CHAPTER XVIII. 



THE KIDNEYS, THE SKIN, AND THE GENERAL 

 PHYSIOLOGY OF EXCRETION. 



We have in the preceding chapters traced the foods 

 through the various changes which they surfer in digestion ; 

 they have been followed through the intestinal walls into 

 the blood or lymph streams. The changes which some of 

 them undergo in the liver has been explained. We have 

 seen them distributed in the blood and carried to the indi- 

 vidual tissues. It was pointed out how at this place these 

 foods were built up into living tissue, and how by the dis- 

 integration of this tissue the energy was produced. There 

 now remains the final question concerning the nature of the 

 waste products which result from this destruction and their 

 final elimination from the body. This closing chapter in 

 the history of the foods and tissues has therefore to do with 

 the skin and the kidneys, the organs whose duty it is to 

 take the final products of metabolism and remove them 

 from the body. 



THE KIDNEYS. 



The kidneys are large, bean shaped organs lying on the 

 back wall of the abdomen. They have a reddish character- 

 istic color, but are usually more or less imbedded in con- 

 nective or adipose tissue. The right and left kidneys do 

 not lie at the same level, the difference in position between 

 the two being often several inches. On account of their 

 proximity to the back wall of the abdomen pains in the 

 kidneys are usually referred to the small of the back. A 

 large branch from the aorta, the renal artery, carries blood 

 to each kidney, and a corresponding vein carries the return- 

 ing blood to the ascending vena cava. 



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