178 



RESPIRATION AND EXCRETION 



easily be felt as the Adam's apple of the throat, divides into two 

 bronchi (br6n'k!). The bronchi within the lungs break up into a 

 great number of smaller bronchial tubes, which divide somewhat 

 like the small branches of a tree. The bronchial tubes are lined 

 with ciliated cells, the cilia of which are constantly in motion. 



They lash with a quick stroke 

 toward the outer end of the 

 tube, that is, toward the mouth. 

 Hence any foreign material will 

 be raised first by the action of 

 the cilia and then by coughing 

 or " clearing the throat. '^ The 

 bronchial tubes end in very 

 minute air sacs, little pouches 

 having elastic walls, into which 

 air is taken when we inspire, or 

 take a deep breath. In the 

 walls of these pouches are nu- 

 merous capillaries. It is 

 through the very thin walls of the 

 air sacs that a diffusion of gases 

 takes place ivhich results in the 

 blood giving up carbon dioxide, 

 and taking up oxygen in its 

 place. 



Changes in the Blood within the Lungs. — Blood leaving the 

 lungs is much brighter red than when entering them. The change 

 in color is due to the combination of oxygen with the haemoglobin 

 of the red corpuscles to form oxyhsemoglobin. The changes 

 taking place in the blood are obviously the reverse of those which 

 take place in the air in the lungs. 



Changes in Air in the Lungs. — Air is much warmer when it 

 leaves the lungs than before it enters them. Breathe on the bulb 

 of a thermometer to prove this. Expired air contains a consider- 

 able amount of moisture, as may be proved by breathing on a cold 

 polished surface. The loss of moisture from the body in expired 

 air is about half a pint in twenty-four hours. Carbon dioxide in 

 expired air may be detected easily by the lime water test. 



The lungs are two masses of many tubes 

 and sacs. 



