276 CIRCULATION 



not made good until the blood has been carried by the veins into 

 the right auricle, passed from this reception house into the body 

 of the pump, the right ventricle (P. II.) and forced by the action 

 of this subsidiary pump into the lung capillaries. There, as we 

 saw in the last chapter, it gets rid of the excess of carbon dioxide 

 and makes up its deficit 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 pump. 



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 

 mechanism. Harvey found that the left ventricle of a i 

 heart held two ozs. of blood without being distended. If onb 

 half the load were discharged at each systole and the heart b( 

 70 times per minute, then 700 ozs. or 44 pints of blood would 

 discharged into the aorta every 10 minutes. The total bl( 

 volume is under 9 pints. From this he urged the necessity 

 some communication between the arteries and veins. That 

 after experiment, observation, analysis and argument coi 



