WAUM-BLOODED CIRCULATION. 251 



pulmonary artery. The left chamber, on the other 

 hand, is completely isolated ; the partition which 

 separates it from the other chambers being entire ; 

 and the whole of the blood which it receives from 

 the left auricle, and which has circulated through 

 the lungs, is sent by it into the left aorta, the chief 

 branches of which supply the head and limbs; 

 while a remaining branch unites with the right 

 aorta to supply the mixed blood from both sources 

 to the principal viscera of the body. It would thus 

 appear, from this arrangement of the vessels both 

 in Sauria and Chelonia, that the brain, or central 

 organ of the nervous system, requires, more than 

 any other part, a supply of oxygenised blood for 

 the performance of its functions. 



In these animals, a partial respiratory system is 

 provided for by the admission, through two canals 

 opening externally, of aerated water into the cavity 

 of the abdomen, where it may act upon the blood 

 which is circulating in the vessels. Traces of canals 

 of this description are also met with in some of the 

 higher classes of vertebrated animals, as, for in- 

 stance, among the Mammalia, in the Monotremata 

 and the 3Iarsupialia. 



It is in warm-blooded animals that the two offices 

 of the circulation are most efficiently performed ; 

 for the whole of the blood passes alternately througli 

 the greater and the lesser circulations ; and a com- 

 plete apparatus is provided for each. There are, 

 in fact, two hearts ; the one on the left side impel- 

 ling the blood through the greater or systemic 

 circulation ; the other, on the right side, appro- 

 priated to the lesser or pulmonary circulation. The 

 annexed diagram (Fig. 359), illustrates the plan of 

 the circulation in warm-blooded animals. From 



