A very important fact regarding the nutrition of the young is that the milk of one race is specifically adapted to the growth of the offspring of that particular race. Bunge2 found that dogs' milk had an ash of exactly the same composition as the ash of the newborn puppy. The ash of the milk was, therefore, perfectly adapted for the construction of new puppy tissue. It was, however, very different in composition from human, or cows', or other milk. Only in the case of iron is the quantity lower than corresponds to the composition of the offspring, but this factor is offset by the fact that the animal when newborn is richer in iron than it is at any other period of life. Not only this, but the caseins of different milks are different in chemical behavior. And besides this, the rennin of the stomach is said to be specifically adapted for the coagulation of the casein produced by the female of the same race.3

2 Bunge: "Zeitschrift fur Biologie," 1874, x, 326. 3 Kiesel: "Pfluger's Archiv," 1905, cviii, 343.

Furthermore, the percentage quantity of the constituents in the milk is dependent upon the rapidity of the growth of the organism. Bunge4 has shown this in the following comparative table:

4 Bunge: "Lehrbuch der physiologischen Chemie," 1898, p. 118.

Camerer1 finds that human milk, drawn three to twelve days after parturition, contains 0.2 milligram of iron (Fe203) per 100 c.c, while the later milk contains 0.1 milligram. The quantity is decreased if the environment or the condition of the mother be poor.2 Edelstein and Csonka3 state that 1 liter of cows' milk contains 0.7 milligram of Fe203 (0.6 to 1 mg.), which is one-third to one-half the quantity contained in human milk. Using the customary methods of infant feeding with cows' milk, the infant obtains too little iron.

Blauberg4 reports the following percentage absorption of the ash of cows' and human milk:

The quantity of calcium in cows' milk is in excess of the needs of the human infant.

The absorption of the energy-containing constituents of the milk is remarkably constant. This is illustrated in the following table made from Rubner's experiments,5 which shows the physiologic utilization of the total calories of milk:

1 Camerer: "Zeitschrift fur Biologie," 1905, xlvi, 371.

2 Jolles and Friedjung: "Arch, fur experimentelle Path, und Pharm.," 1901, xlvi, 247.

3 Edelstein and Csonka: "Biochemische Zeitschrift," 1911-12, xxxviii, 14.

4 Blauberg: "Zeitschrift fur Biologie," 1900, xl, 44.

5 Rubner: Ibid., 1899, xxxviii, 380. For further statistics of absorption consult Tangl: "Pfluger's Archiv," 1904, civ, 453.

As regards the relative composition of average cows' and human milk five and one-half months after parturition, the following comparison may be made:

Percentage Composition Of Cows' And Human Milk

Or, expressed in the relative calorific value of the different constituents, this comparison may be given:5

Percentage Distribution Of Calories In Cows' And Human Milk

Here, then, there are tremendous differences of composition, which fact forces the conclusion that cows' milk is not to be substituted for human milk in rearing a child.

Patein and Daval6 find that human milk after the first month of lactation contains but 0.8 to 1 per cent, of casein.

Another distinction between cows' and human milk is that the former contains but little extractive nitrogen, while the latter may contain 18 to 20 per cent.7 in that form. These nitrogenous extractives contain a considerable amount of carbon. Meigs and Marsh4 state that human milk contains 1 per cent, of unknown extractive substances which are almost free from nitrogen. This is probably one of the causes of the increase of the C/N ratio (see p. 38) to over unity in the urine of breast-fed infants.

1 Rubner: Von Leyden's "Handbuch," 1903, i, 95.

2 Van Slyke, "Modern Methods of Testing Milk and Milk Products," 1907. Average of 5552 American analyses.

3 Rubner and Heubner: "Zeitschrift fur ex. Pathologie und Therapie," 1905, i. 1.

4 Soldner: "Zeitschrift fur Biologie," 1896, xxxiii, 66. Average of the milk of 5 women.

5 Rubner: "Energiegesetze," 1902, p. 418.

6 Patein and Daval: "Journal de Pharm. et de Chimie," 1905, xxii, 193. 7 Rubner and Heubner: Loc. cit.

From the standpoint of chemical analysis Abderhalden2 could find no distinctive quantitative difference between the amounts of various amino-acids in human and bovine milks.

A recent analysis3 presents the following data as regards the probable composition of human milk:

The large protein content of cows' milk may be bad for the child. In the first place it clots in a heavy mass in the baby's stomach; and in the second place, even though it be digested, it is relatively much above the requirement of the organism, and its specific dynamic action increases the amount of heat produced. (See p. 406).

If cows' milk be diluted with 2 or more parts of water its protein content may approach that of human milk and its precipitation by rennin in the stomach is in the form of flakes. The writer's father,4 following a suggestion of Abraham Jacobi, used oatmeal or barley water as a diluent of milk given to babies. The precipitation of cows' casein takes place in very fine flakes when the milk is mixed with barley water, as was shown by Chapin.

Chapin's observations, in which the writer assisted, have been confirmed by White,6 who says that this action is due to the presence of ¾ to 1 per cent, of dissolved starch.

1 Meigs and Marsh: "Journal of Biological Chemistry," 1913-14, xvi, 147. 2 Abderhalden and Langstein: "Zeitschrift fur physiologische Chemie," 1910, lxvi, 8.

3 Bosworth: "Journal of Biological Chemistry," 1915, xx, 707. 4 Lusk, W. T.: "Science and Art of Midwifery," 1891, p. 258.

5 White: "Journal of the Boston Society of Medical Sciences," 1900, v, 130.

The dilution of cows' milk, however, reduces the quantity of fat and carbohydrates, and these must therefore be added to the milk in order to make a proper diet for a child.

To obtain a sufficient fat content, "top milk," rich in fat, may be taken from milk which has been standing, and may be mixed with water. Milk-sugar may then be added.

Such a milk, called "modified milk," was first introduced by Rotch, of Boston. Infants are brought up on it with greater success than was the case when undiluted cows' milk was given.