216 



MICROSCOPIC ANATOMY OF THE ORGANS. 



The connecting tubules, collecting tubules, and the ductus 

 papillares have clear, transparent cells, showing no striated 

 structure. In the beginning they are cubical, but as the 

 canal widens into the papillary ducts they become columnar 

 (Fig. 168). The nucleus is always spherical, and sharply 



FIG. 168 



Bl.- 



Bl. 



Ab. H. 



Af. H 



Ab. H. 



From a transverse section through tin- base of a pyramid of an ape's kidney. S. R., col- 

 lecting tubule ; Ab. H., descending limb of Henle's loop ; Af. //., ascending limb of Henle's 

 loop; BL, blood-vessels: lid., interstitial connective tissue. X 500. 



marked off. The diameter of the papillary ducts is as much, 

 as 100 p. 



Zimmermann found in the cells of all regions of the canal 

 a double centrosome lying near the free surface of the cell. 



The cortical substance may be divided into kidney lobules. 

 These consist of all those Malpighian corpuscles and tubules 

 which go to form one medullary ray. At the boundaries of 

 each lobule there run the interlobular vessels. This is the 

 secretory unit, and its periphery is formed by the beginnings 

 L ., the capsules of Bowman of all the tubules which empty 

 finally into the collecting ducts that run in the medullary rays 

 (Fig. 165). There is more or less overlapping in this lobular 

 division, and there is no definite separation of the lobules. 



