iii. 4. 6^ 



all sanguineous animals, and the reason for this has already been 

 given. For that sanguineous animals must necessarily have blood 

 is self-evident. And, as the blood is fluid, it is also a matter of 

 necessity that there shall be a receptacle for it ; and it is clearly 

 to meet this requirement that nature has devised the blood-vessels. 

 These, again, must necessarily have one primary source. For it 

 is preferable that there shall be one such, when possible, rather 

 than several. This primary source of the vessels is the heart.^ 

 For the vessels clearly are from it, and not through it.^ Moreover, 

 being as it is homogeneous, it has the character of a blood-vessel. 

 Again its position is that of a primary or dominating part. For 

 nature, when no other more important purpose stands in her way, 

 places the more honourable part in the more honourable position ;' 

 and the heart lies about the centre of the body, but rather in its 

 upper than its lower half, and also more in front than behind. 

 This is most evident in the case of man, but even in other animals 

 there is a tendency in the heart to assume a similar position, in 

 the centre of the necessary part of the body, that is to say of the 

 part which terminates in the vent for excrement. For the limbs 

 vary in position in different animals, and are not to be counted 

 with the parts which are necessary for life. For life can be 

 maintained even when they are removed ; while it is self-evident 

 that the addition of them to an animal is not destructive of it. 



There are some^ who say that the vessels commence in the 

 head. In this they are clearly mistaken. For in the first place, 

 according to their representation, there would be many sources 

 for the vessels, and these scattered ; and secondly these sources 

 would be in a region that is manifestly cold, as is shown by its 

 intolerance of chill, whereas the region of the heart is as manifestly 

 hot.' Again, as already said, the vessels extend through all the 

 other viscera, but no vessel extends through the heart.^" From 

 this it is quite evident that the heart is a part of the Vessels and 

 their origin ; and for this it is well suited by its structure. For its 

 centraPi part consists of a thick and hollow body, and is more- 

 over full of blood, as though the vessels took thence their origin. 

 It is hollow to serve for the reception of the blood, while its wall 

 is thick, that it may serve to protect the source of heat. For 

 here, and here alone in all the viscera and in fact in all the 

 body, there is blood without blood-vessels, the blood elsewhere 

 being always contained within vessels.' ^ Nor is this but con- 

 666 a. 



