CHAPTER X. 

 EXCRETION BY THE KIDNEYS AND SKIN. 



EXCRETION of the various foods after they have dis- 

 charged 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 impossible to differentiate strictly be- 

 tween a secretory and excretory fluid. The urine is as typi- 

 cal 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 usually a little lower and a little lighter than the left. The 

 hilum from which the ureter springs looks inward and for- 

 ward. The kidney, as found behind the peritoneum, is cov- 

 ered with a considerable amount of fat, but the substance 

 proper of the organ is closely surrounded by a somewhat re- 

 sistant 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 objtained. The renal sub- 

 stance is seen to be divided into an outer layer, known as the 



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