3i6 



HISTOLOGY 



ascending limb. Thus it happens that a broad inner zone of the medulla 

 (i.e., toward the papilla) contains only thin segments of renal tubules 

 in addition to the large collecting tubules (Fig. 310, D); and the zone 

 so characterized may be distinguished macroscopically. The papilla con- 

 tains only collecting tubules, but the loops of Henle turn back at different 

 levels, and therefore the papillary zone entirely free from loops is not well 

 defined. The outer zone of the medulla contains both thick and thin seg- 



Renal corpuscle. Convoluted tubules. Cortical ray. 



Interlobular vein. 



Rente's loop. Arciform vein. Arciform artery. 



FIG. 312. PART OF A RADIAL SECTION OF A HUMAN KIDNEY. X 5. 

 At x a renal corpuscle has dropped out. 



ments of Henle's loops, in addition to the collecting tubules. In the de- 

 scending limbs the change to thin segments occurs at a more or less definite 

 level within this outer zone, thus subdividing it into a narrow outer band, 

 with few thin segments, and an inner band containing many of both sorts. 

 These zones have only recently been recognized (Peter, Untersuchungen 

 iiber Ban und Entwickelung der Niere, Jena, 1909). 



The renal tubules which have their capsules close to the medulla are 

 the first to develop; the others are formed successively outward, the young- 



