Both 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 the 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. [Illustration: 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 centre. (Peabody.) ] 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. [Illustration: FIG. 259.—BRAIN OF FROG. ] [Illustration: FIG. 260.—NERVOUS SYSTEM OF FROG. ] The =brain= of the frog (Fig. 259) is much like that of a fish (Fig. 224). The _olfactory_, _cerebral_, and _optic lobes_, _cerebellum_ and _medulla_ are in the same relative position, although their relative sizes are not the same. Compared with the other parts, are the olfactory lobes more or less developed than in a fish? The cerebral hemispheres? The optic lobes? The cerebellum? There is a cavity in the brain. It is readily exposed on the under surface of the medulla by cutting the membrane, which is there its only covering (Fig. 259). [Illustration: FIG. 261.—Position of legs in tailless (_A_) and tailed (_B_) amphibian. ] =Frogs and toads are beneficial= (why?) and do not the slightest injury to any interest of man. If =toads= are encouraged to take up their abode in a garden, they will aid in ridding it of insects. A house may be made in a shady corner with four bricks, or better still, a hole a foot deep may be dug to furnish them protection from the heat of the day. A toad’s muzzle is not so tapering as a frog’s (why?), its feet are not so fully webbed (why?), and its skin is not so smooth (why?). In case of doubt, open the mouth and rub the finger along the upper jaw; a frog has sharp teeth, a toad none at all. The tadpoles of frogs, toads, and salamanders are much alike. In toad’s spawn the eggs lie in strings inclosed in jelly; frogs spawn is in masses (Fig. 248).