238 ELEMENTS OF HISTOLOGY. [Chap. xxx. 



310. (5) The so formed descending Henle's loop- 

 tube passes the line between the boundary layer 

 and. papillary portion, and having entered this 

 latter pursues its course for a short distance, when 

 it sharply bends backwards as the loop of Henle's 

 tube ; it now runs back towards the boundary layer, 

 and precisely at the point of entering this becomes 

 suddenly enlarged. Up to -this point the structure 

 and size of the loop are exactly the same as those of 

 the descending limb. 



311. (6) Having entered the boundary layer it 

 pursues its course in this latter to the cortex in a 

 more or less straight direction within the medullary 

 ray as the ascending loop-tube. Besides being bigger 

 than the descending limb and the loop, its lumen 

 is comparatively smaller, and its lining epithelium 

 is a layer of polyhedral, distinctly fibrillated 

 epithelial cells. The tube is not quite of the same 

 thickness all along the boundary layer, but is broader 

 in the inner than in the outer half; besides, the tube 

 is not quite straight, but slightly wavy or even 

 spiral. 



(7) Having reached the cortex, it enters this as the 

 cortical part of the ascending loop-tube, forming one 

 of the tubes of a medullary ray ; it is at the same 

 time narrower than in the boundary layer, and is 

 more or less straight or wavy. Its lumen is very 

 minute, its lining cells are flat polyhedral with a 

 small flattened nucleus, and there is an indication of 

 fibrillation (Fig. 135). 



(8) Sooner or later on its way in the cortex in 

 a medullary ray it leaves this latter to enter the 

 labyrinth, where it winds between the convoluted 

 tubes as an angular irregular tubule (Fig. 135). Its 

 shape is very irregular, its size alters from place 

 to place, its lumen is very minute, its epithelium a 

 layer of polyhedral, pyramidal, or short columnar cells 



