What is "cardinal veins"

Cardinal \Car"di*nal\, a. [L. cardinalis, fr. cardo the hinge of a door, that on which a thing turns or depends: cf. F. cardinal.] Of fundamental importance; pre["e]minent; superior; chief; principal. The cardinal intersections of the zodiac. --Sir T. Browne. Impudence is now a cardinal virtue. --Drayton. But cardinal sins, and hollow hearts, I fear ye. --Shak. Cardinal numbers, the numbers one, two, three, etc., in distinction from first, second, third, etc., which are called ordinal numbers. Cardinal points

1. (Geol.) The four principal points of the compass, or intersections of the horizon with the meridian and the prime vertical circle, north, south east, and west.

2. (Astrol.) The rising and setting of the sun, the zenith and nadir.

   Cardinal signs (Astron.) Aries, Libra, Cancer, and Capricorn.

   Cardinal teeth (Zo["o]l.), the central teeth of bivalve shell. See Bivalve.

   Cardinal veins (Anat.), the veins in vertebrate embryos, which run each side of the vertebral column and returm the blood to the heart. They remain through life in some fishes.

   Cardinal virtues, pre["e]minent virtues; among the ancients, prudence, justice, temperance, and fortitude.

   Cardinal winds, winds which blow from the cardinal points due north, south, east, or west.