166 NATURE OF RESPIRATION. 



the inhaled air takes place, in consequence of which 

 the former is found to have assumed its florid or 

 arterial hue, and to have regained its power of sup- 

 porting life. The blood then enters minute venous 

 ramifications, which gradually coalesce into larger 

 branches, and at last terminate in four large trunks 

 in the left side of the heart ; whence the blood in its 

 arterial form is again distributed over the body, to 

 pursue the same course and undergo the same 

 changes as before. 



There are thus two distinct circulations, each 

 carried on by its own system of vessels : the one, 

 from the left side of the heart to every part of the 

 body, and back to the right side ; and the other, 

 from the right side of the heart to the two lungs, 

 and back to the left. The former has for its object 

 nutrition and the maintenance of life ; and the latter 

 the restoration of the deteriorated blood, and the 

 animalization or assimilation of the chyle from which 

 that fluid is formed. 



As the food cannot become a part of the living 

 animal, or the venous blood regain its lost proper- 

 ties until they have undergone the requisite changes 

 in the air-cells of the lungs, the function of respira- 

 tion, by which these are effected, is one of pre-emi- 

 nent importance in the animal economy, and well 

 deserves the most careful examination. The term 

 respiration is frequently restricted to the mere inha- 

 lation and expiration of air from the lungs ; but 

 more generally it is employed to designate the 

 whole series of phenomena which occurs in these 

 organs. The words sanguification, and aeration of 

 the blood are other forms of expression occasionally 

 used to denote that part of the process in which the 

 blood, by exposure to the action of the air, passes 

 from the venous to the arterial state ; and, as the 

 chyle does not become assimilated to the blood 

 until it has passed through the lungs, the term san- 

 guification, or blood-making, is not unaptly applied. 



