74 2 



THE POPULAR SCIENCE MONTHLY. 



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 him- 

 self 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. 



HOW ANIMALS BREATHE. 



By HERMAN L. FAIRCHELD. 

 II. 



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 branchiae of 

 fishes are analogous in position and structure to those of crabs, but are 

 morphologically different, as they are not developed from the 6kin 

 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 aerating 

 water. The branchiae are comb-like fringes of minute blood-vessels, 



Fig. 1. A, Lam."ret (Petr07nyzon), showing the siickimr-month and the apertures of the gill-saca. 

 B, diagram t<> illustrate the structure of the sills in the Lamprey : a. pharynx : b, tube leading 

 from the pharynx into one of the sill-sacs ; c, one of the pill-sacs, showing the lining mem- 

 brane thrown into folds ; <1, external opening of the gill-sac. (Tn 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 



