COMPARISON OF THE EXCRETORY ORGANS, l/ 



from time to time and expels the liquid which has gath- 

 ered in it, through a passage (4) named the urethra. The 

 kidneys are at work all the time, separating urea from 

 the blood, though the bladder only empties out their 

 secretion a few times a day. 



3. The Kidneys lie at the back of the abdominal cavity, 

 on the sides of the vertebral column, a short way below 

 the diaphragm. Each is about half as big as its owner's 

 clenched fist. The blood is sent to the kidneys for purifi- 

 cation by two large branches of the aorta, named the 

 renal arteries. The kidneys not only take urea from the 

 blood, but help in removing other waste matters. 



4. The Chief Excretory Organs Compared as to their 

 Functions. The skin gets rid of a good deal of water, of 

 some mineral matters which have done their work, and 

 sometimes of a little urea. The duties of the skin as an 

 excretory organ are important, and health cannot be 

 maintained if they are badly performed. But the chief 

 functions of the skin are to protect deeper parts, to 

 regulate the temperature of the body (p. 77), and to 

 give us the sense or feeling of touch (Chap. XXL). 



The lungs get rid of much carbonic acid, of small quan- 

 tities of very poisonous animal vapors, and of some water. 

 They separate no mineral wastes and no urea. The func- 

 tion of the lungs as receptive organs, to supply the body 

 with oxygen, is as important as their excretory function. 



The kidneys are solely excretory organs. To get rid of 



3. Position of the kidneys ? Size ? How is blood carried to them ? 

 What do they do besides taking urea from the blood ? 



4. What is said of the skin as an excretory organ ? Of its other 

 functions? Of the lungs as excretory organs ? Of their other duty ? 

 Of the duty of the kidneys ? What do the kidneys remove from the 

 body ? 



