62 General Biology 



The veins which carry blood to the kidneys constitute the renal- 

 portal ( ) system. The renal-portal vein receives 

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

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



There is also an hepatic-portal system through which blood is 

 brought to the liver. 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 general circulation is brought about by the sinus venosus con- 

 tracting first to force the impure venous blood into the right auricle. 

 Both auricles then contract, and the oxygenated ( ) 



"blood, which was 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, forcing out the impure blood. This impure 

 blood first passes principally into the pulmocutaneous arteries and then 

 to the lungs and skin. The oxygenated blood is pushed out 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 posterior 

 portions of the body to the kidneys where urea and similar impurities 

 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 been mentioned, breathing takes place through the skin, 

 both in water and air, although the lungs are naturally the principal 

 organs of respiration. The air is taken in through the external nares 

 into the olfactory ( ) chamber, then through the 



