Chap, xxx.] KIDNEY, URETER, AND BLADDER. 237 



next the membrana -propria is distinctly striated, 

 owing to the presence of rod-shaped fibrils (Heiden- 

 hain) vertically arranged. The inner part of the cell 

 substance i.e., between the nucleus and the inner 

 free margin appears granular. Epithelial cells the 

 protoplasm of which possesses the above rod-shaped 

 fibrils, will in the following paragraphs be spoken of 

 as fibrillated ceils. 



The proximal convoluted tube appears sometimes 

 thicker than at other times ; in the first case, its 

 lumen is smaller, but its lining epithelial cells are 

 distinctly more columnar. This state is probably con- 

 nected with the state of secretion. 



308. (3) The convoluted tube passes into the 

 spiral tubule (Schachowa). This differs from the 

 former in being situated not in the labyrinth, but in 

 a medullary ray, in which it forms one conspicuous 

 element, and in not being convoluted, but more or 

 less straight, slightly wavy, and spiral. Its thickness 

 and lumen are the same as in the former ; its 

 epithelium is a single layer of polyhedral cells, with 

 distinct indication of fibrillation. 



309. (4) Precisely at the line where the cortex 

 joins the boundary layer, the spiral tube becomes 

 suddenly greatly reduced in thickness ; it becomes at 

 the same time very transparent; its lumen is distinct; 

 its membrana propria is now lined with a single 

 layer of scales, each with an oval flattened nucleus. 

 This altered tubule is the descending loop-tube of 

 Henle, and it pursues its course in the boundary 

 layer as a straight tubule, in the continuation of the 

 medullary ray. 



In aspect and size this part of the urinary 

 tubule resembles a capillary blood-vessel, but differs 

 from it inasmuch as, in addition to the lining layer 

 of flattened epithelial cells, it possesses a membrana 

 propria. 



