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THE URINARY ORGANS. 



tion at the orifices on the apices of the pyramids of Malpighi, and that the 

 urine, the secretion of which commences in the capsule, finds its way through 

 these tubes into the calices of the kidney, and so into the ureter. To recapitulate : 

 the tube first presents a constricted portion, (1) the neck. 2. It forms a wide 

 convoluted tube, the proximal convoluted tube. 3. It becomes spiral, the spiral 

 tubule of Schachowa. 4. It enters the medullary structure as a narrow, straight 

 tube, the descending limb of Henle's loop. 5. Forming a loop and becoming 

 dilated, it ascends somewhat spirally, and, gradually diminishing in calibre, 

 again enters the cortical structure, the ascending limb of Henles loop. 6. It now 

 becomes irregular and angular in outline, the irregular tubule. 7. It then becomes 

 convoluted, the distal convoluted tubule. 8. Diminishing in size, it forms a curve, 

 the curved tubule. 9. Finally, it joins a straight tube, the straight collecting tube., 

 which is continued downward through the medullary substance to open at the apex 

 of a pyramid. 



The Tubuli Uriniferi: their Structure. The tubuli uriniferi consist of base- 

 ment membrane lined with epithelium. The epithelium varies considerably in 

 different sections of the uriniferous tubes. In the neck the epithelium is con- 

 tinuous with that lining the Malpighian capsule, and, like it, consists of flattened 

 cells with an oval nucleus (Fig. 547 A). The cells are, however, very indistinct and 

 difficult to trace, and the tube has here the appearance of a simple basement 

 membrane unlined by epithelium. In the proximal convoluted tubule and the 

 spiral tubule of Schachowa the epithelium is polyhedral in shape, the sides of the 

 cells not being straight, but fitting into each other, and in some animals so fused 



FIG. 548. ' Longitudinal section of Henle's 

 descending limb. a. Membrana propria. 6. 

 Epithelium. 



FIG. 549. Longitudinal section of straight 

 tube. o. Cylindrical or cubical epithelium. 

 b. Membrana propria. 



together that it is impossible to make out the lines of junction. In the human 

 kidney the cells often present an angular projection of the surface next the base- 

 ment membrane. These cells are made up of more or less rod-like fibres, which 

 rest by one extremity on the basement membrane, whilst the other projects toward 

 the lumen of the tube. This gives to the cells the appearance of distinct striation 

 (Heidenhain) (Fig. 547, B). In the descending limb of Henle's loop the epithelium 

 resembles that found in the Malpighian capsule and the commencement of the tube, 

 consisting of flat transparent epithelial plates with an oval nucleus (Figs. 547, A, 

 548). In the ascending limb, on the other hand, the cells partake more of the 

 character of those described as existing in the proximal convoluted tubule, being 

 polyhedral in shape and presenting the same appearance of striation. The nucleus, 

 however, is not situated in the centre of the cell, but near the lumen (Fig. 547, c). 

 After the ascending limb of Henle's loop becomes narrower upon entering the 

 cortical structure, the striation appears to be confined to the outer part of the cell : 

 1 From Handbook for the Physiological Laboratory. 



