THE ABDOMEN 821 



a foveola or small depression, and through these openings the urine 

 escapes into a calix. 



The diameter of the excretory tubes or ducts of Bellini is about 

 the 25^ inch. 



Sammary of a Tubule from Beginning to End. 



1. The capsule of Bowman, containing a glomerulus or Malpighian corpuscle, 

 in the labyrinth. 



2. The neck, in the labyrinth. 



3. The flrst or proximal convoluted tubule, in the labyrinth. 



4. The spiral tubule, in a medullary ray. 



5. The descending tubule of Henle (small), in a boundary zone, and in part 

 of a papillary zone, of a Malpighian pyramid. 



6. The loop of Henle' s tubule, in a papillary zone. 



7. The ascending tubule of Henle (large), in part of a papillary zone, a 

 boundary zone, and the cortex, in which latter it is in a medullary ray. 



8. The irregular or zigzag tubule, in the labyrinth. 



9. The second or distal convoluted (intercalary) tubule, in the labyrinth. 



10. The junctional tubule, in the labyrinth on its way to a medullary ray. 



11. The straight collecting tubule, in a medullary ray of the cortex, and in a 

 boundary zone of a Malpighian pyramid. 



12. The excretory tube, or duct of Bellini, in a papillary zone of a Malpighian 

 pyramid. 



Structure of the Uriniferous Tubules.— The tubules are composed of a base- 

 ment membrane lined with a single layer of epitheUal cells. The basement 

 membrane itself consists of flattened epithelial cells. The tubules vary in 

 size, but theur average diameter may be stated as ,^ inch. The characters 

 of the Uning epithelial cells present differences in the different tubules, which 

 will now be considered in order. 



1. The capsule of Bowman is lined with a single layer of flattened epi- 

 thelium, which is reflected over the glomerulus or Malpighian corpuscle, where 

 it is more distinct in early Ufe than in the adult. 



2. The neck is Uned with cubical epithelium. 



3. The first or proximal convoluted tubule is also lined with cubical epi- 

 thelium, but the cells, which are thick, are peculiar. The inner part of each 

 cell — that is the part next the lumen of the tubule, including the spherical 

 nucleus, has granular protoplasm, whilst the outer part — that is, the part 

 next the basement membrane — has its protoplasm striated or fibrillated, 

 owing to the presence of rod-shaped fibrils disposed vertically to the basement 

 membrane (Heidenhain). These fibrillated cells are provided laterally with 

 processes, by which they embrace one another. The lumen of the tubule is 

 distinct. 



4. The spiral tubule is lined with epitheUum similar to that of the proximal 

 convoluted tubule, and its lumen is distinct. 



5. The descending tubule of Henle is lined with clear, thin, flattened cells. 

 The tubule, though narrow, has a distinct lumen. 



6. The loop Of Henle's tubule is lined with epithelium which resembles that 

 of the descending tubule. 



7. The ascending tubule of Henle is Uned with fibrillated, cubical epithelial 

 cells like those of the proximal convoluted and spiral tubules. Though it 

 is of conaparatively large size, its lumen is small. 



8. The irregular or zigzag tubule is lined with cubical epithelial cells, which 

 are markedly fibrillated, and its lumen is minute. 



9. The second or distal convoluted tubule is lined with epithelium which 

 resembles that of the proximal convoluted tubule, with the following differ- 

 ences : (a) the cells are longer ; and (b) they are highly refractive. In size 

 it corresponds with the proximal convoluted tubule. 



10. The junctional tubule is hued with clear flattened, cubical epithelial 

 cells, and its lumen is large. 



