Il6 THE HISTORY OF BIOLOGY 



the right to the left side of the heart through the lungs, ar.d in this he ac- 

 knowledges the services of Columbus in his explanation of this phenomenon. 

 With regard to the part played by the lungs and the air in this circulation 

 he has not much to add to the hypotheses of his predecessors. After having 

 thus described the small circulation Harvey proceeds to a presentation of 

 the blood's movement in the body itself and it is here that he brings out 

 his most daring originality. According to the old theory, food was converted 

 in the liver into blood, which was driven through the veins partly to the 

 heart, in order to receive the " spiritus vitalis," and partly into the body. To 

 this theory Harvey opposes a mathematical calculation; if the human heart 

 contains two ounces of blood and gives sixty-five beats to the minute, then 

 it drives in less than one minute ten pounds of blood out into the body. 

 Such a quantity of blood cannot incessantly arise from the food consumed, 

 but it must be assumed that the same quantity of blood incessantly circu- 

 lates in the body; it is driven out through the arteries and returns through 

 the veins. Harvey then collects a quantity of evidence in proof of this con- 

 clusion from the relation of the arteries and the veins in the body. He in- 

 vestigates the arterial pulse both in normal individuals and in those having 

 calcinated veins; he opens a live serpent and ties up first the vetia cava and 

 then the aorta; while the vein is emptied between the heart and the ligature 

 and swells up on the other side, the contrary is true of the aorta. He studies 

 the venous valves in a man's arm, which were discovered by Fabrizio, and 

 shows how they swell below a ligature; he severs a vein and an artery par- 

 allel to it and shows that the blood flows from the different ends of the 

 wound. On these and several other grounds, deduced from the study of every 

 possible animal form, he draws the conclusion that the arteries convey the 

 blood from the heart out into the body; there it is transmitted into the rami- 

 fications of the veins and flows from these into the principal vein and thence 

 back to the heart. The arterial blood, he considers, provides nourishment for 

 the body, while that of the veins is impure. How the transition between the 

 arterial and venous system takes place he could not explain; the capillary 

 system he was unable to distinguish, not having access to a microscope, and 

 he therefore assumed that some kind of ramified hollows formed the con- 

 necting link between the two. Another weak point in his theory was that 

 he could never find a satisfactory explanation of how the components of 

 the food are converted into blood, but he had to be content with the old 

 hypothesis that the liver was the medium in this process. He lived to see 

 the discovery by others of the lymphatic and thoracic ducts, but then he 

 was no longer capable of realizing how well these experiences complemented 

 his own discoveries; he desired to know nothing about them and on this 

 point adhered to the old theory. 



If we compare Harvey's account of the circulation of the blood with 



