THE KIDNEYS 125 



others form a long loop, the loop of Henle, extending into the 

 medulla and back to the caj^sule. Here commences the distal 

 tubule, which is tortuous and leads into an initial collecting tubule. 

 This, in turn, unites with others to form a collecting tubule that 

 passes through the medulla to enter the pelvis through the surface 

 of the papilla. Blood capillaries again come into contact with each 

 tubule at certain points in its course, where further excretion and 

 selective reabsorption occur. The excreted fluid, urine, contains 

 characteristic nitrogenous waste niaterials, usually mainly urea 

 but with smaller quantities of other nitrogenous substances such 





A B 



Fig. 72. Corrosion preparations of the larger blood vessels in the kidney 

 of the rabbit: A, veins; B, arteries. The greater density of the vessels in 

 the cortex as compared with the medulla is evident. 



as uric acid or, in most mammals, allantoin. These are formed 

 mainly in the liver and perhaps elsewhere in the body. 



Homologies of Vertehraie Kidneys 



Like all other parts of the urinogenital system, the mammalian 

 kidney affords in its structure and embryonic development a re- 

 markable illustration of the extent to which the adult form and 

 relations of an organ may depend upon ancestry, and of the 

 greatness of the changes which ^may occur before these adult 

 conditions are attained. In the vertebrate subphylum, three 

 successive pairs of kidneys have been recognized. They occur in 

 antero-posterior order in the body, they are of increasing special- 

 ization, and their order of appearance and functional value are 



