RESPIRATION. 



173 



"pulmonary" heart; while the left side is " : systemic " in function. In 

 many of the lower animals, however, no such distinction can be drawn. 

 Thus, in Fish the heart propels the blood to the respiratory organ (gills); 

 l)ii t there is no contractile sac corresponding to the left side of the heart, 

 to propel the blood directly into the systemic vessels. 



It may be well to state here that the lungs are only the medium for 

 the exchange, on the part of the blood, of carbonic acid for oxygen. They 

 are not the seat, in any special manner, of those combustion-processes 

 of which the production of carbonic acid is the final result. These occur 

 in all parts of the body more in one part, less in another: chiefly in the 

 substance of the tissues, but in part in the capillary blood-vessels contained 

 in them. 



THE RESPIRATORY PASSAGES AND TISSUES. 



The object of respiration is the interchange of gases in the lungs; for 

 this purpose it is necessary that the atmospheric air shall pass into them 



and be expelled from them. The lungs are contained in the chest or 

 thorax, which is a closed cavity having no communication with the out- 



