Chullin 76: An Animal's Legs, the Arkuba

A new Mishna teaches that an animal whose hind legs are cut off "from the joint and below" is permitted.  If the legs were cut off "from the joint and above", the animal is considered to be a tereifa.  If an animal's tzomet ha'gidim, convergence of sinews, has been removed then it is also a tereifa.  The Gemara discusses the location of the "cut off" between above and below the joint.  This particularly disturbing set of images includes arguments about the physiology of different animals.  The upper leg, the lower leg and the arkuba, the joint between the bones in the upper and lower legs.