Page:Popular Science Monthly Volume 86.djvu/97

known nature of its functions insures it from bothersome investigation at the hands of medical men ("Die unbekannte Funktion der Nebenniere sichert dieses Organ von lästigen Nachfragen in der Heilwissenschaft").

If we regard the lungs or the individual cells of the body tissues provisionally as ductless glands, then it will be perceived that the truth of the equation formulated by Legallois had already been demonstrated quantitatively when Lavoisier proved that inspired air is converted into carbon dioxide and water, and when Lagrange, through his pupil Hassenfratz, proved that the oxygen in inspired air, being dissolved in the blood, takes up carbon and hydrogen from the body tissues as the blood courses through them (1791). We now know that the respiratory center in the medulla is stimulated by the CO₂ in the venous blood, which Lavoisier and Lagrange had shown to be, in effect, a true metabolite, or waste-product of tissue-oxidation. Their work was in fact the starting point of the chemical study of metabolism, which received its next great advancement in Claude Bernard's study of glycogen; for although the latter may not be, in the strict sense, a true internal secretion, discharged from a gland into the blood, yet its investigation led Bernard to the classical statement of the doctrine of internal secretions as such:

In animals, the glycogenic secretion is an internal secretion because it is discharged directly into the blood. I have considered the liver, as found in the higher vertebrates, as an organ with a double secretory function. It seems to reunite, in effect, two distinct secretory elements and it represents two secretions, one external, the biliary secretion, the other internal, the glycogenic secretion, which is discharged into the blood.^[1]

In the year 1843, Claude Bernard, in his graduating thesis, made the discovery that cane sugar is acted upon by the gastric juice, being converted by it into dextrose. This experimental fact led to a train of reasoning which was to revolutionize the physiology of nutrition and metabolism and at the same time to introduce the new concept of internal secretions and to be the starting point of the experimental production of disease by the artificial use of chemical and physical agencies. All carbohydrates, Bernard reasoned, must get into the blood in the form of dextrose. "What becomes of this dextrose?" he next inquired. Somewhere between the alimentary canal (via the portal vein) and the liver, between the liver (via the right heart) and the lungs, between the lungs (via the left heart) and the various body tissues, this dextrose is either destroyed and disappears or is transformed into some other substance. If the locus of this transformation could be discovered and its activities inhibited, an artificial diabetes might be produced by the induction of excess of sugar in the blood. On feeding a dog on rich sugar diet and killing it at the height of digestion, he found the hepatic veins loaded with dextrose, and although this looked at first as if the

1. Bernard, "Leçons de physiologie expérimentale," Paris, 1855, I., 96.