פירוש על שמות 29:22

החלב THE FAT — This means the fat of the gut, or, (according to the opinion of Rabbi Ishmael,) of the maw (Chullin 49b; cf. Rashi on Leviticus 3:3).

כי איל מלואים הוא, this was the reason that the right thigh was burned on the altar. This practice did not apply to any of the other animal sacrifices. [with the exception of burnt-offerings which were burned up completely. Ed.] The Torah views the right thigh of the animal as equivalent to the right hand of a human being. Wherever in the Torah the word אצבע appears in the same context as the priesthood, the reference is to the finger of the right hand. (Menachot 10).

This refers to the fat of the small intestines. R. Akiva and R. Yishmael disagree over this: one says it is the fat of the small intestines, and one says it is the fat of the stomach.