Page:Natural History, Reptiles.djvu/253

Rh that every globule of blood is distinctly and beautifully visible.

The branchial tufts now begin to diminish, while the Tadpole increases in bulk, until at length the limbs bud forth and assume their form and toes, the hinder pair being developed first. The tail is now gradually absorbed, the absorption commencing at the tip and proceeding towards the base until the last trace has disappeared. The gills are by this time so much reduced as to be withdrawn beneath the skin, and are no more visible; the eyes and mouth are fully formed; the colour has changed to an olive-green, spotted with yellow on the under parts; and, in short, the animal is no longer a Tadpole, but a little Frog.

The respiration is now carried on by means of large cellular lungs; but the action of breathing is not performed as in most other Vertebrata, by alternate contraction and dilatation of the chest, for the Frog has no ribs, by the depression of which to enlarge the thoracic cavity,—but by the actual swallowing of the air, the element being forced down the windpipe into the lungs, by an action perfectly similar to that by which food is forced down the gullet into the stomach.

But there is another agent employed in the respiration of these animals beside the lungs, namely, the whole surface of the skin. Numerous and careful experiments have proved that the blood is purified, and the vital functions go on for a considerable time, after all access of air to the lungs has been cut off, and even (by an experiment, the wicked cruelty of which we cannot