PARTS OF ANIMALS, III. iv. 



proportion" : this is because the nature of the material 

 and its bulk are especially obvious at the first stage 

 of a creature's formation. The heart is present in 

 all blooded animals, and the reason for this has been 

 already stated : It is obviously necessary for all 

 blooded creatures to have blood, and as blood is a 

 fluid, there must of necessity be a vessel to hold 

 it, and it is evidently for this purpose that Nature 

 has contrived the blood-vessels. And these blood- 

 vessels must have a source — one source (one is 

 always better than many where it is possible), and 

 this source is the heart. This is certain, because the 

 blood-vessels come out of the heart and do not pass 

 through it ; and again, the heart is homogeneous and 

 in character identical with the blood-vessels. Further- 

 more, the place in which it is set is the place of 

 primacy and governance. It is in a central position, 

 and rather in the upper part of the body than the 

 lower, and in front rather than at the back ; Nature 

 always gives the more honourable place to the more 

 honourable part, unless something more important 

 prevents it. WTiat I have just said is seen most 

 clearly in the case of man, yet in other animals the 

 heart tends in a similar way to be in the centre of 

 the " necessary body," i.e. the portion of it which 

 is terminated by the vent where the residues are 

 discharged. The limbs vary in the various animals, 

 and cannot be reckoned among the parts that are 

 *' necessary " for life, which is why animals can lose 

 them and still remain alive ; and obviously they could 

 have limbs added to them without being killed. 



Those who suppose that the source of the blood- 

 vessels is in the head are wrong, because : (1) this 

 involves holding that there are many sources, 



235 



