THE MAMMALS I4I 



supplied with blood only through the glomerular arteri- 

 oles. After traversing the glomerular capillaries, the 

 blood emerges through a short 'efferent' arteriole which 

 distributes it to the peritubular capillaries, from which it 

 is collected into a meshwork of veins and returned to the 

 systemic circulation. Perhaps the best reason that can be 

 given for the disappearance of the renal-portal system is 

 that, as glomerular function received increased emphasis 

 in the early mammals, and as the filtration rate came to 

 be stabilized at a high level, an independent blood sup- 

 ply to the tubules became superfluous. 



When the Cenozoic opened the mammals began to ap- 

 pear in greater variety and abundance, and by the end 

 of the Eocene all the modem orders were represented 

 on the stage, warm-blooded, active, and, for the most 

 part, alert for all the year. These modern mammals are 

 usually divided into 15 orders, as represented by: 1) the 

 shrews and hedgehogs; 2) the armadillos and sloths; 3) 

 the scaly anteaters; 4) the rodents; 5) the picas, hares, 

 and rabbits; 6) the bats; 7) the hyenas, cats, dogs, 

 bears, raccoons, pandas, sea lions, walruses, and seals; 

 8) the whales, porpoises, and dolphins; 9) the conies; 

 10) the elephants; 11) the manatee and dugongs; 12) 

 the aardvarks; 13) the horses, tapirs, and rhinoceroses; 

 14) the pigs, hippopotamuses, camels, deer, antelopes, 

 sheep, and oxen; and 15) the tree shrews, lemurs, tar- 

 siers, monkeys, apes, and man. 



The over-all function of the mammalian kidney can 

 readily be visualized in terms of its operations in man, 

 the best-studied species. The two almost identical kid- 

 neys in man each contain about one million nephrons. 

 Each nephron consists of a glomerulus and its sub- 

 joined tubule, this tubule being differentiated into a 

 proximal segment, thin segment, and distal segment. 

 These tubules drain into confluent collecting ducts that 

 empty into the renal pelvis, from which the urine drains 



