A question frequently asked on raw feeding boards is – what are organs?
Organs are the internal parts of an animal such as Liver, kidney, bladder, lungs, brains etc.
Heart is considered a muscle since it pumps blood and actually works hard! The gizzards of poultry are considered a muscle as well. Hearts are usually inexpensive to purchase and often beef heart becomes an inexpensive way of adding red meat to a predominately white meat diet.
The stomach of ruminants (aka cattle, deer, goats etc), also known as tripe, if unwashed and uncleaned, is supposedly really really good for dogs to eat as it contains a whole whack of good stuff in it. It also does apparently have quite the odour to it and if one can get their hands on it, needs to be processed outside and the contents disposed of.
Some people swear by it, I have never tried it.