The milk-glands somewhat resemble the salivary glands in the way in which they are affected by the central nervous system, and by the action of drugs upon them. The action of the central nervous system on the milk-glands, however, has not been made out with anything like the same clearness as in the case of the salivary glands, experiments on animals not having given very definite results. It is chiefly inferred from the effect of mental emotions in checking or altering the secretion of the milk; and from the effect of belladonna locally applied in checking the secretion. The amount of secretion appears to depend on the amount of blood-pressure in the gland, and gentle stimulation of the nipple increases both the flow of blood to the gland and the secretion of milk. It is uncertain whether there are definite secreting nerves affecting the gland-cells apart from the vasomotor nerves.

The character of the milk depends to a great extent upon the feeding and exercise of the mother, and diet is the most important agent in regulating both the quality and the quantity of the milk. As Dolan points out, it not unfrequently happens that a wet nurse, when first she arrives, yields such milk that the child she is nursing thrives well, but the quality soon falls off. In place of much outdoor exercise and plain, nutritious diet, she is fed luxuriously and gets little exercise. In order to restore the quality of the milk in such a case, the woman must be restored as far as possible to her previous conditions of diet and exercise.

Many substances are excreted in the milk, such as ammonia and the aromatic oils to which vegetable substances belonging to Umbelliferae and Cruciferae owe their flavour, probably also all volatile oils are thus excreted. Amongst those which have actually been found to pass into the milk are the oils of anise, cumin, dill, wormwood, and garlic, as well as turpentine and copaiba. The purgative principles of rhubarb, senna, scammony, and castor-oil, pass into the milk. Opium, iodine, and indigo do so also, and metals, such as antimony, arsenic, bismuth, iron, lead, mercury, and zinc. Volatile oils, having an agreeable taste, do not appear to affect the secretion of milk directly, but appear to render it pleasant to children, so that they take the breast eagerly. When lactation is defective they may increase the reflex stimulus to the nipple by making the child suck more vigorously and thus increase the quantity of milk. For this reason such volatile oils as anise and dill may be useful as galac-tagogues. Garlic, on the contrary, renders the milk disagreeable to children, so that they will not take it. Copaiba also renders the milk disagreeable. The nearest approach to a true galacta-gogue is jaborandi, but it affects the gland only temporarily. Beer and porter stimulate the secretion for a short time, but they produce no proportionate benefit in the child, and nursing mothers are, as a rule, much better without alcohol, and should rather take milk instead. When the milk of the mother is deficient in saline constituents they may be supplied by giving the appropriate salts to the mother.

Various physiological actions may be produced in the child by administering drugs to the mother. The administration of acids to nursing mothers is generally to be avoided, as they are apt to cause griping in the child. Neutral salts as a rule pass into the milk and cause looseness of the bowels in the child. Senna, castor-oil, rhubarb, scammony, sulphur, and probably jalap, act as purgatives to the child. Salts of potassium administered to the mother will act as diuretics to the child. Turpentine administered to the mother also can be detected in the urine of the child; and this is also the case with copaiba and iodide of potassium. Opium administered to the mother may act as a narcotic to the child, and mercury, arsenic, and iodide of potassium may all be given to nursing children by administration to the mother.