RESPIRATORY CIRCULATION. 207 



with the materials they require for the performance 

 of their functions, undergoes during these pro- 

 cesses a considerable change in its chemical com- 

 position. In all the higher animals, the blood 

 i which is adapted for nutrition is of a bright Ver- 

 million colour ; but, after it has circulated in the 

 vessels of the body, it contains an excess of carbon, 

 and assumes in consequence a dark purple hue. 

 In this state it is not merely unfit for the purposes 

 of nutrition, but, if it were again circulated, it 

 would exert a noxious and even poisonous influence 

 on the organs. It must therefore be deprived of 

 this redundant carbon : and the separation of this 

 element is effected by the exposure of the blood 

 either immediately to the air of the atmosphere, or 

 to a fluid containing this air. Hence arises the 

 twofold function which devolves on the organs of 

 circulation : their first office being to collect the 

 vitiated, or carbonised blood from all parts of the 

 body into one or more central vessels, and to 

 convey it thence to the respiratory organs appro- 

 priated to its purification by the agency of oxygen : 

 the second, to bring back from the respiratory 

 organs this purified or oxygen ised blood, to collect 

 it into other central vessels, and thence to dis- 

 tribute it afresh over every part of the system. 



When viewed with reference to these offices, the 

 two sets of circulating organs may be designated, 

 the one the nutrient, and the other the oxygenisin<y 

 system. Each, in its complete form, may be com- 

 pared to a tree, of which the roots represent that 

 set of vessels, in either system, which are termed 

 veins; and which, commencing by minute radicles, 

 gradually join to form larger and larger vessels. 



