RESPIRATION. 2(J3 



the annals of our race as one of its most distin- 

 guished benefactors. The consideration, also, that 

 it had its source in the study of comparative ana- 

 tomy and physiology, affords us a convincing proof 

 of the great advantages that may result from the 

 cultivation of these sciences ; to which Nature, 

 indeed, seems, in this instance, expressly to have 

 invited us, by displaying to our view, in the organs 

 of the circulation, an endless diversity of combina- 

 tions, as if she had purposely designed to elucidate 

 their relations with the vital powers, and to assist 

 our investigations of the laws of organized beings. 



Chapter XI. 



RESPIRATION. 



<^ 1. Respiration in General. 



The action of atmospheric air is equally necessary 

 for the maintenance of animal and vegetable life. 

 As the ascending sap of plants cannot be perfected 

 unless exposed to the chemical agency of air in 

 the leaves; in like manner the blood of animals 

 requires the perpetual renovation of its vital pro- 

 perties by the purifying influence of respiration. 

 The great importance of this function is evinced 

 by the constant provision which has been made by 

 Nature, in every class of animals, for bringing each 

 portion of their nutritive juices, in its turn, into 

 contact with air. Even the circulation of these 

 juices is an object of inferior importance, compared 



