The earliest scientific observations concerning nutrition were founded upon the commonly noted fact that in spite of the ingestion of large quantities of food, a normal man did not vary greatly in size from year to year. It was understood early in the history of physiology that the weight added by the ingestion of food and drink was lost in the urine, the feces, and the "insensible perspiration." The "insensible perspiration" was partly in evidence when moisture of the warm breath condensed upon a cold plate. By it were meant the usually invisible exhalations from the body, which are now known to be carbon dioxid and water.

Sanctorius1 made many experiments upon himself and others to determine the amount of insensible perspiration. An old cut shows him sitting in a chair suspended from a large steelyard. As a matter of routine he determined his own weight previous to each meal and then weighted the steelyard so as to counterbalance the additional food he proposed to eat. During the meal when the chair dipped he ended his repast.

In Section I, Aphorism II, Sanctorius gives the following curious advice: "If a physician who has the care of another's health is acquainted only with the sensible supplies and evacuations, and knows nothing of the waste that is daily made by the insensible perspiration, he will only deceive his patient and never cure him." Aphorism III reads: "He only who knows how much and when the body does more or less insensibly perspire will be able to discern when or what is to be added or taken away either for the recovery or preservation of health".

1 Sanctorius: "De medicina statica aphorismi," Venice, 1614. Translation by John Quincy, M.D., London, 1737.

In 1668 John Mayow, writing in London, stated that the atmosphere contained a constituent which supported combustion as well as animal life.

The modern era of the science of nutrition was opened by Lavoisier in 1780. He was the first to apply the balance and the thermometer to the investigation of the phenomena of life, and he declared "La vie est une fonction chimique." The work of today is but the continuation of that done a century and more ago. Lavoisier and Laplace made experiments on animal heat and respiration. The great German chemist Liebig received his early training in Paris, residing there in 1822. Liebig's conception of the processes of nutrition fired the genius of Voit to the painstaking researches which laid the foundation of his Munich school. These have been repeated and extended by his pupils, of whom Rubner is chief, and by others the world over. Thus the knowledge often transmitted personally from the master to the pupil, to be in turn elaborated, had its seed in the intellect of Lavoisier. It was he who first discovered the true importance of oxygen gas, to which he gave its present name. He declared that life processes were those of oxidation, with the resulting elimination of heat. He believed that oxygen was the cause of the decomposition of a fluid brought to the lungs, and that hydrogen and carbon were produced in this fluid and then united with oxygen to form water and carbon dioxid. He said that perspiration regulated the quantity of heat lost from the body and that digestion replenished the blood with the materials eliminated through respiration and perspiration. It was he who first made respiration experiments on man, the results of which are briefly described in a letter to Monsieur Terray,1 written in Paris and dated November 19, 1790. There is no existing record of the apparatus with which Lavoisier worked and early obtained the following results. The more important conclusions Lavoisier sums up as follows:

1. The quantity of oxygen absorbed by a resting man at a temperature of 260 C. is 1200 pouces de France2 hourly.

2. The quantity of oxygen required at a temperature of 120 C. rises to 1400 pouces.

3. During the digestion of food the quantity of oxygen amounts to from 1800 to 1900 pouces.

4. During exercise 4000 pouces and over may be the quantity of oxygen absorbed.

These remarkable results are in strict accord with the knowledge of our own day. We know more details, but the fundamental fact that the quantity of oxygen absorbed and of carbon dioxid excreted depends primarily on (1) food, (2) work, and (3) temperature was established by Lavoisier within a few years after his discovery that oxygen supported combustion. Writing in 1849 Regnault and Reiset say, "Les recherches modernes ont confirme ces vues profondes de lillustre savant".

It was, however, quickly noted that if carbon and hydrogen burned in the lungs, the greatest heat would be developed there, a result not in accordance with observation. It was then suggested that the blood dissolved oxygen, and that the production of carbon dioxid and water took place through oxidation within the blood. In 1837 Magnus discovered that the blood did hold large quantities of oxygen and carbon dioxid, which gave apparent support to this theory. Ludwig in his later years believed that the oxidation took place in the blood.3 Through the critical studies of Liebig, which were published in 1842, it was seen that it was not carbon and hydrogen which burned in the body, but protein, carbohydrates, and fat. Liebig's original theory was that while oxygen caused the combustion of fat and carbohydrates, the breaking down of protein was caused by muscle work. It will be shown later that oxygen is not the cause of the decomposition of materials in the body, but that this decomposition proceeds from unknown causes, and the products involved unite with oxygen. The sum of these chemical changes of materials under the influence of living cells is known as metabolism. This process may involve two factors, catabolism, or the reduction of higher chemical compounds into lower, and anabolism, or the construction of higher substances from lower ones.

1 Report of the British Association for the Advancement of Science, Edinburgh, 1871, p. 189.

21 cubic pouce = 0.0198 liter. 3Oral statement to the writer.