CHAPTER X. 

 EXCRETION BY THE KIDNEYS AND SKIN. 



EXCRETION of the various foods after they have discharged 

 their several functions in the body is effected mainly by the 

 kidneys, skin, lungs and alimentary canal. The excretory action 

 of the last two named is considered under Respiration and 

 Digestion. Attention is again called to the fact that it is impos- 

 sible to differentiate strictly between a secretory and excretory fluid. 

 The urine is as typical of the excretions as any fluid to be found. 

 But it will be convenient to speak of the "secretion" of urine 

 when reference is made to the act of separating its constituents 

 from the blood. 



THE KIDNEYS. 



Anatomy. The kidneys, one on each side of the body, are 

 behind the peritoneum in the lumbar region. The right is usu- 

 ally a little lower and a little lighter than the left. The hilum 

 from which the ureter springs looks inward and forward. The 

 kidney, as found behind the peritoneum, is covered with a con- 

 siderable amount of fat, but the substance proper of the organ 

 is closely surrounded by a somewhat resistant fibrous capsule 

 which in health can be easily stripped away. At the hilum the 

 capsule is continued inward to line the pelvis, infundibula and 

 calyces. 



The kidney belongs to the class of compound tubular glands. 

 If it be cut into two halves by an incision passing through the two 

 borders (and, therefore, through the hilum) an idea of its gross 

 divisions is obtained. The renal substance is seen to be divided 



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