420. The same thing is imaged in the lungs, whose arteries and veins correspond to the affections of love, and whose respirations correspond to the perceptions and thoughts of the understanding, as has been said above. That the heart's blood is purified of undigested matters in the lungs, and nourishes itself with suitable food from the inhaled air, is evident from much observation. (1) That the blood is purified of undigested matter in the lungs, is evident not only from the influent blood, which is venous, and therefore filled with the chyle collected from food and drink, but also from the moisture of the outgoing breath and from its odor as perceived by others, as well as from the diminished quantity of the blood flowing back into the left ventricle of the heart. (2) That the blood nourishes itself with suitable food from the inhaled air is evident from the immense volumes of odors and exhalations continually flowing forth from fields, gardens, and woods; from the immense supply of salts of various kinds in the water that rises from the ground and from rivers and ponds, and from the immense quantity of exhalations and effluvia from human beings and animals with which the air is impregnated. That these things flow into the lungs with the inhaled air is undeniable: it is therefore undeniable also that from them the blood draws such things as are useful to it; and such things are useful as correspond to the affections of its love. For this reason there are, in the vesicles or innermost recesses of the lungs, little veins in great abundance with tiny mouths that absorb these suitable matters; consequently, the blood that flows back into the left ventricle of the heart is changed into arterial blood of brilliant hue. These facts prove that the blood purifies itself of heterogeneous things and nourishes itself with homogeneous things. That the blood in the lungs purifies and nourishes itself correspondently to the affections of the mind is as yet unknown; but in the spiritual world it is very well known, for angels in the heavens find delight only in the odors that correspond to the love of their wisdom, while the spirits in hell find delight only in the odors that correspond to a love opposed to wisdom; these are foul odors, but the former are fragrant. It follows that men in the world impregnate their blood with similar things according to correspondence with the affections of their love; for what the spirit of a man loves, his blood according to correspondence craves and by respiration attracts. From this correspondence it results that man as regards his love is purified if he loves wisdom, and is defiled if he does not love it. Moreover, all purification of man is effected by means of the truths of wisdom, and all pollution of man is effected by means of falsities that are opposite to the truths of wisdom.