XXXIII. RESPIRATION 



Necessity for Respiration. — We have seen that plants and animals 

 need oxygen in order that the life processes may go on. Food is 

 oxidized to release energy, just as coal is burned to give heat to 

 run an engine. As a draft of air is required to make a fire under 

 the boiler, so, in the human body, oxj^gen must be given so that 



foods or tissues may be oxidized to re- 

 lease energy used in growth. Blood, in 

 its circulation to all parts of the body, is 

 the medium which conveys the oxygen to 

 that place in the body where it will be 

 used. But where does the blood get 

 this supply of oxygen? We have al- 

 luded, in Circulation, to the fact that 

 the lungs are the organs which give 

 oxygen to the blood and take from it 

 carbon dioxide. Let us examine the 

 organs used by the frog in breathing, 

 and see if this matter becomes any 

 plainer. 



Lungs of a frog ; baglike exten- 

 sions of the windpipe. 



Study of the Organs of Respiration in the Frog; Comparison with Man. — 

 Notice the pumping movement in the throat as a frog breathes. The frog 

 swallows air. Does this method differ from breathing in man? How 

 and where are the nostril holes (nares) placed? How is this of advantage 

 to the animal? Do you notice any movement of the nares in breathing? 

 Use a preserved frog for the following exercise • Open the mouth of the frog, 

 and find the openings in the roof of the mouth, leading from the anterior 

 nares. Find a vertical slit in the floor of the throat. This is the glottis 

 or opening to the windpipe. If the muscles be carefully removed from the 

 ventral surface of the body just beneath the arms, and a careful incision 

 made, the windpipe may be seen. Notice that it branches into two smaller 

 tubes, each of which leads to a lung. These tubes are the bronchi. The 

 lungs are two spongy bags, the walls of which are filled with tiny blood 

 vessels. Inflate the lungs by inserting a blowpipe in the glottis and blow- 

 ing into it. Are the lungs elastic? 



The Organs of Respiration in Man. — The course of air passing 

 from the outside of the lungs in man is much the same as in the 



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