Rubner,2 in apparent ignorance of this work of Dr. Wilson, has arrived at essentially the same conclusions, and he finds that the law is true regarding all species (horse, calf, sheep, pig, dog, cat, rabbit) except man. He formulates the "law of constant energy expenditure" as follows: The amount of energy (calories) which is necessary to double the weight of the newborn of all species (except man) is the same per kilogram no matter whether the animal grows quickly or slowly. To construct one kilogram of normal body substance containing 30 grams of nitrogen and 1722 calories, 4808 calories are required except in the case of man, where six times that amount is needed. This is almost in exact agreement with the experiments of Wilson.

1 Rost: "Arbeiten aus dem kaiserlichen Gesundheitsamte," 1901, xviii, 206.

2 Rubner: "Das Problem der Lebensdauer und seiner Beziehung zu Wachs-tum und Emahrung," 1908.

The same principles apply to the growth of rats, as may be seen from the following, calculated from the results of Funk and Macallum,1 who fed these animals during twenty-eight days:

In the work of Hopkins2 different sets of rats were given the same food in different quantities, and the following table has been calculated from the results obtained after nine days of food ingestion:

Another lot of rats when given 65 calories per hundred grams live weight refused to eat all their food. It is of great interest that, notwithstanding the restriction of the dietary below the limits set by the appetite in some of the experiments, yet the gain in the weight of the rats is nearly proportional to the calories in the dietary. Evidently, ample protein, together with calcium and other salts, was present for the construction of new tissue in all the rats. Aron1 has shown that when growing dogs are given too little energy in their food, the skeleton may grow and the weight increase, though the caloric content of the animal may diminish.

1 Funk, C, and Macallum, A. Bruce: "Journal of Biological Chemistry," 1915, xxiii, 413.

2 Hopkins: "Journal of Physiology," 1912, xliv, 425.

Rubner finds in all species the constant retention of approximately the same percentage of the energy ingested, which averages 34.3 per cent., except in the case of man, where the energy retained for growth is only 5.2 per cent. He states that 40 per cent, of the energy ingested may be retained for the growth of pigs, whereas Dr. Wilson found only 20 per cent, so retained. This is because the pigs in the latter case were given skimmed milk, and the added tissue substance was found on analysis to have a heat value of only 866 calories per kilogram, instead of 1722 as assumed by Rubner.

It is therefore evident that while it requires the same energy equivalent to construct one kilogram of new substance in young animals, the percentage of energy retained for growth may depend upon the amount of fat in the diet.

Rubner states that if the requirement for energy in the various animals be placed at 100, then the amount of energy in the food actually ingested by them averages 202. This corresponds to Dr. Wilson's computation of the energy ingested by the growing pigs, which averaged 2100 calories per square meter of surface, as compared with a normal requirement of 1089. Dr. Wilson explained this high energy requirement as being partly due to growth and partly to the extreme activity of the little animals. A human infant does not require this large excess of energy in his food, probably because he is kept warm and sleeps much of the time.

Finally, Rubner has calculated that the quantity of energy metabolized in a kilogram of living cells from maturity to death is the same in different animals, except in the case of man, who again occupies an exceptional position.

1 Aron: "Biochemische Zeitschrift," 1910-11, xxx, 207.

This is represented in the following table:

Rubner finds that among the animals each kilogram of adult body substance metabolizes an average of 191,600 calories and then dies. Man alone has power in his protoplasm to use a much larger share of energy in the furtherance of his activities.

Bunge1 has recalled the relationship between rapidity of growth and longevity, as orginally suggested by Flourens in 1856. This writer believed that if the time of reaching the end of growth be multiplied by 5, the average term of life might be computed. This relationship may be tabulated as follows:

Table Showing Flourens' Law Of Longevity

Bunge calls attention to the fact that a horse more often lives to be forty than a man to be a hundred. Either the law is false, or man is a too early victim of an improper heredity or environment.

1 Bunge: "Pfluger's Archiv," 1903, xcv, 606.

Very little has been accomplished upon the subject of the mineral metabolism of growing children, so the following work of Jundell1 is of especial interest. Two boys, K. and N., were given a diet during ten days containing 2.9 grams of protein, 2.7 grams of fat, 10.8 grams of carbohydrate, and in all 81 calories per kilogram of body weight daily. The mineral metabolism as calculated per kilogram of body weight daily was as follows:

Mineral Metabolism Of K. (5½ Years Old, Weight 18.4 Kg.) And Of N. (7¾ Years Old, Weight 23.1 Kg.) In Grams Per Kg. Per Day

It may be calculated from this table that the older boy took 1.8 grams of calcium oxid in his food daily and retained about 0.07 gram. If the intake had been solely in the form of cows' milk, not far from a liter would have been required. One of the most important questions of the time concerns the determination of the quantity of salts in the food necessary to prevent malnutrition in children, and it would be well to know the quantity of cows' milk which should be prescribed in the daily diet of children in order to satisfy the mineral requirements for normal growth.

For metabolism in youth, see page 559.

1 Jundell: "Nordiskt Medicinskt Arkiv," 1914, xlvii, Abth. 2, 1.