THE SECRETION OF URINE 



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surface of the kidney. All parts of the kidney are made up of branched 

 tubules embedded in scanty connective tissue and richly supplied with 

 blood-vessels. Each tubule begins by a blind dilated extremity in the 

 cortex, known as Bowman's capsule, which surrounds a bunch of capil- 

 lary blood-vessels, the glomerulus, the two together forming the Mal- 

 pighian capsule. From Bowman's capsule a short neck leads into a 

 proximal convoluted tubule, and this into a U-shaped portion which 



Cortex 



Boundary zone 



Medulla 



FIG. 527. Diagram showing course of urinary tubules, and the distribution 

 of the blood-vessels. (From YEO.) 



passes down in a medullary ray into the underlying portion of the 

 medulla, and consists of straight descending and ascending limbs and 

 the loop of Henle. The ascending limb passes into a distal convoluted 

 tubule, and this by a ' junctional tubule ' joins with a number of 

 others to form a straight ' collecting tubule.' Several of these unite 

 to form the papillary ducts, which open on the surface of the papilla 

 in the expanded part of the renal duct or ureter (Fig. 527). The whole 

 tubule consists of epithelium lying on a basement membrane ; the 

 epithelium varies in structure in different parts of the tubule. The 

 bunch of glomerular capillaries is covered with a very thin layer of endo- 

 thelial cells, and a similar layer forms the lining of Bowman's capsule. 

 The convoluted tubules contain cells which are roughly cubical or 

 cylindrical in cross-section, but do not present very definite cell out- 

 lines. These cells, which are similar in the two sets of convoluted 

 tubules, have long been distinguished as ' rodded epithelium ' (Fig. 528) 

 on account of the ease with which a radial disposition of rods or granules 

 is demonstrated in their protoplasm. As ordinarily prepared, the 



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