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passes backward in the trunk, where it is united again to its fellow. (Colored Fig. 2.)

Both of the pulmonary veins, returning to the heart with pure blood from the lungs, empty into the left auricle. Veins with the impure blood from the body empty into the right auricle. Both the auricles empty into the ventricles, but the pure and impure blood are prevented from thoroughly mixing by ridges on the inside of the ventricle. Only in an animal with a four-chambered heart does pure blood from the lungs pass unmixed and pure to all parts of the body, and only such animals are warm-blooded. The purer (i.e. the more oxygenated) the blood, the greater the oxidation and warmth.

The red corpuscles in a frog's blood are oval and larger than those of man. Are all of them nucleated? (Fig. 258.) The flow of blood in the web of a frog's foot is a striking and interesting sight. It may be easily shown by wrapping a small frog in a wet cloth and laying it with one foot extended upon a glass slip on the stage of a microscope.

Fig. 257.—Plan of Frog's Circulation.

Venous system is black; the arterial, white. AU, auricles; V, ventricle; L, lung; LIV, liver. Aorta has one branch to right, another to left, which reunite below. Right branch only persists in birds, left branch in beasts and man.

Fig. 258.—Frog's Blood (magnified 2500 areas). Red cells oval, nucleated, and larger than human blood cells. Nuclei of two white cells visible near center. (Peabody.)