THE FROG i'-V6 



Organs of Respiration. — Although the young frog (tadpole) 

 uses gills for absorbing the oxygen dissolved in water, the adult 

 breathes also air. Even the adult doubtless absorbs the 

 oxygen of the water through the thin skin, and perhaps also 

 through the lining of the pharynx ; but the main organ of 

 respiration is the pair of lungs (ring and Ling). The lungs 

 are sacs whose inner surface is divided by partitions into air- 

 cells which greatly increase the respiratory surface. Through 

 the thin walls of the air-cells there flows a constant stream of 

 blood, gaining oxj^gen from the air in the lungs. 



Organs of Circulation. — The impure blood from the body 

 collects in a single vessel, and then enters a chamber of the 

 heart called the right auricle. Thence it passes through an 

 opening, guarded by valves, to a chamber that has thick mus- 

 cular walls, and is called the ventricle. When the ventricle 

 contracts, the blood is prevented by the valves from returning 

 to the auricles, and passes out of the ventricle into a great 

 arterial trunk, or " conus " (Fig. 314). Alongside of the 

 right auricle, but completely separated from it by a par- 

 tition, is the left auricle. This contains richly oxygenated 

 blood freshly received from the lungs. The left auricle con- 

 tracts at the same time with the right, and pours its blood into 

 the same ventricle. It would seem inevitable that the pure 

 and impure blood should completely mingle, but this is pre- 

 vented by the prompt contraction of the ventricle, assisted by 

 a rather complicated mechanism. The conus lies on the right 

 side of the ventricle, so that it is nearer to the stream of impure 

 blood emerging from the right auricle, and is first filled by it. 

 The first outlets from the conus that this impure blood meets 

 are the arteries that go to the lungs, and so, with the assistance 

 of certain valves in the conus, the impure blood flows to the 

 lungs. As the ventricle continues to empty itself, a mixed 



