CLASS AMPHIBIA 489 



renal portal system. The renal portal vein (Fig. 415, rn.pt) 

 receives the blood from the hind legs by means of the sciatic 

 (sc) and femoral (fm) veins, and blood from the body-wall by 

 means of the dorso-lumbar vein (ds.lmb). 



The liver receives blood from the hepatic portal system. The 

 femoral veins (fm) from the hind limbs divide, and their branches 

 unite to form the abdominal vein (abd). The abdominal vein 

 also collects blood from the bladder, ventral body-wall, and 

 heart. The portal vein carries blood into the liver from the 

 stomach, intestine, spleen, and pancreas. 



Circulation takes place in the following manner: The sinus 

 venosus contracts first, forcing the impure venous blood into 

 the right auricle (Fig. 413 b, C, 75). Then both auricles contract, 

 and the oxygenated blood brought into the left auricle by the 

 pulmonary veins is forced into the left part of the ventricle, 

 and the impure blood from the right auricle is forced into the 

 right side of the ventricle. The ventricle then contracts and 

 the impure blood is forced out frrst, passing principally into the 

 pulmocutaneous arteries and thence to the lungs and skin, and 

 the oxygenated blood is forced out later through the carotid 

 and systemic arteries to the other parts of the body. The blood 

 is prevented from flowing back, and the oxygenated blood and 

 impure blood are distributed as stated above, by means of valves 

 (Fig. 413 b, C, 18, ip). 



The blood that is thus forced through the arteries makes its 

 way into tubular blood-vessels that become smaller and smaller 

 until the extremely narrow capillaries are reached. Here food 

 and oxygen are delivered to the tissues, and waste products are 

 taken up from the tissues. The renal portal system carries 

 blood to the kidneys, where urea and similar impurities are taken 

 out. The hepatic portal system carries blood to the liver, where 

 bile and glycogen are formed. The blood brought to the lungs 

 and skin is oxygenated and then carried back to the heart. The 

 passage of blood through the capillaries can easily be observed 

 in the web of the frog's foot or in the tail of the tadpole. 



