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THE URINARY APPARATUS. 



[CH. xxxvi. 



Each begins in the cortex as a dilatation called the Capsule 

 o/' Bowman ; this encloses a tuft or glomerulus of capillaries called 

 a Malpighian corpuscle. The tubule leaves the capsule by a neck, 

 and then becomes convoluted (first convoluted tubule), but soon 

 after becomes nearly straight or 

 slightly spiral (spiral tubule] ; 

 then rapidly narrowing it passes 

 down into the medulla as the 

 descending tubule of Henle ; this 



Fig. 418. Transverse section of a deve- 

 loping Malpighian capsule and tuft 

 (human), x 300. From a fcetus at 

 about the fourth month ; a, flattened 

 cells growing to foim the capsule ; 

 b, more rounded cells ; continuous with 

 the above, reflected round c, and finally 

 enveloping it: c, mass of embryonic 

 cells which will later become developed 

 into blood-vessels. ("W. Pye.) 



Fig. 419. Epithelial elements of a Malpi- 

 ghian capsule and tuft, with the com- 

 mencement of a urinary tubule showing 

 the afferent and efferent vessel ; a, layer 

 of flat epithelium forming the capsule ; 

 I, similar, but rather larger epithelial 

 cells, placed in 'the walls of the tube ; 

 c, cells, coveringtbe vessels of the capil- 

 lary tuft; d, commencement of the 

 tubule, somewhat narrower than the 

 rest of it. (W. Pye.) 



turns round, forming a loop (loop of Henle), and passes tip to the 

 cortex again as the ascending tubule of Henle. It then becomes 

 larger and irregularly zigzag (zigzag tubule) and again convoluted 

 (second convoluted tubule). Eventually it narrows into a junctional 

 tubule, which joins a straight or collecting tubule. This passes 

 straight through the medulla, where it joins with others to form 

 one of the ducts of Bellini that open at the apex of the pyramid. 

 These parts are all shown in fig. 417. 



The character of the epithelium that lines these several parts 

 of the tubules is as follows : 



In the capsule, the epithelium is flattened and reflected over 

 the glomerulus. 



The way in which this takes place in process of development 

 is shown in figs. 418 and 419. 



