_Human Milk_.--Between the time of birth and the beginning of lactation there is always an interval during which the breasts secrete colostrum, just as they do throughout pregnancy. Although the nutritional value of this fluid is not great, it is doubtful if colostrum serves any other essential purpose than as nourishment. Possibly it also stimulates the intestines to expel the material which has collected within, them during fetal development, yet we know the bowels will move without a purgative; and often do so long before the infant is placed at the breast. Typically, the secretion of milk begins the third day after delivery; yet in perfectly normal patients it may appear as early as the second or as late as the fifth, and occasionally lactation does not begin until the baby is more than a week old. As to what starts the secretion of milk we have only a vague idea; but we know that when the flow is once established its continuation depends primarily upon the sucking efforts of the infant. If nursing is discontinued the secretion dwindles and the breasts dry up. On the other hand, the strong, persistent stimulus of the infant's suckling gradually brings the secretion to a high degree of efficiency. Within the first two weeks, therefore, the daily secretion increases from a few ounces to a pint or more. Subsequently the output fluctuates between one and two quarts daily, according to the demands made upon the breasts; the secretion is larger, consequently, if there are twins. Astounding yields of milk have been recorded, as in the case of a wet-nurse in a German institution who nursed a number of infants and became capable of supplying three to four quarts daily. That newborn infants thrive better on human milk than on any other nourishment is a conviction that must come home to every one who has had even a limited experience. It keeps the babies in health, serves to make them grow, and promotes the development of all their organs as nothing else will. Because there are present in this fluid all the elements necessary for nutrition, physiologists have called it a perfect food. Quantitatively its most important ingredient is water, which constitutes about 86 per cent. of its weight. It also contains about 7 per cent. of milk-sugar, 4 per cent. of butter fat, 2 per cent. of protein, and 0.2 per cent. of mineral matter.