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To do this he compares the volume of the heart muscle with that of the 

 temporal and masseter muscles and the weight they can support. He 

 makes acute observations on the How of blood in the arteries. His 

 observations in this regard bring one to the time of Stephen Hales, 

 who was the first to measure accurately the blood-pressure in the 

 arteries of a horse. 



Harvey also applied a numerical method in connection with the 

 amount of blood passing through the heart, and his calculation formed 

 part of the evidence he adduces that led him to think that the blood 

 might, "as it were, move in a circle." Here is the passage : 



" But what remains to be said upon the quantity and source of the blood which thus 

 passes is of so novel and unheard of a character, that I not only fear injury to myself 

 from the envy of a few, but I tremble lest I have mankind at large for my enemies, so 

 much doth wont and custom, that become as another nature, and doctrine once sown 

 and that hath struck deep root, and respect for antiquity influence all men. Still the die 

 is cast, and my trust is in my love of truth, and the candour that inheres in culti- 

 vated minds. And sooth to say, when I surveyed my mass of evidence, whether derived 

 from vivisections and my various reflections on them, or from the ventricles of the heart 

 and the vessels that enter into and issue from them, the symmetry and size of these 

 conduits for nature, doing nothing in vain, would never have given them so large a 

 relative size without a purpose or from the arrangement and intimate structure of the 

 valves in particular, and of the other parts of the heart in general, with many things 

 besides, I frequently and seriously bethought me, and long revolved in my mind, what 

 might be the quantity of blood which was transmitted, in how short a time its passage 

 might be effected, and the like ; and not finding it possible that this could be supplied by 

 the juices of the ingested aliment without the veins on the one hand becoming drained, and 

 the arteries on the other getting ruptured through the excessive charge of blood, unless 

 the blood should somehow find its way from the arteries into the veins, and so return to 

 the right side of the heart ; I began to think whether there might not be a motion, as it 

 were in a circle. Now this I afterwards found to be true ; and I finally saw that the 

 blood, forced by the action of the left ventricle into the arteries, was distributed to the 

 body at large, and its several parts, in the same manner as it is sent through the lungs, 

 impelled by the right ventricle into the pulmonary artery, and that it then passed 

 through the veins and along the vena cava, and so round to the left ventricle in the 

 manner already indicated. Which motion we may be allowed to call circular, in the 

 same way as Aristotle says that the air and the rain emulate the circular motion of the 

 superior bodies ; for the moist earth, warmed by the sun, evaporates ; the vapours drawn 

 upwards are condensed, and, descending in the form of rain, moisten the earth again ; 

 and by this arrangement are generations of living things produced ; and in like manner 

 too are tempests and meteors engendered by the circular motion, and by the approach 

 and recession of the sun." 



It is a singular fact that, notwithstanding the laws of optics were 

 well known to the ancients, the invention of spectacles and the micro- 

 scope came very late in the history of human progress. Magnifying 

 glasses were in use in the sixteenth century, but with the invention of 

 the compound microscope a new and potent instrument was added to 

 the investigators' armamentarium. In this connection we shall recall 

 the work of some early pioneers, Malpighi, Grew, Swammerdam, 

 Leeuwenhoek, Redi, and others. 



