RESPIRATORY CIRCULATION. 



191 



within us, during the whole period of our lives, in every, 

 even the minutest, portion of our frame. How inadequate, 

 then, must be any ideas we are capable of forming of the in- 

 calculable number of movements and of actions, which are 

 conducted in the living system; and how infinite must be 

 the prescience and the wisdom, by which these multifarious 

 and complicated operations were so deeply planned, and so 

 harmoniously adjusted! 



353 



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 a very important office to perform. The 

 blood undergoes, in the course of its circulation, considera- 

 ble 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 pur- 

 ple, from its containing an excess of carbon, and is conse- 

 quently unfit to be again circulated. Whenever, from some 

 derangement in the functions, 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 circu- 

 late, destroy life. Hence, it is ne- 

 cessary that the blood which returns 

 by the veins should undergo purifi- 

 cation, by exposure either to the air 

 itself, or to a fluid containing air, for 

 the purpose of restoring and pre- 

 serving its salutary qualities. The 

 heart and vascular system have, 

 therefore, the additional task as- 

 signed them of conveying the vi- 

 tiated venous blood to certain or- 

 gans, where it may have access to the air, and receive its 



