THE UEINAEY TRACT. 741 



certain depths in the pyramidal substance, the narrow tubule 

 produces a loop (the loop of Henle), and takes an upward course 

 as the ascending branch of the narrow tubule, this being on the 

 whole slightly wider than the descending portion. Again, the 

 ascending branch widens, with snort, irregular curves and angles 

 (the irregular portion), and at the most peripheral part of the 

 cortex, in which there exist no tufts, it resumes the width and 

 aspect of the convoluted tubule, being in this situation termed 

 the convoluted tubule of the second order, or the intercalated 

 tubule, which inosculates with the straight collecting tubule. 

 By the union of several intercalated and collecting tubules arches 

 are formed. Henle insists upon the arched arrangement of the 

 collecting tubules themselves, to which the intercalated tubules 

 are joined. The collecting tubule occupies the center of the medul- 

 lary ray in the cortex. The groups of collecting tubes in the 

 pyramis are situated between groups of the narrow tubules, 

 decreasing in number, by continuous union at acute angles of 

 analogous formations, until, lastly, a limited number of wide col- 

 lecting tubules (eight to fifteen) open at the point of the pyramis 

 the papilla which protrudes into the calyx. Their mouths are 

 visible to the naked eye, and are called the foramina papillaria 

 the " blessed sieve n (cribrum benedictum) of old anatomists. 



The uriniferous tubule may in its course, though not constantly, exhibit 

 slight variations in its caliber and structure, and thus a number of subdivi- 

 sions of their portions may be admissible. E. Klein, for instance, gives the fol- 

 lowing complex diagram of the course of a uriniferous tubule : (1) capsule of 

 the Malpighian corpuscle ; (2) narrowed neck ; (3) the proximal convoluted 

 tubule ; (4) the spiral tubule of Schachowa, which is already part of the 

 medullary ray; (5) the descending branch of Henle's loop; (6) the loop 

 itself, which, turning upward and reaching the boundary layer, becomes 

 suddenly enlarged and slightly wavy, thus forming (7) the first thick portion 

 of the ascending limb of Henle's loop, and (8) the spiral part of the ascend- 

 ing limb ; ( 9^) then it becomes narrow again, and more or less straight or 

 slightly curving; (10) produces the irregular tubule, with angular convolu- 

 tions; (11) the intercalated segment ; (12) the curved part of the collecting 

 tube ; (13) the straight part of the collecting tube of the cortex in the medul- 

 lary ray; (14) the collecting tube enters the boundary layer, and lastly, the 

 papillary portion; by confluence becomes (15) the large collecting tube, or 

 tube of Bellini, until (16) it opens as one of the ducts on the free surface of 

 the papilla. 



Each portion of the uriniferous tubule has its peculiar epithe- 

 lial lining. This, in general, can be defined as being cuboidal in 

 the convoluted tubules, flat in the narrow tubules, and columnar 

 in the collecting tubules. 



