CeU 



1 



Broncm 

 al tubes 



Pul ^ 



vein 



Air 



Pulmonary 

 *S^ artery 





^y^-'^ 



^s^ 



^2t^ ^ f^-^->^ J 





O2 Lymph 



Blood ■ 

 vessel 



EXTERNAL AND INTERNAL RESPIRATION 



The external respiration consists of all the processes that bring oxygen to the sev- 

 eral millions of cells in the body, and remove from them the carbon dioxide which 

 they excrete. The internal respiration consists of the gas-exchange between any 

 body cell and the surrounding lymph. The external respiration thus includes the 

 muscular activities of pumping air into and out of the lungs; the actual movement of 

 air into and out of the lungs; and the osmotic movements of oxygen and carbon 

 dioxide between the air sacs and the blood, and between the blood vessels and the 

 lymph 



Gills^ The simplest kind of blood respiration is found in such animals 

 as the earthworm. In this, the respiration takes place by osmosis through 

 the moist epidermis, or skin. In some worms there are extensions of the 

 skin surface into little outgrowths, called gills. In clams and oysters there 

 are special outgrowths that multiply the breathing surface in much the 

 same way (see illustration opposite). We may think of the gills in lobsters, 

 crabs, and other water animals as structures in which the blood is brought 

 close to a great expansion of surface within a comparatively small space. 



Although insects are in general "air-breathers", some make their abode 

 in water for at least a part of the life cycle. The diving beetle comes to the 

 surface and takes down a supply of air under its wings. So does the water 

 boatman. Mosquitoes, in the larval and pupal stages, live in water; they get 



^See No. 8, p. 213. 

 208 



