BLOODVESSELS OF THE KIDNEY. 165 



nuclei of which cause a slight bulging at the centre of the 

 cells. As the nuclei and bulging portions of the cells alter- 

 nate on opposite sides of this tubule its lumen appears slightly 

 sinuous. 



The ascending limb of Henle's loop is situated partly in the 

 Malpighian pyramid and partly in the pyramid of Ferrein. 

 It is straight, parallel with the descending limb, and is lined 

 with a single layer of columnar epithelium. 



The irregular tubule is the portion of the uriniferous tubule 

 succeeding the ascending limb, and is situated in the inter- 

 pyramidal cortical substance. It pursues an irregular angular 

 course, and is lined with striated, simple columnar epithelium. 



The distal convoluted tubule succeeds the irregular portion, 

 is situated in the interpyramidal cortex, and is similar in all 

 respects to the proximal convoluted portion of the uriniferous 

 tubule. 



The curved or arched tubule is a short, curved tube, in which 

 the distal convoluted tubule terminates. It passes into a 

 pyramid of Ferrein and empties into one of the straight col- 

 lecting tubules. It is lined with low cuboidal epithelium 

 resembling that of the straight tubes. 



The straight collecting tubes begin in the pyramids of Fer- 

 rein and pass in straight lines through the medulla of the 

 kidney to the apices of the Malpighian pyramids, where, hav- 

 ing successively united into a smaller number of ducts, of 

 larger size, they empty into the calices as above stated. 

 These largest discharging ducts are called the tubes of Bellini. 

 Into each collecting tube opens, in the pyramids of Fer- 

 rein, a number of the curved tubules by which the secretory 

 portions of the uriniferous tubules are connected with the col- 

 lecting tubes. 



In this manner the urine secreted by the secretory tubules 

 is collected and discharged into the pelvis of the kidney. 



The collecting tubes have large open lumina, and are lined 

 with a single layer of clear columnar epithelium cells, which 

 are shorter in the cortical portion of the kidney and become 

 longer in the papillary region and tubes of Bellini. 



The bloodvessels of the kidney (Fig. 68) are branches of 

 the renal artery and vein. These vessels enter the kidney 

 at its hilum and pass into the areolar tissue within and lin- 



