myelin sheath vs sheath

myelin sheath and sheath both are nouns.

Word	Noun	Adjective	Verb	Adverb
myelin sheath	Yes	No	No	No
sheath	Yes	No	No	No

As nouns, sheath is a hypernym of myelin sheath; that is, sheath is a word with a broader meaning than myelin sheath:

myelin sheath: a layer of myelin encasing (and insulating) the axons of medullated nerve fibers
sheath: an enveloping structure or covering enclosing an animal or plant organ or part

Other hypernyms of myelin sheath include case.

myelin sheath (noun)	sheath (noun)
a layer of myelin encasing (and insulating) the axons of medullated nerve fibers	a protective covering (as for a knife or sword)
	a dress suitable for formal occasions
	an enveloping structure or covering enclosing an animal or plant organ or part