According to AAFCO, meat can be derived from any skeletal muscle of any slaughterd animal. It can come from the tongue, diaphragm, heart, or esophagus, and it can include fat or skin. "If it bears a name descriptive of its kind," AAFCO's guideline goes on to say of meat, "it must correspond thereto."
An investigative report by the Animal Protection Institute of America (APIA), a national nonprofit animal advocacy organization, is both impressive in its scholarshop and utterly depressing in its conclusions. A book by Ann Martin titled Food Pets Die For is even more damning.