PARTS OF ANIMALS, III. vi.-vii. 



the possession of a lung is one of their essential 

 characteristics, just as there are certain char- 

 acteristics which are included in the essence of 

 a " bird," the name which is applied to another such 

 class. 



VII. Some of the viscera appear to be single why the 

 {e.g. the heart and the lung) ; others double arrdoubie. 

 {e.g. the kidneys) ; and some it is difficult to place 

 under either heading. The liver and the spleen ap- 

 parently are intermediate ; they can be considered 

 either as each being a single organ, or else as two 

 organs taking the place of one and having a similar 

 character. In fact, however, all of them are double. 

 And the reason for this is that the structure of the 

 body is double, though its halves are combined under 

 one source. We have upper and lower halves, front 

 and back halves, right and left halves. Thus even the 

 brain as well as each of the sense-organs tends in all 

 animals to be double ; so does the heart — it has 

 cavities. In the Ovipara the lung is so much divided 

 that they appear to have two lungs. The kidneys 

 are obviously double ; but there is fair room for 

 hesitation about the liver and spleen. This is be- Liver and 

 cause in those animals which of necessity have a "P^^®^ 

 spleen, the spleen looks rather like a bastard liver, 

 while in those which have a spleen though not of 

 necessity — i.e. a very small one, as it were by way 

 of a token — the liver is patently double, and the 

 larger part of it tends to lie towards the right, the 

 smaller towards the left. Still, there are cases even 

 among the Ovipara where this division is less distinct 

 than in those just described, while in some Vivipara 

 the division is unmistakable — e.g. in some districts 



2G1 



