The selection of a wet nurse should be based upon the following data:

Her own general health and digestion must be good, and her bowels not habitually constipated. All evidence of syphilis, scrofula, tuberculosis, or other disease must be rigidly excluded. A primip-ara is to be preferred to a multipara as having milk somewhat richer in fat.

Her best age is between twenty-two and thirty years, and she should be of good temperament and have cleanly habits. Her period of lactation should not be too widely different from the age of the infant to be nursed.

The breasts should be firm, and the nipples fairly prominent and free from fissures. Breasts which are always oozing spontaneously are by no means always the best, for their milk is soon exhausted.

The nurse's own child, if living, should be examined, for if it has been nursing at the breast its condition is an even better index of suitableness of the mother's milk than her own appearance.

The child should be in good flesh and firm, without evidence of gastric catarrh, fever, or indigestion, and of good development for its age.

The question whether it is possible for a wet nurse to transmit her own mental and physical characteristics to the child at her breast has given rise to much discussion, but there is no more reason why she should transmit an evil temper through the food she gives than that a cow should transmit a bland one. An ill-tempered, irascible wet nurse may sometimes give milk which disagrees with the child's stomach, but beyond that there is nothing in the belief that she can affect it mentally or morally by this agency.

The milk which can be expressed from the breast should have the following properties: Reaction alkaline, color an opaque blue-white, specific gravity 1.031, taste sweetish. Examined microscopically, the fat corpuscles should abundantly fill the field and be of nearly equal size.

Human breast milk on an average has a specific gravity of 1.031 at 70° F., but it varies between 1.017 and 1.036 (Holt). The fat varies less than the sugar and proteids.

If the specific gravity of the milk is high, and at the same time the percentage of fat is considerable, the gravity must be due to a larger quantity of proteids than usual, because fat tends to lower it. If the fat percentage is small and the specific gravity of the milk is low, the proteids must also be reduced. Holt furnishes the following convenient table for determining the richness of human milk in fats and proteids, which gives results that are sufficient for practical purposes in examining the milk of a wet nurse:

To calculate the actual fat from the cream multiply by three fifths.

In examining the composition of the milk of a wet nurse it is only fair to the nurse to remember that, like cow's milk, her own contains less fat when the milk is first drawn than after the breast has been nursed for a few moments.

The quantity of the milk may be best estimated by weighing the infant immediately before and after suckling, when the gain should be between three and six ounces.