CHAPTER X 



EXCRETION 



EVERY substance taken into the body, in whatever form, must, in the 

 end, be cast off again, no matter how great the change that may be wrought 

 during its sojourn. We have already found that in the lungs the expired 

 air, and in the intestine the feces, carry from the body waste matters of no 

 further use. We have now to find that the urine separated by the kidney 

 and the sweat and sebum of the skin are likewise channels by which the 

 body throws off water, salts, and broken-down organic matters of no further 

 use to the organism. Of these two organs, the skin and the kidney, the 

 latter is by far the more important in so far as the quantity and complexity 

 of its secretion is concerned. 



STRUCTURE AND FUNCTION OF THE KIDNEYS. 



General Structure. The kidneys are. two in number, and are 

 situated deeply in the lumbar region of the abdomen on either side of the 

 spinal column behind the peritoneum. They correspond in position to the 

 last two dorsal and two upper lumbar vertebrae, the right slightly below 

 the left in consequence of the position of the liver on the right side of the 

 abdomen. They are about 4 inches long, 2^ inches broad, and i^ inches 

 thick. The weight of each kidney is about 4^ ounces, 140 grams. 



On dividing the kidney into two equal parts by a section carried through 

 its long convex border, figure 286, the main part of its substance is seen to 

 be composed of two chief portions called respectively cortical and medullary, 

 the latter being also sometimes called pyramidal, from the fact of its being 

 composed of about a dozen conical bundles of uriniferous tubules, each bun- 

 dle forming what is called a pyramid. The upper part of the ureter, or duct 

 of the organ, is dilated into the pelvis ; and this, again, after separating into 

 two or three principal divisions, is finally subdivided into 8 to 12 smaller 

 portions, calyces, each of which receives the pointed extremity or papilla of 

 a pyramid. Sometimes, however, more than one papilla is received by a 

 calyx. 



The kidney is a compound tubular gland. Both its cortical and its medul- 

 lary portions are composed essentially of numerous tubes, the tubuli urinijeri t 



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