sheath vs myelin sheath

sheath and myelin sheath both are nouns.

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

As nouns, myelin sheath is a hyponym of sheath; that is, myelin sheath is a word with a more specific, narrower meaning than sheath:

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

Other hyponyms of sheath include theca, lorica, medullary sheath, neurilemma, neurolemma, husk.

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