rumen vs tummy

rumen and tummy both are nouns.

Word	Noun	Adjective	Verb	Adverb
rumen	Yes	No	No	No
tummy	Yes	No	No	No

As nouns, tummy is a hypernym of rumen; that is, tummy is a word with a broader meaning than rumen:

rumen: the first compartment of the stomach of a ruminant; here food is collected and returned to the mouth as cud for chewing
tummy: an enlarged and muscular saclike organ of the alimentary canal; the principal organ of digestion

Other hypernyms of rumen include breadbasket, stomach, tum.

rumen (noun)	tummy (noun)
the first compartment of the stomach of a ruminant; here food is collected and returned to the mouth as cud for chewing	an enlarged and muscular saclike organ of the alimentary canal; the principal organ of digestion
	slang for a paunch