350 THE ANIMALS AND MAN 



dusted, the amount of dust in the air may be positively in- 

 jurious. The dust may, in addition, contain germs of dis- 

 eases. This is especially true in the sick room. Therefore 

 the sick room should have special care and no one should 

 enter except the nurse or caretaker. 



EXCRETION 



Besides the carbon dioxide that is thrown into the blood by 

 the tissues, during their activity, certain nitrogenous com- 

 pounds (the wastes) are also eliminated by the cells. As 

 there is a special apparatus for removing the carbon dioxide 

 from the blood so there are special organs for removing 

 the nitrogenous wastes. These are the organs of excretion. 



They consist of the skin and kidneys. The manner of 

 removal of nitrogenous waste from the blood is quite as 

 interesting as the manner of removal of carbon dioxide. 



THE KIDNEYS 



Structure of the kidneys.- -The human kidneys are 

 located in the dorsal (lumbar) region next to the backbone 

 and immediately beneath the diaphragm. There is one 

 kidney on each side. A duct, or ureter, passes from each 

 kidney to the urinary bladder. If a kidney is cut through 

 crosswise (fig. 177) the ureter (u) is seen to widen into a 

 cavity (p), the pelvis of the kidney. From the pelvis, 

 branches, called calices, extend into the kidney substance. 

 Of the solid part of the kidney there are two layers, the 

 cortex (c) and the medulla (m) . The medulla is divided into 

 pyramids (pyramids of Malpighi) whose small ends (o) are 

 directed into the pelvis. 



The renal artery (R. A.) enters the kidney at the hilus, or 

 depression on the inner side of the kidney below the ureter, 

 and the branches of the renal vein (R. V.) leave it above the 

 ureter. 



The pyramids in the medulla are composed of hundreds of 

 microscopic tubules (T). These open by minute pores at 



