Answer:
   
   Certain parts are listed as being prohibited, such as the fat of and
   by the tail, the fat on the kidneys, the sinew that surrounds the
   sciatic nerve, and the blood that was in veins or arteries at the time
   of death. [Note that blood found within tissues, and is clearly not
   within a part of the circulatory system, is kosher.] Kidneys are
   generally considered non-kosher, as there is no way to get all the
   non-kosher blood out of them. Brain is kosher, as are sweet-breads
   (thymus or pancreas).
   
   The thigh is generally not eaten in the US, as it is not cost
   effective to remove all the branches of the prohibited tendon. We sell
   that part of the cow to the non-kosher market.