bundle of His vs cardiac muscle

bundle of His and cardiac muscle both are nouns.

Word	Noun	Adjective	Verb	Adverb
bundle of His	Yes	No	No	No
cardiac muscle	Yes	No	No	No

As nouns, cardiac muscle is a hypernym of bundle of His; that is, cardiac muscle is a word with a broader meaning than bundle of His:

bundle of His: a bundle of modified heart muscle that transmits the cardiac impulse from the atrioventricular node to the ventricles causing them to contract
cardiac muscle: the muscle tissue of the heart; adapted to continued rhythmic contraction

Other hypernyms of bundle of His include heart muscle.

bundle of His (noun)	cardiac muscle (noun)
a bundle of modified heart muscle that transmits the cardiac impulse from the atrioventricular node to the ventricles causing them to contract	the muscle tissue of the heart; adapted to continued rhythmic contraction