Hi Faith,
Losul did give you some good information, as always. The others have as well. 🙂
Heart is technically considered a muscle meat. It is loaded in natural CoQ10 as well as a host of vitamins. Giving any dog fresh food will make for a healthier dog. I do feed mine a combination of fresh and kibble, simply because I have too many pounds of dogs at my house, otherwise it would all be fresh/raw. Here’s another link from Primal, a reputable raw food company confirming that heart is a muscle meat, even though it’s considered an organ.
http://blog.primalpetfoods.com/post/19313616133/muscle-meat-vs-organ-meat
Another raw website explaining what the organs are:
http://dogsdinner2.webs.com/offalororgan.htm
Good Luck with your dog! I wish you the best! 🙂
