CHAPTER XIX 



RESPIRATION 



Tracheae of insects. 



Gills of fish. 



What is the relation of respiration to excretion? How do the lungs 

 perform their function f 



Most of the carbon dioxide and some of the water, formed by 

 the body cells during the oxidation of food materials, are eliminated 

 through the lungs. There are no organs in a higher plant similar 

 to the lungs of man, although the stomata, openings for the in- 

 take and outgo of air, in leaves are concerned with the function 

 of respiration in the plants. The abundance of these stomata 

 make up for their microscopic size. They are found in the epi- 

 dermal tissues of a leaf and they lead to air spaces among cells 

 between the two layers of epidermis. These openings are not 

 adapted for securing large quantities of air at one time. They 

 may be compared to the nostrils of man rather than to the lungs. 



Lower animals have various devices for securing air. An insect 

 has small openings in the abdomen and thorax which lead into 

 branching tubes. These tubes subdivide until they can reach the 

 smallest cell in the body. By compressing and releasing their 

 body regions, somewhat like a bellows, the animals are able to 

 take in oxygen and give out carbon dioxide. Watch the abdomen 

 of a fly or a bee and note the breathing movements. A fish has 

 sills which are branched structures with such thin walls that air 

 can pass directly from the water into the blood stream. The air 



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