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Page:Popular Science Monthly Volume 20.djvu/764

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the logic of Aristotle, was utilized in the service of the Church, and the union of the Church and philosophy was irresistible, and enthralled the human mind for three centuries.

Thus the logic of Aristotle, which was never intended as anything but a defense against philosophical error, was turned into a system for the discovery of truth and scientific investigation. Aristotle himself would have been the first to protest against this misuse of it. The labors of the men were prodigious, but they were utterly barren of results—as barren as the labor on the tread-mill.



SPECIAL ORGANS OF THE FOOD-TRACT.—Another class of respiratory organs may now be distinguished, namely, those developed directly from the alimentary canal. Here belong the more highly specialized organs of the vertebrates.

Aquatic Organs of the Food-Tract.—The gills or branchiæ of fishes are analogous in position and structure to those of crabs, but are morphologically different, as they are not developed from the skin directly, but from the lining of the pharynx. A powerful heart impels the blood rapidly through the gills, while these are bathed by water-currents produced by the pumping action of the mouth; so that rapid and constant changes are effected in both the blood and the aërating water. The branchiæ are comb-like fringes of minute blood-vessels,

PSM V20 D764 Lamprey showing the sucking mouth and gills.jpg
Fig. 1. A, Lamprey (Petromyzon), showing the sucking-month and the apertures of the gill-sacs. B, diagram to illustrate the structure of the sills in the Lamprey: a, pharynx: b, tube leading from the pharynx into one of the gill-sacs; c, one of the pill-sacs, showing the lining membrane thrown into folds; d, external opening of the gill-sac. (In reality the gill-sacs do not open directly into the pharynx, but into a common respiratory tube, which is omitted for the sake of clearness.

placed on bony arches, having a complex structure, and beautifully adapted to their purpose of exposing a great amount of blood in small space and in brief time.

It is impracticable to describe at length the various arrangements