62 GENERAL BIOLOGY 



the blood from the hind legs by means of the sciatic and femoral veins, 

 and blood from the body wall by means of the dorso-lumbar vein. 



"The liver receives blood from the hepatic-portal system. The 

 femoral veins from the hind limbs divide, and their branches unite to 

 form the abdominal vein. 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." 



\^The sinus venosus contracts first, forcing the impure venous blood 

 into the right auricle; then both auricles contract and the oxygenated 

 ( ) blood brought to the left auricle by the pulmonary 



veins is forced into the left part of the ventricle, while 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, first passing 

 principally into the pulmocutaneous arteries and thence to the lungs and 

 skin, and the oxygenated blood is pushed but later through the carotid 

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

 passes through the various blood vessels which become smaller and 

 smaller. These minute vessels are called capillaries. It is here that the 

 food and the oxygen of the blood bathe the tissues, and waste products 

 are taken up. 



The renal-portal system carries the blood from the legs and pos- 

 terior portions of the body to the kidneys where urea and similar impuri- 

 ties are taken out. The hepatic-portal system carries all the blood from 

 the digestive tract into the liver where bile and glycogen are formed. 

 All blood brought to the lungs and skin is oxygenated and carried back 

 to the heart. 



The liquid in which the blood corpuscles float is called blood plasma 

 as long as it is contained within the walls of the blood vessel. When 

 it leaves the blood vessel and bathes various parts of the intervening 

 spaces, it is called lymph; while, if it should be taken out of the body 

 entirely, it would be called serum. 



The lymph spaces in the frog's body are very large and communi- 

 cate with one another as well as with the veins. There are four so-called 

 lymph-hearts (Figs. 11, 347) ; two near the third vertebra, and two near 

 the end of the vertebral column. These lymph-hearts force the lymph 

 into the internal jugular and transverse iliac veins by their pulsation. 

 The lymph itself is colorless, and whatever corpuscles it may contain are 

 likewise colorless. 



RESPIRATION 



As has already been mentioned, breathing takes place through the 

 skin, both in water and air, although the lungs are naturally the prin- 

 cipal organs of respiration. The air is taken jn through the external 

 nares into the olfactory ( ) chamber, then through 



the internal or posterior nares into the mouth cavity. The valves, which 



