111. 9. 81 



allow the superfluous fluid to pass from the blood-vessel into the 

 kidney, and the resulting renal excretion to collect, by the perco- 

 lation of the fluid through the solid substance of the organ, in its 

 centre, where as a general Tule there is a cavity. (This by the way 

 explains why the kidney is the most ill-savoured of all the viscera.) 

 From the central cavity the fluid is discharged into the bladder by 

 the ducts just mentioned, having already assumed in great degree 

 the character of residuum.^ The bladder is as it were moored^" to 

 the kidneys ; for, as already has been mentioned, it is attached to 

 them by strong ducts. These then are the purposes for which the 

 kidneys exist, and such the functions of these organs. 



In all animals that have kidneys, that on the right is placed 

 higher than that on the left.*^ For, inasmuch as motion commences 

 from the right, the organs on this side become stronger than those 

 on the left, and must all push forward in advance of their opposite 

 fellows ; as may be seen in the fact that men even raise the right 

 eyebrow more than the left, and that the former is more arched 

 than the latter.^^ The right kidney being thus drawn upwards is 

 brought into contact with the liver ; for the liver in all animals lies 

 on the right side. 



Of all the viscera the kidneys are those that have the most 

 fat.^^ This is in the first place the result of necessity, because 

 the kidneys are the parts through which the residual matters 

 percolate. For the blood which is left behind after this excretion, 

 being of pure quality, is of easy concoction, and the final result 

 of thorough blood-concoction is lard and suet. For just as a 

 certain amount of heat is left in the ashes of. solid substances, 

 such as wood, after combustion, so also does a remnant of the 

 heat that has been developed remain in fluids after concoction ; 

 and this is the reason why oily matter is light, and floats on 

 the surface of other fluids.^* The fat is not formed in the 

 kidneys themselves, the density of their substance forbidding 

 this, but is deposited on their external surface. It consists 

 of lard or of suet, according as the animal's fat is of the 

 former or latter character. The difference between these two 

 kinds of fat has already been set forth in other passages.^^ 

 The formation, then, of fat in the kidneys is the result of 

 necessity ; being, as explained, . a consequence of the necessary 

 conditions which accompany the possession of such organs. But 

 at the same time the fat has a final cause, namely to ensure 

 672 a. 6 



