PARTS OF ANIMALS, III. v. 



now go on to explain why there are two of these blood- 

 vessels, why they begin from a single source, and why 

 they extend all over the body. 



The reason why finally they both coincide in one 

 source and also begin from one source is this. The 

 sensory Soul is, in all animals, one actually ; there- 

 fore the part which primarily contains this Soul is 

 also one (one potentially as well as actually in the 

 blooded animals, but in some of the bloodless animals 

 it is only actually one °), and for this reason the source 

 of heat also must of necessity be in the selfsame 

 place. But this concerns the blood, for this source 

 is the cause of the blood's heat and fluidity. Thus 

 we see that because the source of sensation and the 

 source of heat are in one and the same part, the blood 

 must originate from one source too ; and because 

 there is this one origin of the blood, the blood-vessels 

 also must originate from one source. 



The blood-vessels are, however, two in number, be- 

 cause the bodies of the blooded creatures that move 

 about are bilateral : we can distinguish in all of 

 them front and back, right and left, upper and lower. 

 And just as the fore part is more honourable and 

 more suited to rule than the back part, so is the Great 

 Blood-vessel pre-eminent over the Aorta. The Great 

 Blood-vessel lies in front, while the Aorta is at the 

 back. All blooded creatures have a Great Blood- 

 vessel, plainly visible ; but in some of them the Aorta 

 is indistinct and in others it cannot be detected. 



The reason why the blood-vessels are distributed 

 all over the body is that blood (and in bloodless 

 creatures, its counterpart) is the material out of 

 which the whole body is constructed, and blood- 

 vessels (and their counterparts) are the channels in 



21-9, 



