^6 iii, 6 — iii. 7. 



These animals, speaking generally, are also distinguished from 

 others by their smaller bulk.^^ For heat promotes growth, and' 

 abundance of blood is a sure indication of heat. Heat again 

 tends to make the body erect ; and thus it is that man is the 

 most erect of animals, and the vivipara more erect than other 

 quadrupeds.^6 For no viviparous animal, be it apodous '^'^ or be it 

 possessed of feet, is so given to creep into holes as are the ovipara. 



The lung then exists for respiration ; and this is its universal 

 office. But the amount of blood it contains, and its structure 

 generally, varies with the requirements of different groups of 

 animals.^^ There is however no one term to denote all animals 

 that have a lung, no designation, that is, like the term Bird, 

 applicable to the whole of a certain class. Yet the possession of 

 a lung is a part of their essence, just as much as the presence of 

 certain characters constitutes the essence of a bird. 



(Ch. y.) Of the viscera some appear to be single, as the heart 

 and lung ; others to be double, as the kidneys ; while of a third 

 kind it is doubtful in which class they should be reckoned. For 

 the liver and the spleen would seem to lie half-way between 

 the single and the double organs. For they may be regarded 

 either as constituting each a single organ, or as a pair of organs 

 resembling each other in character.^ 



In reality however all the organs are double.^ The reason for 

 this is that the body itself is double, consisting of two halves, 

 which are however combined together under one supreme centre. 

 For there is an upper and a lower half, a front and a rear, a 

 right side and a left. 



This explains why it is that even the brain and the several 

 organs of sense tend in all animals to consist of two parts ; 

 and the same explanation applies to the heart with its cavities. 

 The lung again in ovipara is divided to such an extent, that 

 these animals look as though they had actually two lungs. ^ As 

 to the kidneys no one can overlook their double character. But 

 when we come to the liver and the spleen, any one might 

 fairly be in doubt. The reason of this is, that, in animals that 

 necessarily have a spleen,* this organ is such that it might be 

 taken for a kind of bastard liver ; while in those, in which a 

 spleen is not an actual necessity but is merely present, as it 

 were, by way of token, in an extremely minute form, the liver 

 plainly consists of two parts ; of which the larger tends to lie 

 669 b. 



