iii. 4. 69 



organ in other respects. For the definitive characteristic of an 

 animal is the possession of sensation ; and the first sensory part 

 is that which first has blood ; that is to say is the heart, which 

 is the source of blood and the first of the parts to contain it. 



The apex of the heart is pointed and more solid than the rest 

 of the organ. It lies against the breast, and entirely in the 

 anterior part of the body, in order to prevent that region from 

 getting chilled. For in all animals there is comparatively but 

 little flesh over the breast, whereas there is a more abundant 

 covering of that substance on the posterior surface, so that the 

 heat has in the back a sufficient amount of protection. In all 

 animals but man the heart is placed in the centre of the pectoral 

 region ; but in man ^^ it inclines a little towards the left, so that 

 it may counterbalance the chilliness of that side. For the left 

 side is colder in man, as compared with the right, than in any 

 other animal. ^^ It has already been stated that even in fishes 

 the heart holds the same position as in other animals ; and the 

 reasons have been given why it appears not to do so. The 

 apex of the heart, it is true, is in them turned towards the 

 head, but this in fishes is the front aspect, for it is the direction 

 in which their motion occurs. ^^ 



The heart again is abundantly supplied with sinews,-" as might 

 reasonably be expected. For the motions of the body commence 

 from the heart, and are brought about by traction and relaxation. 

 The heart therefore, which, as already said, is as it were a living 

 creature inside its possessor, requires some such subservient and 

 strengthening parts. 



In no animals does the heart contain a bone, certainly in none 

 of those that we have ourselves inspected, '-^^ with the exception of 

 the horse and a certain kind of ox. In these exceptional cases 

 the heart, owing to its large bulk, is provided with a bone as a 

 support ; just as the bones serve as supports for the body 

 generally. 23 



In animals of great size the heart ^^ has three cavities ; in 

 smaller animals it has two ; and in all has at least one. The 

 reason for this, as already stated, is that there must be some 

 place in the heart to serve as a receptacle for the first blood ; 

 which, as has been mentioned more than once, is formed in this 

 organ. But inasmuch as the main blood-vessels are two in 

 number,^* namely the so-called great vessel and the aorta, each of 

 666b. 



