The Instructor 1.6.39

But if some want to argue further, saying that milk signifies the first lessons, as the first food, and solid food spiritual knowledge, raising themselves up to knowledge, let them know that when they call solid food meat and the flesh and blood of Jesus, they are brought down by their own boastful wisdom to the true simplicity. For blood is found to be the first thing formed in a human being, and some have even dared to call it the substance of the soul. This blood, changed by a natural digestion when the mother conceives, blooms and ages through sympathetic love, so that the child need not fear. Blood is more liquid than flesh, being a kind of liquid flesh; and milk is more nourishing than blood and finer in texture. For whether the blood supplied to the embryo, previously sent through the mother's navel, or the menstrual flow itself, shut off from its proper course, is commanded by God, who nourishes all and brings all to birth, to go by a natural diffusion to the breasts, now swelling, and, changed by warm air, is prepared as desirable food for the infant, it is the blood that is changed. For of all the members the breasts are in closest sympathy with the womb. So when, at birth, the vessel by which blood was carried to the embryo is cut off, its passage closes, and the blood takes its course toward the breasts; and when much of it flows there, they are distended and the blood is changed into milk, just as blood is changed into pus in an ulcer. Or again, the veins beside the breasts open when they are stretched in pregnancy, and the blood pours into the natural cavities of the breasts; mixed with the air sent down from the neighboring arteries, while the underlying substance of the blood remains intact, it foams and turns white, and by such agitation is changed into froth, much as the sea, which the poets say, when winds strike it, “spits out the foam of the brine.” But its substance remains blood.