KIDNEYS. 213 



convoluted tubule of the first order. At the beginning of this 

 there is a slight constriction. After taking a very tortuous 

 course between the medullary rays, and forming what is called 

 the labyrinth of the kidney, these convoluted tubules become 

 much narrower and enter the pyramids of Ferrein. Here they 

 take a straight course as far as the border of the medulla, and 

 then turn abruptly on themselves, become considerably thicker 

 again, and proceed upward toward the surface of the kidney, 

 always remaining, however, in the medullary rays. This 

 straight tubule is known as Henle's loop, of which there are the 

 descending and the ascending arms (ramus descendens et 

 ascendens). The ascending arm of Henle's loop passes over 

 into the intermediate tubule or convoluted tubule of the second 

 order, which leaves the medullary ray arid takes a tortuous 

 course in the labyrinth similar to but much shorter than that 

 pursued by the convoluted tubules of the first order. From 

 the labyrinth the canal passes back into the medullary ray as 

 the connecting tubule. Similar tubules enter the pyramids of 

 Ferrein from all sides and open into the larger collecting tu- 

 bules, which run down through the medulla and join near the 

 apex of the Malpighiari pyramid to form the so-called papil- 

 lary ducts, which open out in the area cribrosa of the papilla 

 in from ten to twenty orifices. 



Each of these different parts of the urinary canal has a 

 characteristic structure. Everywhere there is a single row of 

 epithelium with a fine structureless membrane, the membrana 

 propria. 



The capsule of Bowman is related to the glomerulus in such 

 a way that the latter is covered closely by the inner wall of the 

 capsule, while the outer wall passes over into the wall of the 

 convoluted tubule of the first order (Fig. 166). The walls of 

 the capsule are made up of a layer of flat epithelial cells with 

 a membrana propria composed of reticulum (Mall). The cap- 

 sule of Bowman, together with the glomerulus, forms what is 

 known as the Malpighian corpuscle, which has a diameter of 

 from 130 to 220 /u. A reconstruction of the glomerulus of a 

 human kidney has been made by W. B. Johnston by the Born 



