THE METANEPHROS 



199 



tubules. The human kidney may have twenty renal pyramids, 

 but usually they are not so numerous. The kidneys of marsupials, 

 insectivoi's, rodents, and earnivors have usually only a sino;le renal 

 pyramid. In the medullary zone between the renal pyramids are 

 branehes of the renal artery and vein embedded in connective tissue; 

 these interi)yramidal regions are known as the columns of JJertini. 

 With the aid of a hand lens or low-power objective, an examination 

 of the cortex reveals alternating light and dark radial striations. 

 The lighter striations are called pars radiata. The darker striations 

 are called i)ars convoluta and show a series of small dot-like struc- 



^fSr ',» n> ',< V '" 



Pars convoluta 



Pars radiata ~-r~\ J'- \ \-- , , AN /^7 



Papilla /'*' ■ ' ■ • — -^.-^v \ a / 



Pelvis 



Renal pyramid 



Interlobar artery 



Interlobar vein 

 Primary calyx 

 Secondary calyx 



Renal artery ^~-\C 



Renal vein- 



Fig. 125. — Diagram of a longitudinal section of a human kidney. 



tures, renal corpuscles, arranged along an imaginary radial axis. 

 Before progressing further it is essential to understand in detail 

 the structure and distribution of a nephridial tubule which is the 

 microanatomical structural unit of the kidney. Each kidney 

 possesses thousands of these nephridial tubules. 



The Nephridial Tubule.— Attention has already been called to the 

 dot-like structures, the renal or Malpighian corpuscles, visible in 

 the pars convoluta striations. Each nephridial tubule begins with 

 one of these structures, as in the case of the mesone])hric tubule. 

 Each renal corpuscle consists of a capillary complex, known as a 

 glomerulus, which is surrounded by Bowman's capsule, a double- 

 walled structure of squamous epithelium. An afferent arteriole 

 carries blood into the glomerulus and an efferent arteriole carries 

 blood away, the two arterioles being adjacent where they continue 



