Milk, the normal food of the young of all mammals, is prepared by the milk glands of each species to meet the specific and peculiar growth and developmental needs of the young of the particular species. The milk of one species, while it may be used by the young of another, is not adapted to the needs of the young of any other species. The great differences between the chemical compositions of the milk of the various species, differences that are accompanied by physical differences equally as great, may be partially seen by a glance at the following tables:

COMPOSITION OF HUMAN AND ANIMAL MILKS

Average Chemical Composition Percent

	Water	Protein	Fat	Carbo- hydrate	Mineral Matter	Total
Human	87.75	1.60	3.95	6.25	0.45	34.70
Cow's	87.30	3.55	3.70	4.88	0.71	55.50
Dog's	75.60	11.20	9.60	3.10	1.35	55.30
Swine's	82.30	5.90	6.90	3.80	1.10	65.00
Sheep's	80.80	6.50	6.90	4.90	0.90	47.40
Goat's	85.70	4.30	4.50	4.40	0.80	70.00
Mare's	91.50	1.30	1.20	5.70	0.41	48.20
Rabbit's	69.40	15.50	10.50	2.00	2.60	85.00
Buffalo's	82.20	4.40	7.10	4.70	0.85	48.00
Camel's	87.10	3.60	2.70	5.30	0.75	58.15

Composition of mineral matter in 1,000 parts of water
free substance of human and animal milks.

 Acid binding Acid forming
 elements | elements
 |
 Pot- Sod-- | Cal- Mag- Iron Pho- Sul- Sil- Chl-
 ass- ium | cium nes- sph- phur icon orine
 ium | ium orus 
 |
Human 11.73 3.16 | 5.80 0.75 0.07 7.84 0.33 0.07 6.38
Cow's 13.70 5.34 | 12.24 1.69 0.30 15.79 0.17 0.02 8.04
Dog's 5.70 3.25 | 18.20 0.80 0.08 19.75 0.25 0.15 6.55
Swine's 9.35 6.95 | 21.35 1.20 0.25 16.20 0.30 0.25 9.15
Goat's 15.60 3.45 | 13.90 2.30 0.60 21.05 0.30 0.20 13.50
Mare's 12.05 1.60 | 14.25 1.50 0.20 15.00 0.15 0.05 3.65
Rabbit 11.40 7.75 | 33.60 1.75 0.07 22.80 0.40 0.30 7.00
Buffalo 6.60 2.88 | 15.95 1.50 0.08 16.15 1.37 --- 3.47
Camel's 10.50 2.00 | 15.48 2.70 0.12 17.20 2.05 --- 8.10

Studying the tables it will be noticed that the protein content of the various milks varies from 1.30 to 15.50, being highest in that of the rabbit, the young of which is one of the most rapidly growing mammals. Note, also, that the milk of the rabbit contains the highest percent of fat and mineral matter. The human infant is the slowest growing animal on the earth. Note that the milk of the human mother is lowest in protein and low in mineral matters and fat. The milk of the various species is adapted to meet the particular growth needs of each species.

Human milk resembles cow's milk but differs from it in several important particulars. It is much sweeter than cow's milk, has no odor, and varies in color from a bluish white to a rich, creamy yellow. One, however, cannot judge of the quality of milk from its appearance, for the yellowest milks owe their color to a substance called carotin which is found in certain vegetables used for food.

The composition of human milk is very much the same throughout the whole of the nursing period. The greatest variation is in its protein content which diminishes as time passes. The composition of the milk varies from day to day and even from one feeding to the next, as well as from the beginning to the end of each nursing.

Human milk, on an average, contains about 7 per cent milk sugar, 3 to 4 per cent fat, 1.50 per cent protein, and 0.20 per cent of salts. The percentage of whey or soluble proteins in human milk is much more easily utilized by the baby than are those of the cow's milk. Its salts are in a form much more easily utilized by the baby than are those of the cow's milk. There is sufficient of these salts for the baby's needs except that of iron. But, since, the child is born with a good supply of iron stored in the liver, it does not suffer, at least for many months, due to this deficiency. This is, indeed, a remarkable instance of the precise adaptation of the milk to the needs of the child.

The most important differences between human milk and the milk of cows or goats are qualitative rather than quantitative. Cow's milk contains too much casein and not sufficient albumen for the human infant. It is also deficient in milk sugar. The amino-acid content of its proteins is different, also. Mother's milk is peculiarly adapted to the needs of the human infant. Cow's milk is not.

While the human infant is normally of very slow growth, the human brain is larger at birth than that of any other animal and its rate of growth is greater. In mother's milk nature has provided for the greater demands made by the rapid growth and important size of the human brain in infancy and she has not made these provisions in the milk of any other animal. In other words, mother's milk is peculiarly fitted to nourish the brain of the infant, while that of the cow and goat are not. Not merely is human milk more digestible, but it is more complex than the substitutes. In it are found lecithin bodies in peculiar properties, which serve for the construction of the large human brain.

The minerals of cow's milk are not readily assimilated by the human infant so that one-third of the mineral elements of cow's milk is lost in the bowel discharges. This may not be as important as it appears at first glance, as cow's milk is richer in most minerals than mother's milk.

In view of the superior fitness of mother's milk to nourish the human brain, it comes as no surprise to learn that mentally, breast-fed children are brighter and indefinitely superior to bottle-fed children. Bottle-fed babies are more neurotic, have more of the "diseases of childhood," and a higher death-rate. It has been repeatedly shown that breast-fed babies are physically and mentally superior to bottle-fed babies. Except for height, bottle-fed babies rank lowest in all physical traits measured.

Human milk is peculiarly and specifically adapted to the needs of the human infant. No other milk is so adapted to the nutritive requirements of the baby. Owing to the peculiar composition of human milk, it is impossible to secure a substitute that is "just as good." It is a terrible thing for a mother to fall down on the duty of nursing her baby. Cow's milk, despite all the virtues attributed to it, is a terrible food for child as well as adult.

Human milk is secreted for the use of the human infant and under normal conditions, in healthy mothers, will be secreted in sufficient quantity, and proper quality and over a sufficiently long period of time to supply the entire milk-needs of the infant.

The secretion of the breasts during the first few days after birth is somewhat different to ordinary milk and is called colostrum. It is scanty in amount, thicker than milk and of a deep lemon-yellow color. Its chemical composition differs greatly from that of the later secretion. It is supposed to have a laxative effect upon the child.

Colostrum changes gradually into true milk which is thinner and bluer. The flow of milk is usually well-established by the end of the first week while the complete change is finished by the end of the second or third week.

As the child grows the secretion of milk gradually increases in response to his demands. Much of the milk is actually formed while the baby nurses and is secreted in proportion to the vigor, strength and persistence with which he sucks.

Mother's milk varies with the food eaten and with the season. Her glands secrete about 2¼ to 3 pints of milk a day between the third and sixth months of lactation. This amount gradually increases up to the end of one year. If the mother's diet is rich in phosphates, or in lecithins, the milk will possess large amounts of these constituents. Her milk is also readily affected by tobacco, alcohol, coffee, teas, narcotics, and nearly all drugs. These should be avoided.

Analyses of mother's milk to determine its quality are of no practical value unless the whole of several nursings are used. Samples taken in the evening are likely to be different from those taken in the morning. At the beginning of the nursing the fat in the milk varies from 6 to 10 per cent.

Considerable variation in the composition of the milk of various women is found. But babies thrive well on all of these. A baby that was thriving well on its mother's milk will thrive equally as well on the milk of a wet nurse. It is also true that one baby may thrive well on milk which, for some reason, another baby failed on. A baby may even take the milk of several wet-nurses and thrive well on all of them.

There can be no absolute standard for good milk. Unless some extreme variation exists, chemical analysis of the milk cannot determine its fitness or unfitness for the baby. Most of this laboratory monkey-work is just part of commercial medicine.

There is only one test for the adequacy or inadequacy of milk, and this is the feeding test. If a child is growing normally and thriving on the breast milk it is receiving, it is quite evident that the supply is adequate. But if it is not growing it is possible that the supply of milk is insufficient.

The amount of milk the baby receives may be determined by weighing it before and after nursing. Usually the baby receives one-half its meal during the first five minutes of sucking. During the second five minutes it gets an added quarter of its meal.