PARTS OF ANIMALS, III. iv. 



must be some place in the heart Avhich will be a 

 receptacle for the blood when first formed. (As we 

 have stated several times, blood is first formed in 

 the heart.) Now there are tΛvo chief blood-vessels, 

 the so-called Great Blood-vessel, and the Aorta ; each 

 of these is the source of other blood-vessels ; and 

 the two diifer from each other (this Avill be discussed 

 later) ; hence it is better for them to have separate 

 sources. This result can be obtained by ha\^ng two 

 separate supplies of blood, and thus we find two 

 receptacles wherever this is possible, as in the larger 

 animals which of course have large hearts. But it is 

 better still to have three cavities, and then there is 

 an odd one in the middle which can be a common 

 source for the other two ; since, hoAvever, this requires 

 the heart to be particularly large, only the very 

 largest hearts have three cavities. 



Of these cavities it is the right-hand one which 

 contains the most blood and the hottest (that is why 

 the right side of the body is hotter than the left) ; 

 the left-hand cavity contains least blood, and it is 

 colder. The blood in the middle cavity is inter- 

 mediate both in amount and heat, although it is the 

 purest of them all ; this is because the source must 

 remain as calm as possible, and this is secured Λvhen 

 the blood is pure, and intermediate in its amount and 

 heat. 



The heart has also a sort of articulation, which 

 resembles the sutures of the skull. By this I do not 

 mean to say that the heart is a composite thing, 

 consisting of several parts joined together, but an 

 articulated Avhole, as I said. This articulation is more 

 distinct in animals whose sensation is keen, and less 

 distinct in the duller ones, such as swine. There are 



243 



