420. A similar thing is portrayed in the lungs, whose arteries and veins correspond to the love's affections, and whose respirations correspond to the understanding's perceptions and thoughts, as has been said above. That the heart's blood is purified from disorderly matters in the lungs and is nourished with the beneficial properties of the air inhaled, is evident from much observation. That the blood is purified of disorderly matters in the lungs, is evident not only from the inflowing blood which is venous, and therefore filled with the chyle collected from food and drink, but also from the expirations which are moist, and from their odour, as perceived by others, as well as from the diminished quantity of blood flowing back into the left ventricle of the heart. That the blood is nourished with the beneficial matters of the air inhaled, is evident from the immense volumes of odours and exhalations continually flowing forth from meadows, gardens and woods; and from the copious supplies of various kinds of salts dissolved in the waters rising out of the earth, and from rivers and ponds; and from the vast quantities of exhalations and effluvia from human beings and animals, with which the air is laden. That these things flow into the lungs with the indrawn air is not to be denied, nor can it be denied that the blood draws therefrom such things as are beneficial to it; and such things are beneficial as correspond to the affections of its love. Hence it is that there are in the air-cells, or inmost of the lungs, small veins with little mouths, which absorb such things; thereupon the blood flowing back into the left ventricle of the heart is changed into arterial blood and is in good condition. These facts prove that the blood purifies itself from 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 is very well known in the spiritual world; for angels in the heavens enjoy only those odours that correspond to the love of their wisdom, but spirits in hell enjoy only those odours that correspond to the love opposed to wisdom; these odours are stenches, but the former are perfumes. It follows from this that men in the world impregnate their blood with similar things according to the correspondence with their love's affections; for what man's spirit loves, the blood in accordance with the correspondence craves, and attracts by respiration. From this correspondence it comes that man is purified in respect of his love if he loves wisdom, and is defiled if he does not love it. Moreover, all purification of man is effected by truths of wisdom, and all defilement by falsities opposed to the truths of wisdom.