100 



THE ESSENTIALS OF HISTOLOGY 



but before reaching the end of the papilla it turns round in the form of 

 a loop (loop of Henle, 6) and passes upwards again towards the cortex, 

 parallel to its former course and somewhat larger than before (ascend- 

 ing tubule of Henle, 7, 8, 9). Arrived at the cortex, it at first becomes 

 irregularly zigzag (zigzag tubule, 10), and then again convoluted as at 

 first (second convoluted tubule, 11), eventually, however, narrowing into 

 a vessel (functional tubule, 12) which joins a straight or collecting tubule 

 (is). This now passes straight through the medullary substance of 

 the kidney (14) to open at the apex of the papilla as one of the ducts 

 of Bellini (15). 



The tubules are throughout bounded by a basement -membrane, 



FlQ. '201. TUBULES FROM A SECTION OF THE DOO's KIDNEY. 



a, Capsule, enclosing the glomerulus ; n, neck of the capsule ; c, c, convoluted tubules ; 

 b, irregular tubules ; d, collecting tube ; e, e, spiral' tubes ; /, part of the ascending limb 

 of Henle's loop, here (in the medullary ray) narrow. 



which is lined by epithelium, but the characters of the epithelium-cells 

 vary in the different parts of a tubule. In the capsule the epithelium 

 is flattened and is reflected over the glomerulus (fig. 201, a). In the 

 first convoluted and spiral tubules it is thick, and the cells show a 

 marked fibrillar structure (figs. 202, 203). Moreover, they interlock 

 laterally and are difficult of isolation ; in many animals they have been 

 shown to be ciliated. In the narrow descending limb of the looped 

 tubule (fig. 204, c), and in the loop itself, the cells are clear and flat- 

 tened and leave a considerable lumen ; in the ascending limb they 

 again acquire the striated structure and nearly fill the lumen. The 

 fibrillations of the cells are still more marked in the zigzag tubules 

 (fig. 201, b), and a similar structure is present also in the second 



