9. It is for this reason too that I maintain that while inspiration into the left cavity is effected through the veins, in the right cavity it is effected through an artery; for vessels which are soft have more attractive power, being more capable of distension. Now there was a need for that part of the heart which enfolds it like a cloak to receive less cooling, for it is not on the right side that the heat is situated. Hence it received and organ which would be favourable to this defect, to prevent it from being entirely overcome by the air entering it.