Feet of calves or oxen
Slit them in two, and take away the fat between the claws. The proportion of water to each heel is about a quart; let it simmer gently for eight hours (keeping it clean skimmed); it will make a pint and a half of strong jelly, which is frequently used to make calves’ feet jelly or to add to mock turtle and other soups. This jelly evaporated will give about three ounces and a half of strong glaze.
From The Cook’s Oracle by William Kitchiner, MD, New York, 1829
Comment: This recipe is not merely a tribute to the days when no part of a butchered animal was allowed to go to waste, it was an important source of jelling material in the days before the advent of the modern chemical industry. The best was isinglass, made from the air bladder of the Russian sturgeon; the more common rennet (used in cheesemaking) comes from the lining of the “abomasum” (fourth stomach) of a newborn calf or lamb–but we may be wandering into Too Much Information territory. At any rate, now you know how mock turtle soup is made, and what to do with ox heels should any ever come into your possession.