CHAPTER XXIV 

 CIRCULATION 



" The circling streams, once thought but pools of blood, 

 (Whether life's fuel or the body's food) 

 From dark oblivion Harvey's name shall save." 



DRYDEN. 



THE inland transport system that we have had under consideration 

 differs materially from our canal system. Not only are the barges 

 submersed in the plasma but the force which carries them along 

 is the force which causes the plasma itself to move. The water- 

 ways are a series of elastic-walled tubes forming a closed circuit. 

 In this circuit is a central pumping station, the heart which keeps 

 the blood in motion. The accompanying figure (Fig. 58) is a 

 diagrammatical view of a vertical-mesial section through the 

 heart. From it we learn that the heart is not a simple structure. 

 In the diagram four distinct cavities can be seen, viz. : right and 

 left ventricles, left auricle and aortic space the right auricle is 

 not shown. The heart is really a double pump consisting of a 

 main pump or systemic heart (left auricle and ventricle) and 

 a subsidiary pump or pulmonary heart. In Fig. 59 is given 

 a scheme of the circulation. By contraction of the left ventricle 

 the blood is forced along a series of conducting tubes or arteries 

 (Art.) which lead to every part of the body and end in the substance 

 of the tissues in a network of innumerable hair-like canals, the 

 capillaries (Cap). These capillary vessels are the wharves of the 

 tissues. Through their walls takes place the exchange of imports 

 and exports by which we measure the metabolism of the tissues. 

 Consequently it is found when the capillaries join together to 

 form the wider conducting canals, venules and veins, that the 

 blood has lost some of its cargo of oxygen and nutrient matter, 

 and has gained a certain amount of waste matter. This with- 

 drawal of nutrient material is made good by the diversion of 

 some of the blood from an arterial canal to the capillaries in the 



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