362 CIRCULATION 



and makes up its deiicit of oxygen. Finally the blood, with its 

 fresh supply of nourishing substances from the alimentary canal 

 and of oxygen from the lungs, is poured into the receiving chamber 

 of the main pump — again to pass into the left ventricle and so 

 to the tissues. 



From the capillaries some of the constituents of the plasma 

 are forced into the spaces between the cells as lymph. From 

 these spaces the fluid either passes back into the capillaries or 

 flows away in a series of lymph vessels which carry it through 

 lymph glands {Ly.) from which it gains certain necessary consti- 

 tuents and finally bring it back to the central pinnp. 



This, in brief, is the circulation as we know it to-day, and this 

 knowledge is due in great part to the labours of Harvey. Before 

 his time little was known of how the blood was distributed in 

 the body. Of one point the old physiologists were sure, and that 

 was that there was no circulation of the blood, only an ebb and 

 flow. Harvey's work is a perfect example of how scientific work 

 should be carried out. First of all, he cleared his mind of all 

 preconceived ideas and got down to bedrock. Then he stated 

 his method. The method employed was that now made famous 

 by the author of Sherlock Holmes, viz., induction, based on careful 

 investigation. He examined the valves of the veins, and using 

 them as sign-posts, traced the course of the blood. Similarly, 

 the valves of the heart permit a current to flow in one direction 

 only. There never was a more complete argument than the one 

 that Harvey pressed for the circulation of the blood. There 

 could be no ebb and flow where all the valves were " one way." 



No scientific work is complete without a reference to quantities. 

 The test of truth must rest with the balance or measuring mecha- 

 nism. Harvey found that the left ventricle of a man's heart 

 held 2 oz. of blood without being distended. If only half the 

 load were discharged at each systole and the heart beat 70 times 

 per minute, then 700 oz. or 44 pints of blood would be discharged 

 into the aorta every 10 minutes. The total blood volume is under 

 9 pints. From this he argued the necessity of some communication 

 between the arteries and veins. That is, after experiment, 

 observation, analysis and argument, come reasoned hypotheses. 

 Four years after Harvey's death the great Italian anatomist 

 Malpighi saw under the microscope these capillaries which the 

 physician had seen with the eye of faith. The demonstration 

 of the actual passage of blood from arteries to veins through 

 capillary channels was given in 1688 by Leeuwenhoek, the illiterate 

 janitor of the aldermen of Delft. 



Haemodynamics. Dynamically considered, the blood acts in 



