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 kidneys, separating the urine, 

 and the skin, separating the sweat and the sebum, 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 col- 

 umn 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 ij inches thick. The 

 weight of each kidney is about 4} ounces, 140 grams. 



On dividing a 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 

 medullary portions are composed essentially of numerous tubes, the tubuU 

 uriniferi, which begin at the opening on the Malpighian pyramid and, after 

 a devious course, end in the capsule of the glomerulus. 



421 



