THE LUXGS. 337 



endosmose and exosmose, which enable the solu- 

 tion of a gas to pass throTigh a moist mem- 

 brane with considerable facility. 



The requisite conditions, then, for the che- 

 mical changes of respiration to take place be- 

 tween the air and the blood are, access of fresh 

 air, and the circulation of dark blood on one 

 side of a moist animal membrane. In the lungs 

 these conditions exist to their fullest degree. 

 There, perpetual influxes of fresh air play upon 

 an enormous surface of animal membrane, which 

 is covered with a dense mesh of blood-vessels, 

 all carrying dark blood. To these organs the 

 streams of blood from the remotest parts of the 

 body are directed by the propulsive energies of 

 the heart. At the same time, muscular arrange- 

 ments, externally and internally disposed with 

 miraculous skill, and kept in action by an un- 

 tiring power, continually partly fill and empty 

 these organs, which are subdivided into innu- 

 merable tubes, terminated by minute cells, 

 producing the ordinary phenomena of inspi- 

 ration and expiration, or, in other words, of 

 breathing. By this means, a measured quan- 

 tity of air is admitted to the chest, and then 

 expelled again, and so on alternately, about 

 eighteen times in each minute. The pure air 

 is thus received, and the impure is discharged. 



At this stage, two facts about respiration are 



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