amputated, does the animal remain fit to eat, or has it sustained a wound severe enough to render it a tereifah?
The opening ruling: "Behema shenichtechu ragleha" - an animal whose hind legs were cut through - "min ha'arkuva ulematah, kesheira." The arkuva here is the joint that corresponds to what we would call the knee. Should the severing have occurred anywhere from that joint downward, the animal retains its fitness: it may be slaughtered and its meat eaten, since no tereifah has been created.
"Min ha'arkuva ulema'la, pesula." Sever the limb higher up, at a point beyond that joint, and the animal is disqualified. Such a wound belongs to the category of injuries from which there is no recovery; the creature has no prospect of surviving out twelve months, and consequently even shechitah performed perfectly cannot permit its flesh.
"Vechen shenital tzomet hagiddin." The identical verdict applies when the tzomet hagiddin has been removed. Three sinews stretch down the rear portion of the leg and gather together at a spot slightly higher than the joint. Once that place of convergence is gone, the animal has become a tereifah, and the ruling holds even where not a single bone has been damaged.
A fresh case follows, in which nothing was cut off at all: a bone fractured, leaving the limb suspended from the body. "Nishbar ha'etzem." The break occurred beneath the arkuva, yet skin and flesh continue to hold the limb in place. What, then, is the halachic standing of that dangling portion?
"Im rov habasar kayam, shechitatah metaharto." Should the greater part of the flesh around the fracture remain whole, so that the broken bone is still wrapped in flesh and skin, the limb is reckoned as belonging to the animal itself. The shechitah accordingly purifies it as it purifies the animal's body, and the limb may be eaten.
"Ve'im lav, ein shechitatah metaharto." Where most of the flesh at the fracture has been destroyed, the area torn apart and crushed, the shechitah accomplishes nothing for that suspended limb. Even though something still binds it to the body, halachah views it as though it had already fallen away, and slaughtering the animal leaves that status untouched.