642 STRUCTURE OF TUBULI URINIFERI. [BOOK 11. 



descending limb of the loop of Henle into which, as it 

 passes down the medullary ray, the spiral tubule suddenly 

 changes, is very unlike all the rest of the tubule, and presents 

 special features which call to mind those of a ductule of a 

 salivary or pancreatic alveolus. It is very narrow, 10 to 15/i, and 

 the cells which line it are somewhat oval cells placed length- 

 wise, each with an oval nucleus also placed lengthwise, and 

 a clear cell-substance which is thicker round the nucleus than 

 elsewhere. In a longitudinal section of the tubule, optical or 

 other, the cell appears spindle-shaped with the part round the 

 nucleus projecting into the lumen ; the projections thus formed 

 on one side of the tubule alternate with those on the other side so 

 that the lumen winds in a wavy course between the projections. 

 A transverse section shews corresponding bulgings of the cells into 

 the lumen. Hence this part of a tubule is not wholly unlike a 

 capillary, but may be distinguished by being somewhat larger, by 

 having a basement membrane distinct from the cells, and by the 

 cells, though clear in comparison with other parts of the tubule, 

 being not so transparent as and staining more readily than the 

 epithelioid plates of a capillary. 



The ascending limb over the greater part of its course presents 

 very different characters, the exact point at which the change 

 takes place varying, as we have said, a good deal. The tubule is 

 now wider, 30/i, but not so wide as the convoluted tubule. The 

 cells, which leave a narrow but regular lumen, vary a good deal in 

 form but are composed of cell-substance which always stains 

 deeply, and which in its outer part is frequently striated. Very 

 commonly the cells as seen in a longitudinal section of the tubule 

 overlap each other so as to present an imbricated thatched 

 appearance ; the nuclei are usually oval. 



The irregular or zigzag tubule, in which the ascending limb, 

 running up the medullary ray and leaving the ray at one or other 

 level to plunge into the cortex, ends, is a wide tubule which takes 

 a course bending on itself several times at somewhat sharp angles. 

 The cells are irregular in form, with nuclei which also appear to 

 be irregular, stain very deeply with the staining reagents, and are 

 often conspicuously striated in their outer part ; the lumen is very 

 irregular. This part of the tubule may perhaps be considered as 

 an enlarged ascending limb with exaggerated features. 



The second convoluted tubule so exactly repeats the features of 

 the first convoluted tubule that the description given for that 

 may be applied to this. 



The collecting tubule, in which the second convoluted tubule 

 making its way once more to the medullary ray ends, is a narrow 

 tubule- with a relatively wide lumen. The cells which line it are 

 low short cubical cells, with small rounded nuclei and clear 

 transparent cell-substance. They stain much less readily than 

 the cells in any of the preceding parts of the tubule, even in the 



