72 iii. 5. 



are but offshoots from them. Now that these vessels exist on 

 account of the blood has already been stated. For every fluid" 

 requires a receptacle, and in the case of the blood the vessels 

 are that receptacle. Let us now explain why these vessels are 

 two, and why they spring from one single source, and extend 

 throughout the whole body. 



The reason, then, why these two vessels coalesce into one 

 centre, and spring from one source, is that the sensory soul is 

 in all animals actually one ; (in sanguineous animals one not 

 only actually but potentially, but in some bloodless animals 

 one actually only and not potentially ;2) and this one-ness of 

 the sensory soul determines a corresponding one-ness of the 

 part in which it primarily abides. Where, however, the sensory 

 soul is lodged, there also and in the self-same place must 

 necessarily be the source of heat ;* and, again, where this is 

 there also must be the source of the blood, seeing that it 

 thence derives its warmth and fluidity. Thus, then, in the one- 

 ness of the part in which is lodged the prime source of sensation 

 and of heat is involved the one-ness of the source in which the 

 blood originates ; and this, again, explains why the blood-vessels 

 have one common starting-point. 



The vessels, again, are two, because the body of every sanguineous 

 animal that is capable of locomotion * is bilateral ; for in all such 

 animals there is a distinguishable before and behind, a right and 

 left, an above and below. Now as the front is more honourable 

 and of higher supremacy than the hinder aspect, so also and in 

 like degree is the great vessel superior to the aorta. For the 

 great vessel is placed in front, while the aorta is behind ; the 

 former again is plainly visible in all sanguineous animals, while 

 the latter is in some indistinct and in some not discernible at all. 



Lastly, the reason for the vessels being distributed throughout 

 the entire body is that in them, or in parts analogous to them, 

 is contained the blood, or the fluid which in ex-sanguineous 

 animals takes the place of blood, and that the blood or analogous 

 fluid is the material from which the whole body is made. Now 

 as to the manner in which animals are nourished,^ and as to the 

 ways and means by which they absorb nutriment from the stomach, 

 these are matters which will be more suitably considered and 

 explained in the treatise on Generation and Development. But 

 inasmuch as the parts are, as already said, formed out of the blood, 

 668a. 



