Page:Popular Science Monthly Volume 83.djvu/423

Rh mentioned, he did not employ the microscope, he was unable to work out the full details of the subject. It remained for Marcello Malpighi (1628-1694), of Bologna, a pioneer microscopist and one of the first and greatest of histologists, in 1666 to lay down finally the essential features of the minute structure and mechanism of the main glandular organs as they are accepted at the present time.

The lactealslacteals were discovered in 1622 (published 1627) by Gaspare Aselli (1581-1626), professor at Pavia, who recognized that they conveyed the chyle away from the intestine, but regarded them as emptying into the liver, then thought to be the organ in which the food materials were converted into blood. The discovery of the receptaculum chyli and the thoracic duct, and the connection of these with the lacteals on the one hand, and the venous system on the other, was made independently by Jean Pecquet (1622-1674), of Dieppe and Paris, and Jan van Home (1621-1670), of Leyden, whose observations were published in 1651 and 1652, respectively.

In the ancient and middle ages, the stomach was looked upon as the principal organ of digestion. The process of digestion was by some (Hippocrates and others) regarded as a coction, or πέψς (cooking), a sort of maturation effected with the aid of heat; by others it was considered as akin to putrefaction; and by still others as a mechanical process. It came to be the general doctrine that the food material absorbed from the alimentary tract was first acted upon by the liver and endowed with "natural spirits"; in the heart, by the action on the blood of the inspired air, the natural spirits were converted into "vital spirits"; finally, in the brain the vital spirits were converted into "animal spirits," which were then conveyed by the nerves to all parts of the body.

The beginnings of our modern knowledge of digestion can be traced back to the observations of the Belgian savant Jean Baptiste van Helmont (1577-1644), whose work formed a landmark in the history of chemistry. He regarded the chemical activities of the body as a form of fermentation, analogous to the familiar alcoholic or vinous fermentation; he assigned ferment action as a cause of a wide range of vital processes, thus anticipating theories that at the present time are frequently advanced. In van Helmont's view the digestion of food was accomplished by fermentative action. He recognized only two stages of digestion in the alimentary tract, namely, in the stomach and in the duodenum; the action of the salivary glands and pancreas was not 5'et known. Gastric digestion he regarded as being effected by a ferment derived from the spleen, associated with an acid principle which was necessary to its action. When the chyme passed into the duodenum the acid ferment was neutralized, and the second stage of digestion was effected by the bile.

The next developments in the knowledge of digestion came from