SCIENTIFIC METHOD 79 



" Whence A it is demonstrated that there is a con- 

 tinuous motion of the blood in a circle, affected by 

 the beat of the heart." 



It was not till 1628 that Harvey published his 

 Anatomical Disquisition on the Motion of the Heart 

 and Blood in Animals. It gives the experimental 

 basis of his conclusions. If a live snake be laid open, 

 the heart will be seen pulsating and propelling its 

 contents. Compress the large vein entering the heart, 

 and the part intervening between the point of constric- 

 tion and the heart becomes empty and the organ pales 

 and shrinks. Remove the pressure, and the size and 

 color of the heart are restored. Now compress the 

 artery leading from the organ, and the part between 

 the heart and the point of pressure, and the heart 

 itself, become distended and take on a deep purple 

 color. The course of the blood is evidently from the 

 vena cava through the heart to the aorta. Harvey in 

 his investigations made use of many species of ani- 

 mals at least eighty-seven. 



It was believed by some, before Harvey's demon- 

 strations, that the arteries were hollow pipes carry- 

 ing air from the lungs throughout the body, although 

 Galen had shown by cutting a dog's trachea, inflat- 

 ing the lungs and .tying the trachea, that the lungs 

 were in an enclosing sack which retained the air. 

 Harvey, following Galen, held that the pulmonary 

 artery, carrying blood to the lungs from the right 

 side of the heart, and the pulmonary veins, carrying 

 blood from the lungs to the left side of the heart, in- 

 tercommunicate in the hidden porosities of the lungs 

 and through minute inosculations. 



In man the vena cava carries the blood to the right 



