Excerpted from Quaestiones de animalibus III, q. 1,

We should say that a vein is equivocal of two things or is twofold, namely one that pulses, and this specifically is called an artery, and the other that does not pulse, and in a man it is called by the specific name "vein." A pulsing vein or rather an artery is necessary for the continuation of heat and the carrying away of the spirit to each of the limbs, for these are, so to speak, the center of life, and therefore these arteries are born from the part of the heart in the left side, in which abounds the maximum heat of the heart and vital spirit. But a vein that does not pulse is necessary for the delegation of nutrients, namely of humors, to each of the limbs and for sustaining of warmth and spirits, and for the restoration of what is lost, because when nourishment is received in a defined part, unless it had passage to each limb, the loss would not be able to be repaired in each.

original latin

Dicendum, quod arteriae sunt necessariae, quia ad conservationem animalis duo requiruntur, scilicet restauratio deperditi et continuatio vel conservatio acquisiti. Sed propter restaurationem deperditi ponuntur venae, quae sunt quietae et non pulsatiles, ut per eas sanguis sive nutrimentum ad singula membra deferatur.

Sed propter conservationem acquisiti ponuntur arteriae, quae sunt venae pulsatiles, per quas virtus vitalis et calor naturalis et spiritus, sine quibus non fit operatio, a corde derivantur ad singula membra. Unde sicut arteriae sunt necessariae ad attrahendum aerem propter infrigidationem et temperamentum cordis, sic arteriae a corde procedentes sunt necessariae propter virtutem suae influentiae ad alias partes.

Unde Philosophus in xiv huius dicit, quod omnis operatio corporalis fit mediante corde et influentia cordis ad alias partes -- sentire est operatio corporalis et vivere -- ergo fit per medium corporale, et istud medium est arteria, quae plena est spiritu et calore naturali. Et ideo quaelibet arteria duas habet tunicas propter continuum pulsum spiritus in corde et arteriis. Sed vena tantum unam tunicam habet praeter illam quae dirigitur a corde ad pulmonem. Ipsa enim non solum contentiva est sanguinis, sed spiritus, haec ideo duas habet sicut arteria.