CIRCULATION. 95 



animals this movement is called the circulation of the blood, 

 in plants the circulation of the sap ; in both, however, na- 

 ture has the same object in view, the conveyance into all 

 parts of the organism, of the elements which serve to their 

 increase and nutrition. 



In the higher order of animals the circulation is double, 

 that is to say, the blood which has deposited in every part 

 of the body the materials of nutrition, returns to its point 

 of departure, or to the heart, by a series of vessels named 

 veins, from whence it is driven by the muscular contrac- 

 tions of the heart into the lungs, where, by contact with 

 the atmosphere it acquires new properties and qualities, 

 and returns a second time to the point from whence it set 

 out. There is, therefore, two circulatory circles, and at 

 the base of each an agent for impelling the blood into each 

 of them. This agent consists in a double cavity with 

 muscular walls ; one of these cavities is named the auricle, 

 and the other the ventricle ; both of them united consti- 

 tute the heart, or organ of impulsion. 



The auricle and ventricle of the right side of the heart, 

 contain the dark venous blood, which returns impoverished 

 from all parts of the body ; the auricle and ventricle of the 

 left side contain the bright red arterial blood, which arrives 

 enriched from the respiratory organs. 



In the mammifera, with the exception of the Dugong, 

 -and in birds, there is no direct communication between the 

 right and left cavities of the heart, and consequently no 

 mixture of the arterial with the venous blood. Respiration 

 is therefore complete, that is to say, the whole of the blood 

 which returns from the different parts of the body passes 

 into the lungs, and is aerated before it is poured again into 

 the torrent of the circulation. These animals are there- 

 fore capable of preserving a proper temperature, indepen- 



