OK, that was very clear, thank you very much Faldage. I get the picture now.

The OED includes liver and kidneys in its definition of offal, by the way, despite descibing it as waste stuff etc. I like good quality liver, but I understand it can raise cholesterol levels and that has become an issue.

Regarding rats, I recall reading that:

a) They were used by some hardy souls to supplement the meat in the diet on naval vessels in particular.

b) Some sailors specialised in catching them and sold them to others.

c) The practice was forbidden by most naval authorities on health grounds.

I hadn't heard the suggestion that they would fend off scurvy.