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^ 3. Respiratory Circulation, 



The object of the circulation is not merely to 

 distribute the blood through the general system 

 of the body ; it has also another and very im- 

 portant office to perform. The blood undergoes, 

 in the course of its circulation, considerable 

 changes, both in its colour and its chemical 

 composition. The healthy blood transmitted by 

 the arteries is of a bright scarlet hue ; that 

 brought back by the veins is of a dark purple, 

 from its containing an excess of carbon, and 

 is consequently unfit to be again circulated. 

 Whenever, from some derangement in the func- 

 tions, this dark blood finds its way into the 

 arteries, it acts as a poison on every organ which 

 it reaches, and would soon, if it continued to 

 circulate, destroy life. Hence it is necessary 

 that the blood which returns by the veins should 

 undergo purification, by exposure either to the 

 air itself, or to a fluid containing air, for the 

 purpose of restoring and preserving its salutary 

 qualities. The heart and vascular system have 

 therefore the additional task assigned them of 

 conveying the vitiated venous blood to certain 

 organs, where it may have access to the air, and 

 receive its vivifying influence ; and to this office 

 a distinct set of arteries and veins is appro- 



