vejno

The direction is the definition. A vejno carries blood back towards the koro; an arterio carries it away. That is the whole distinction and it is worth fixing early, because English speakers use vein loosely for any blood vessel and Esperanto medical writing does not. The neutral word covering both is sangovaskulo or simply vaskulo. Arterio has no page on this site yet, but the word is exactly what it looks like, from Greek artēria.

It sits in the body set. Sango at 432 is the blood, koro at 122 the heart, haŭto at 1,300 the skin, osto at 1,326 the bone and muskolo at 2,232 the muscle. This chapter's article on the body collects them; vejno is the one that only shows up once you are looking at the back of your own hand.

And it is not only anatomy. A leaf has vejnoj, and so does a piece of marble, and so does a seam of ore in a mine, which is the sense English keeps in a rich vein of coal. Esperanto allows all three, exactly as Latin vena did. What Esperanto does not do is the mood sense: in that vein has no vejno in it at all, and you would say en tiu spirito or simile.