RELATIVE SIZE OF THE HEART. 



7:3 



From this arrangement it is evident, that not only is partially aerated blood dif- 

 fused throughout the system, but, also, that a moiety only of the whole amount of 

 blood is sent to the lungs and exposed to the action of the atmosphere at each con- 

 traction of the ventricle of the heart. 



From the consideration of the heart and circulatory apparatus of the Chelonia 

 and Sauria, we pass very naturally to that of warm-blooded animals. 



The circulatory apparatus differs in no essential respect in the two great classes 

 of warm-blooded animals, birds and mammals. 



In these higher animals we have a double heart, and two distinct and complete 

 circulations of the blood. Each portion of blood which has passed through the 

 capillaries of the system and become vitiated, is aerated in the lungs before its dis- 

 tribution over the body. This is one of the most important of all distinctions 

 between warm and cold-blooded animals. 



The right heart is devoted to the circulation of venous bloock and the left heart 

 to the circulation of oxygenated or aerated blood. 



The auricle and ventricle of one heart have no communication with the auricle 

 and ventricle of the other except through the bloodvessels and capillaries. The 

 vessels of each heart are distinct, and perform distinct offices. 



The right auricle receives the venous blood from all parts of the system and 

 transmits it to the right ventricle. The contraction of the right ventricle distri- 

 butes the venous blood to the lungs. The oxygenated or arterial blood is conveyed 

 from the lungs to the left auricle, and thence to the left ventricle, and the con- 

 tractions of this distributes it throughout all parts of the system. 



As the circulatory apparatus is developed, the influence and importance of the 

 nervous system are increased, and corresponding arrangements established for its 

 perfect preservation. 



Another consideration to be taken into account is the relative size of the heart 

 and the rapidity of its action in different animals. 



I obtained the following results by carefully weighing the entire body of an 

 animal, and then ascertaining the weight of its heart upon a delicate balance 

 capable of turning to the y^^th part of a grain. 



Comj>arative Weights of the Hearts of Fishes. 



10 



