136 



PRINCIPLES OF ANIMAL BIOLOGY 



In the lower vertebrates (up to the amphibians) much of this embryonic 

 state is retained in the adult, particularly the repetition of the tubules 

 in a serial arrangement. In the higher vertebrates the segmental 



arrangement is completely lost in the gross 

 form of the system. Yet in all of them the 

 uriniferous tul)ule with its renal corpuscle is 

 the functional unit. 



The adult kidney in the frog, in cross sec- 

 tion, is arranged as in Fig. 111. The renal 

 corpuscles are located toward the ventral side. 

 The uriniferous tubules from them pass up- 

 ward, downward, and upward again, with 

 many convolutions, and empty into collecting 

 tubules, a number of which traverse the 

 kidney near the dorsal surface. The collect- 

 ing tubules begin in Bidder's canal, which 

 extends along the median (inner) edge of the 

 kidney, and end in the ureter, which extends 

 along the lateral edge of the kidney, and then 

 on to the cloaca and bladder. At the ventral 

 side are nephrostomes, remnants of the 

 embryonic openings into the coelom, but end- 

 ing blindly in the adult. An important addi- 

 tional feature of the kidney is the abundant 

 supply of blood vessels ; the tubules are every- 

 where in close contact with capillaries. 

 The corresponding system in man is shown in Fig. 112. The kidney 

 is bean-shaped, with the ureter emerging from the "eye" of the bean 



A 



Fig. 110. — Structures 

 from vertebrate kidney, dia- 

 grammatic. A, renal cor- 

 puscle; B and C, cross-sec- 

 tions of uriniferous tubules at 

 different levels; av, afferent 

 vessel; be, Bowman's capsule; 

 cap, capillary; cil, cilia (found 

 in amphibia, not man) ; ev, 

 efferent vessel; gl, glomer- 

 ulus; ut, neck of uriniferous 

 tubule. 



Fig. 111. — Diagrammatic representation of a cross section of the kidne.\- of a frog. 

 B, Bidder's canal; C, collecting tubule; D, dorsal, L, lateral margin of kidney; AI, renal 

 corpuscle; A'^, neijhrostoinc; 7', uriniferous tubule; U, ureter; V, renal portal vein. (Modi- 

 fied f 10771 Holmes, " Biol 00 U of the Froy.") 



and discharging below into the bladder. A copious blood supply is 

 furnished by l)ranches of the main artery and veiij. Inside the kidney 

 are typical uriniferous tubules. Their renal corpuscles are massed toward 



