438 THE URINARY SYSTEM 



thelial cells likewise occurs, so that the lumen of the straight tubules 

 of the medulla is not only actually greater than that of those of the 

 cortical rays, but the walls of the former tubules are also considerably 

 thicker. The extreme of this progression is found in the broad lumen 

 and tall epithelium of the papillary ducts. 



The epithelium of the straight tubules, like that of the arched, 

 possesses a clear cytoplasm, distinct and deeply staining chromatic 

 nuclei, and well defined cell outlines. Beginning in the cortical rays with 

 a low columnar type, it gradually increases in height in the course of 

 the tubule until, in the papillary zone, the epithelium acquires a tall 

 columnar form. The clear cytoplasm and distinct nuclear membranes 

 of the epithelium of the collecting tubules stand out in sharp contrast to 

 the granular cytoplasm and the evenly distributed chromatin in the 

 nuclei of the lining cells in the secreting portions of the uriniferous 

 tubules. 



10. Papillary Ducts. (Ducts of Bellini}. These are the wide 

 mouths of the uriniferous tubules which are formed by the dichot- 

 omous union of from ten to thirty collecting tubules, and which empty 

 their secretion into the renal calyces at the apex of the renal pyramids. 

 They attain a diameter of from two hundred to three hundred microns. 

 They are lined by tall columnar cells with an exceptionally clear cyto- 

 plasm which has an affinity for the basic in preference to the acid class 

 of dyes. The nuclei are spheroidal or ovoid in shape and lie in the 

 basal portion of the cell. At their termination several papillary ducts 

 frequently open into a common depression or foveola which is lined by 

 an involution of the layer of transitional epithelium, derived from that 

 of the renal calyx, by which the free papillary portion of the renal pyra- 

 mid is clothed. 



It is obvious that the entire renal tubule, from the glomerular capsule 

 to the papillary duct, is a continuous canal whose epithelial wall, sup- 

 ported by a thin homogeneous basement membrane varies in character 

 in each succeeding portion. Thus the proximal and distal convoluted 

 portions and the typical ascending limbs of Heule's loops possess a 

 granular, rodded or striated, acidophil, secreting epithelium; the capsule 

 has thin cells of an endothelioid type ; the typical descending limb and loop 

 of Henle are lined by flattened finely granular and faintly acidophil 

 epithelium ; the curved and straight collecting tubules and papillary ducts 

 possess a clear columnar epithelium. It should also be noticed that the 

 several portions of the renal tubule occur in different topographical sub- 



