260 BIOLOGY FOR BEGINNERS 



lungs and skin has been relieved of its carbon dioxide and re- 

 supplied with oxygen, and this returns by the pulmonary veins 

 to the left auricle of the heart. The blood from both the general 

 and the pulmonary circulation then enters the ventricle, but by 

 means of a complicated valve, that having most oxygen is sent to 

 the head and brain. The next best goes out into the aorta, while 

 that with most carbon dioxide is diverted into the pulmonary 

 arteries and goes to the lungs and skin. 



On each complete trip, some of the blood passes through the 

 kidneys, so that all of the nitrogenous waste can be removed as 

 urine. Really the purest blood in an animal's body is that which 

 has just left these very important organs, even though it may 

 have more carbon dioxide than when leaving the lungs. 



The blood which returns from the digestive tract is gathered 

 into a large vein (portal) and passes through the liver, where 

 some food substances may be stored, and certain impurities re- 

 moved, after which it flows back to the right auricle. 



Several important differences will be noted in the frog's circula- 

 tory system, as compared with the fish. The frog's heart is three 

 chambered and is located farther back in the body; the blood 

 leaves the heart in two circuits, the pulmonary and the general, 

 while in the fish, it makes only one continuous trip. In other 

 words, the blood twice returns to the heart of the frog in any single 

 complete circulation, and only once in the fish. 



Respiration. In the larval form, as a tadpole, the young frog 

 breathes by means of gills but the adult develops a pair of simple 

 lungs, opening into the throat by a trachea and glottis. These 

 lungs are rather cone-shaped, sac-like organs, whose inner walls 

 are honey-combed with delicate air cells provided with many blood 

 capillaries so that the conditions for osmosis are fulfilled. 



The frog has no ribs or diaphragm to expand the lungs so that 

 air may come in, and is therefore forced to " swallow " whatever 

 air it gets by a sort of pumping motion of the throat which can be 

 observed in any living frog. Air is taken in through the nostrils, 

 which are then closed and the air " swallowed " by the action of 

 the abdominal muscles. The elasticity of the lung tissue forces 



