186 



ANIMAL BIOLOGY 



Thus as we ascend the Vertebrate series three distinct kidney 

 systems appear, in each case by the development and grouping of 

 nephridium-like elements into a definitive organ. In this process 

 the primitive communication of the individual nephridia with the 

 body cavity is lost and the functions of the tubular portion in- 

 creased, until, in the higher forms, all the waste products are taken 

 solely and directly from the blood. It is therefore apparent that 

 each of the relatively large, compact kidneys of the higher Verte- 

 brates consists essentially of an enormous number of nephridium- 



-Tunic 

 -Pelvis 



Tunic 



Cortex -"* 



y Renal artery 

 ■ — Renal vein 



-Ureter 



Medullary 

 region 



From renal 

 artery 



("Glomerulus 

 v within 



I capsule 



To renal vein 



Collecting 

 tubules 



"Cortex 



Pyramid of medullary 

 region 



A 



Tip of pyramid 



B 



Fig. 131. — Human kidney. A, longitudinal section; B, diagram of the 

 course of the tubules in the kidney. The cortex is the region in which the 

 tubules come into functional association with the capillaries. Tubules extend 

 through the medullary region to open on the summits of the pyramids. 



like elements, the tubules, bound together by connective tissue and 

 covered with a protective coat. The tubules within the kidney de- 

 liver the materials taken from the blood, the urine, to the pelvis of 

 the kidney, from which it passes down the ureter, into the urinary 

 bladder, and finally to the exterior. (Figs. 130, 131.) 



Such, in broad outline, is the historical viewpoint from which 

 the kidneys of Man must be interpreted. As a matter of fact, 

 however, the evolutionary transformation is still further compli- 

 cated by anatomical, though not physiological, relations with the 



