THE KIDNEY 437 



tions which are confined to the region of the cortical hiliyrinth, enters 

 an arched collecting tubule. The distal is much shorter than the proxi- 

 mal convoluted portion. Its caliber is subject to great irregularities, so 

 much so that its early turns have been characterized as the irregular or 

 zigzag portion of the uriniferous tubule. The epithelium of this section 

 resembles that of the proximal convoluted portion but is lower, more 

 cuboidal, and striations are indistinct. 



This portion terminates the typically secretory portion of the urinif- 

 erous tubule. Beyond here the tubule possesses more the function of a 

 duct, hence its epithelium shows a decided change in character. Hith- 

 erto it has possessed the peculiar character, the typically granular cyto- 

 plasm, of a secreting type of cell. Beyond this section the epithelium is 

 no longer so granular but possesses a characteristically clear appearance. 

 The secretory and excretory portions of the renal tubule have a separate 

 embryonic origin, and only secondarily unite to form a continuous duct. 



8. The Arched Collecting- Tubule (Junctional Tubule). This is 

 a short portion of the uriniferous tubule which connects the distal con- 

 voluted portion in the cortical labyrinth with the straight collecting tu- 

 bules of the cortical rays. Its course is characteristically arched. 



The epithelium of the arched tubule consists of clear cuboidal cells 

 with distinct outlines and deeply stained, sharply defined nuclei. The 

 chromatin of the nucleus is irregularly distributed, forming numerous 

 karyosomes, and the nuclear membrane is distinct. The cytoplasm is 

 relatively devoid of granules, and unlike that of the secreting epithelium 

 does not possess a strong affinity for the acid dyes. 



9. The Straight Collecting Tubules. These portions of the tu- 

 bules begin in the cortical rays, where they receive the arched tubules, 

 and, proceeding to the medulla, become considerably increased in size. 

 They penetrate the boundary zone of the medulla, all pursuing a parallel 

 or slightly convergent course, and occasionally uniting with each other. 

 On entering the papillary zone, by frequent union they become rapidly 

 larger, and in the apex of each pyramid finally form about a score of large 

 terminal papillary ducts. Though shorter than the convoluted tubules, 

 the straight collecting portions, because of their direct course, traverse 

 a broader area of the renal tissue; beginning near the peripheral end of 

 the cortical rays, in these columns they cross nearly the whole breadth 

 of the renal cortex and entering the medulla extend from base to apc\ 

 of the renal pyramid. 



Throughout their whole course they progressively increase in si/e and 

 caliber. A corresponding progressive increase in the height of their epi- 



