PARTS OF ANIMALS, III. vii. 



tliose who make water excessively : the fluids are 

 drawn back again into the stomach. But in animals 

 where the amount of residue produced is small, as in 

 birds and fishes, the spleen is either small or present 

 simply by way of a token. In the oviparous quadru- 

 peds, too, the spleen is small and compact, and 

 like a kidney, because the lung is spongy and the 

 animals drink little, and also because the residue 

 which is produced is applied for the benefit of the 

 body itself and of the scaly plates which cover it, 

 just as in birds it is applied for the benefit of the 

 feathers. 



In those animals, however, which possess a bladder, 

 and whose lung contains blood, the spleen is watery. 

 The reason already given partly explains this. An- 

 other is that the left side of the body is generally 

 more watery and colder than the right. As we know, 

 the opposites are divided up into two columns," so 

 that each is classed with those that are akin to it, e.g. 

 right is in the opposite column to left, and hot to 

 cold ; and thus some of them stand together in the 

 same column, as I have just indicated.^ 



Kidneys are present in some animals, but not 

 of necessity ; they are present to serve a good pur- 

 pose ; that is to say, their particular nature enables 

 them to cope with the residue which collects in the 

 bladder, in those cases where this deposit is somewhat 

 abundant, and to help the bladder to perform its 

 function better. 



Since the bladder is present in animals to serve 

 precisely the same purpose as the kidneys, we must 

 now say something about it. This will involve a 

 departure from the serial order in which the parts 

 actually come, for we have said nothing so far about 



267 



