228 ANATOMY OF VEETEBEATES. 



The texture of the kidneys is much more frail than in Mammalia, 

 readily yielding under the pressure of the finger, to which they 

 give a granular sensation as their substance is torn asunder. 



In colour they resemble the human spleen. Besides being 

 di\dded into lobes, the surface of the kidneys may be observed to 

 be composed of innumerable small lobules, separated by continuous 

 gyrations like the convolutions of the cerebral substance. 



The tubuli uriniferi originate from every part of the internal 

 substance of the lobules, extending to the gyrations, uniting in the 

 pinnatifid form, and coursing to the margins of the lobules, all the 

 inflexions of which they follow. The pinnatifid ramification of 

 the uriniferous tubules is sometimes ' opposite,' sometimes ' alter- 

 nate,' sometimes the branches are simple, sometimes dichotomously 

 divided ; but these ramuli appear scarcely smaller than the branches 

 from which they spring, and never intercommunicate.^ The 

 uriniferous ducts from the convoluted lobules unite dichotomously, 

 and ultimately escape by a single duct — the ureter. 



The arteries and veins of the kidneys have already been described. 

 Where the entire stream of the venous blood is not sent to the 

 lungs, but part is diverted to the arterial system, then also a portion 

 of the venous blood circulates through the kidneys before it reaches 

 the heart ; but in Birds, where not only the whole venous current 

 is sent to the lungs, but mth peculiar energy and frequency, such 

 vicarious office of removing efiete particles directly from the venous 

 blood is not required. A certain retention of the oviparous type 

 in the apparent entry of veins into the lower ends of the kidneys 

 is shown, but a reniportal vein does not exist : the connection of 

 the lower veins coming from the kidneys with the ihaco-mesen- 

 teric is of such a kind that the renal venous blood may flow to 

 the portal system of the liver when that system and digestion are at 

 work ; or it may floAV by the upper emulgent veins to the inferior 

 cava and so to the lungs, when respiration is unusually active. 



The ureter, figs. 85, y, 108, h, is continued down along the 

 anterior surface of the kidney toward the mesial side ; here and 

 there imbedded in its substance, forming a series of dilatations 

 corresponding to the principal lobes or enlargements of the gland, 

 and recei\dng the branches of the tubuli uriniferi as it passes 

 along. Below the kidney the ureters pass behind the rectum, 

 becoming connected to, and after a short distance involved in, its 

 coats; they ultimately terminate upon valvular eminences in a 

 depression at the lower part of the urinary sac, ib. d ; the terminal 

 papillas of the ureters are situated with the orifices of the genital 



' cxxii. p. 92. 



