192 THE VITAL FUNCTIONS. 



vivifying influence; and to this office a distinct set of arte- 

 ries and veins is appropriated, constituting a distinct circu- 

 lation. This I have endeavoured to illustrate by the dia- 

 gram, Fig. 353, where D represents the auricle, and E the 

 Ventricle of the heart; and A and c, the main arterial and 

 Venous trunks; and where the two circulations are, for the 

 sake of distinctness, supposed to be separated from one ano- 

 ther, so that the two systems of vessels may occupy diffe- 

 rent parts of the diagram. The vessels which pervade the 

 body generally (B,) and are subservient to nutrition, belong 

 to what is termed the greater, or systemic circulation: those 

 which circulate the blood through the respiratory organs, 

 (R,) for the purpose of aeration, compose the system of the 

 lesser, or respiratory circulation. 



Few subjects in Physiology present a field of greater in- 

 terest than the comparison of the modes in which these two 

 great functions are, in all the various classes of animals, ex- 

 actly adjusted to each other. So intimately are the organs 

 of circulation related to those which distribute the blood to 

 the respiratory organs, that we never can form a clear idea 

 of the first, without a close reference to the last of these sys- 

 tems. While describing the several plans of circulation 

 presented to us by the different classes, I shall be obliged to 

 assume both the necessity of the function of respiration, and 

 of a provision of certain organs for the reception of air, ei- 

 ther in its gaseous fornv or as it is contained in the water, 

 where the blood may be subjected to its action. It is ne- 

 cessary, also, to state that the organs for receiving atmosphe- 

 ric air, in its gaseous state, are either lungs, or pulmonary 

 cavities, while those which are constructed for aquatic res- 

 piration are termed gills, or branchiae; the arteries and the 

 veins which carry on this respiratory circulation, being 

 termed pulmonary, 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 circulating blood 

 which undergoes aeration; the pulmonary or branchial arte- 

 ries and veins being merely branches of the general system 



