On Food and Cooking

include a number of different muscles and
their connective-tissue sheaths. They are
therefore relatively tough. The tenderloin is
appropriately named because it is a single
muscle with little internal connective tissue
that runs along the back and gets little action;
it’s tender. Bird legs are tougher than breasts
for the same reasons; the protein in chicken
legs is 5–8% collagen compared to 2% in the
breast. Younger animals — veal, lamb, pork,
and chicken all come from younger animals
than beef does — have tenderer muscle fibers
because they are smaller and less exercised;
and the collagen in their connective tissue is
more rapidly and completely converted to
gelatin than older, more cross-linked collagen.