RESPIRATORY CIRCULATION. 267 



systems. While describing the several plans 

 of circulation presented to us by the different 

 classes, I shall be obliged, therefore, to assume 

 both the necessity of the function of respiration, 

 and of a provision of certain organs for the 

 reception of air, either in its gaseous form, or 

 as it is contained in water, where the blood 

 may be subjected to its action. It is necessary, 

 also, to state that the organs for receiving atmos- 

 pheric air in its gaseous state are either huigSy 

 or pulmonary cavities, while those which are 

 constructed for aquatic respiration are termed 

 ^'ills, or branchice ; the arteries and the veins 

 which carry on this respiratory circulation, being 

 termed pulmonart/, or branchial, according as 

 they relate to the one or the other description 

 of respiratory organs. 



In many animals it is only a part of the cir- 

 culating blood which undergoes aeration ; the 

 pulmonary or branchial arteries and veins being 

 merely branches of the general system of blood 

 vessels : so that in this case, which is that 

 represented in the preceding figure (353), the 

 lesser circulation is included as a part of the 

 general circulation. But in all the higher classes 

 the whole of the blood is, in some part of its 

 circuit, subjected to the influence of the air ; 

 the pulmonary, being then distinct from the 

 systemic circulation. In the Annelida, for in- 

 stance, the venae cavse, which bring back the 



