MEDULLA OF THE KIDNEY. 



159 



into the cavities of the subdivisions of which (calices) their 

 apices project as papillae. The pyramids (or the renal medulla) 

 have a light- colored, shining, striated appearance, the striae 

 being radiating or nearly parallel. Two zones in the medulla 

 are sometimes distinguished, an outer "boundary layer 7 ' and 



FIG. 65. 



Lobe 



Diagrammatic sketch of structure of kidney (Dunham), a, 6, epithelium lining 

 the calix ; c, column of Berlin ; d, interlobular artery; e, arcuate artery. 



an inner " papillary layer." The substance of the pyramids 



consists chiefly of straight urinary tubules converging to the 



apices ; these produce the striated appearance of the pyramids. 



The pyramids are often separated from one another by 



