scleral veins vs vena

scleral veins and vena both are nouns.

Word	Noun	Adjective	Verb	Adverb
scleral veins	Yes	No	No	No
vena	Yes	No	No	No

As nouns, vena is a hypernym of scleral veins; that is, vena is a word with a broader meaning than scleral veins:

scleral veins: tributaries of the anterior ciliary veins that drain the sclera of the eye
vena: a blood vessel that carries blood from the capillaries toward the heart

Other hypernyms of scleral veins include vein, venous blood vessel.

scleral veins (noun)	vena (noun)
tributaries of the anterior ciliary veins that drain the sclera of the eye	a blood vessel that carries blood from the capillaries toward the heart