140 AJTATOMY AND PHYSIOLOGY. 



1st. That the venous blood arrives from all parts of the body by 

 the general system of veins ; 



2d. That from these veins it enters the right auricle of the heart; 



3d. That from the right auricle it passes into the right ventricle ; 



4th. That from the right ventricle the venous blood passes through 

 the pulmonary artery to the lungs ; 



5th. That in the capillary vessels, which form the termination 

 of the pulmonary artery, and commencement of the pulmonary 

 veins, this liquid is changed into arterial blood ; 



6th. That this arterial blood returns from the lungs, through the 

 pulmonary veins, and enters the left auricle of the heart ; 



7th. That from the left auricle it descends into the ventricle of 

 the same side ; 



8th. That from the left ventricle it is forced into the aorta, by 

 which it is distributed to all parts of the body. 



And 9th. and finally, that in the capillary terminations of the 

 system of canals formed by the aorta, the arterial blood acts upon 

 the organs, is changed there into venous blood, and enters the veins 

 to be carried again to the heart. 



In accomplishing the circulatory circle, the blood then passes 

 twice through the heart, in the state of venous b]ood on the right 

 side, and in the state of arterial blood in the left side of this organ ; 

 yet the circulation is complete, because the pulmonary and aortic 

 cavities of the heart do not open one into the other, and the venous 

 blood passes through the entire respiratory apparatus to be trans- 

 muted into arterial blood. 



From the description given, it must be plain that the office 

 filled by the heart, with its accurately-working valves, is essentially 

 that of a forcing-pump. And with what inimitable precision and 

 regularity does it perform this all-important duty ! Unweariedly 

 during the whole term of a long life it sends out daily its one hun- 

 dred thousand waves of healthful fluid to refresh and renovate every 

 corner of the system ; and small as each wave may be individually, 

 the aggregate amount is enormous. Thirteen thousand pounds of 

 blood pass out of the left ventricle of the heart of an ordinary man 

 every twenty-four hours. But the aorta of man is not an inch in 

 diameter, whereas the aorta of a whale is three feet two inches in 

 circumference. Well, therefore, might Dr. Paley say, that the. cir- 

 culation is a serious affair in such an animal. " The aorta of the 

 whale," says he, " is larger in the bore than the main pipe of the 

 water-works at London Bridge ; and the water roaring in its pas- 



