RESPIRATION 379 



collected into two large tubes, called lymphatic ducts. By these it is 

 discharged into the blood. The lymph of the higher animals resembles, 

 in many ways, the colorless blood of the lower animals, such as insects. 

 In each case the absence of a pumping organ and of definite tubes 

 makes the circulation slow. 



380. Excretion. As we have already learned (cf. 

 358), glands secrete liquids from the blood. If the secre- 

 tion is waste that needs to be expelled from the body, it 

 is called an excretion. Excretion is just as necessary as 

 any body function, for the wastes are poisons to the body. 

 The most important wastes to be removed, aside from the 

 solid waste of the alimentary canal, are water, carbon 

 dioxide, uric acid, urea, and certain salts (cf. 220). The 

 carbon dioxide is removed chiefly by the lungs. Most of 

 the other materials are in solution, and are removed 

 by the kidneys. There are 2 kidneys, and they are 

 attached at the back of the abdomen, one on each side of 

 the backbone. They have the shape of beans, and weigh 

 about 5 ounces each. The kidneys receive a large supply 

 of blood from the heart. After removing the waste, they 

 return the purified blood to the body. As the solution 

 containing body waste is collected by the kidneys, it is 

 passed on to the bladder, where it is stored until it is 

 expelled from the body. Alcohol drinkers are especially 

 liable to dangerous diseases of the kidneys (cf. 371); 

 Bright's disease is one of them. 



381. Respiration. The need of respiration has already 

 been given, and respiration has been defined (cf. 52). 

 We need to respire 



(1) To bring oxygen to the blood. 



