A HISTORY OF SCIENCE 



than above it, and than they usually appear when 

 there is no bandage upon the arm. 



"It therefore plainly appears that the ligature pre- 

 vents the return of the blood through the veins to the 

 parts above it, and maintains those beneath it in a 

 state of permanent distention. But the arteries, in 

 spite of the pressure, and under the force and impulse 

 of the heart, send on the blood from the internal parts 

 of the body to the parts beyond the bandage." 5 



This use of ligatures is very significant, because, as 

 shown, a very tight ligature stops circulation in both 

 arteries and veins, while a loose one, while checking 

 the circulation in the veins, which lie nearer the surface 

 and are not so directly influenced by the force of the 

 heart, does not stop the passage of blood in the arteries, 

 which are usually deeply imbedded in the tissues, and 

 not so easily influenced by pressure from without. 



The last step of Harvey's demonstration was to 

 prove that the blood does flow along the veins to the 

 heart, aided by the valves that had been the cause 

 of so much discussion and dispute between the great 

 sixteenth-century anatomists. Harvey not only de- 

 monstrated the presence of these valves, but showed 

 conclusively, by simple experiments, what their func- 

 tion was, thus completing his demonstration of the 

 phenomena of the circulation. 



The final ocular demonstration of the passage of the 

 blood from the arteries to the veins was not to be 

 made until four years after Harvey's death. This 

 process, which can be observed easily in the web of a 

 frog's foot by the aid of a low-power lens, was first 



